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Dampness penetration problems in granite buildings in Aberdeen, UK: Causes and remedies
Construction and Building
MATERIALS
Construction and Building Materials 21 (2007) 1846–1859
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Dampness penetration problems in granite buildings in Aberdeen, UK: Causes and remedies Maureen E. Young
*
Scott Sutherland School, Robert Gordon University, Garthdee Road, Aberdeen AB10 7QB, United Kingdom Received 23 June 2005; received in revised form 13 January 2006; accepted 31 May 2006 Available online 10 October 2006
Abstract Many of the buildings in Aberdeen constructed prior to the mid 20th century are constructed of granite. The impermeable nature of this stone puts heavy demands on the performance of bedding and pointing mortars and affects the nature of penetrating damp problems. The particular dampness problems associated with granite buildings, especially with gable walls was investigated by case studies and a questionnaire survey of building professionals. While a variety of defects contributed to investigated damp problems, a high proportion were found to be involve mortar performance issues and decayed mortar debris blocking void spaces. A relatively high proportion of practitioners reported problems caused by incorrect specification of mortars and had encountered problems that originated at the time of repointing of buildings. A significant minority were of the opinion that the use of power tools for raking out joints contributed to debris build-up in voids. On granite gables the demands placed on mortar performance are particularly severe since the mortar has to do all the work in dealing with moisture movement. Many of the penetrating damp problems encountered in granite buildings can be attributed to debris dislodged by the use of power tools in raking out of joints, loss or removal of harling (rendering) and to incorrect specification of pointing mortars. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Damp; Penetration; Granite; Masonry; Mortar
1. Introduction Many of the older dwellings in Aberdeen (pre-1945) are of solid granite construction rather than sandstone which is commonly used elsewhere in Scotland. Granite buildings are no more likely to suffer from penetrating damp problems than sandstone buildings – the rates for both construction types are 10% [1]. But, despite the similarity in numbers, the nature of problems encountered in granite buildings is different to those in sandstone buildings. Rising damp affects a significant number of sandstone properties (of those with penetrating dampness, approximately 40% are affected by rising damp [1]) – this is much less of a problem in granite buildings – implying that a greater proportion of the problems in granite buildings are from non*
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rising damp sources. The extent of damp penetration problems in Aberdeen City is at about the Scottish national average (6%) [1]. The city is below the national average for problems with condensation at 8% compared to a national average of 11% [1]. Due to changes in construction methods, date of construction has a very significant effect on subsequent damp problems. Data clearly indicates that the older the building, the greater the likelihood of penetrating damp problems [1]. This backs up anecdotal information from Aberdeen City Council that many notifications of damp problems involve Victorian granite tenements. These buildings are constructed of a solid masonry wall (granite ashlar and/ or rubble), with lime-based bedding mortar, variable pointing composition and a void space of a few centimetres width between the external wall and internal lath and plaster. On ashlar walls, pointing mortar constitutes only a small percentage of the surface area, but on rubble walls
M.E. Young / Construction and Building Materials 21 (2007) 1846–1859
more than 50% of the exposed surface may be pointing mortar (Fig. 1). The performance of pointing and bedding mortar is therefore critically important in relation to moisture penetration. Concern at the number of such buildings affected by penetrating damp problems which were proving difficult to resolve, prompted Aberdeen City Council to establish the research programme outlined here. 2. Previous research Unless it is significantly weathered, granite is impermeable – it does not absorb rainwater and moisture cannot flow through it. All moisture movement in granite walls therefore occurs in the mortar joints. Similar problems have also been noted in brick structures [2] where low water absorption rates in brick concentrated moisture movement in mortar joints. In such structures cement-rich mortars have caused problems due to water access through shrinkage cracks [2]. Although it is the general rule that harder mortars are used for harder masonry, this rule does not apply where the stone is impermeable, as is the case with granite. The Society for the Protection of Ancient Buildings (SPAB) [3] recommend using the softest, most permeable mortar possible. A relatively high fines or lime content may be necessary to ensure adequate bonding between the mortar and
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granite [4]. In permeable masonry, moisture transfer from the mortar draws fine binding agents to the interface zone to contribute to a good mechanical bond [2] – this does not happen with impermeable stones. This advice must however be tempered by consideration of the degree of exposure of a wall since a very soft, permeable mortar may be unsuitable for such a situation. Lime-rich mortars are relatively soft, permeable and are able to accommodate more strain than cement-rich mortars. They can also self-heal: effectively resetting after movement occurs – an important property with respect to preventing moisture ingress. However, lime-rich mixes can be a significant source of calcium salts which have been shown to be harmful to stone by contributing to salt decay processes [5–7]. Gypsum is a known cause of granite decay and black, gypsum-rich soiling crusts and associated blistering, scaling and other forms of decay are commonly observed in association with pointing mortars (Fig. 2). While lime-rich mortars may be preferred for their permeability and self-healing properties, lime–sand mortars are more difficult to use in practice than mixes which incorporate cement and the free calcium from lime-rich pointing mortars is a concern with respect to soluble salts [8]. Natural hydraulic lime mortars, like non-hydraulic limes, are able to accommodate movement in a structure [4,9]. Hydraulic lime contains reactive silica and alumina compounds which allow a more rapid early gain in strength
Fig. 1. On ashlar walls (left), pointing mortar constitutes only a small percentage of the surface area, but on rubble walls (right) more than 50% of the exposed surface may be pointing mortar.
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veyors, building consultants, chartered surveyors and individuals and companies involved in architectural services and damp control) in the Grampian area. The aim of the questionnaire survey was to establish the degree of knowledge among correspondents regarding current damp problems (both penetrating damp and condensation) and solutions to these problems. Sixty-one completed questionnaires were returned. 4. Results
Fig. 2. Build-up of calcium sulphate (gypsum) crusts on granite surfaces lead to decay of the granite surface by blistering and granulation of the substrate.
than occurs with non-hydraulic lime mortars and their use may be preferred where there is concern about the possible harmful effects of free lime [3]. Hydraulic limes also have good vapour permeability [4]. Cement-rich mortar mixes have relatively high strength. They can be prone to development of shrinkage cracks which can be routes for rapid ingress of rainwater to the core of a wall [3]. The characteristics and workability of cement-rich mortars can be altered by the addition of lime, pozzolan, plasticizers (which improve air entrainment) or other additives and by altering the proportion of cement or water in the mix. Some authors [5,10] have investigated variants of cement-rich mixes to overcome their known disadvantages while retaining their low soluble calcium content. 3. Research methodology The Masonry Conservation Research Group was commissioned by the Aberdeen Property Repair Initiative to examine the particular problems associated with granite buildings, especially with gable walls. Investigations consisted of 12 case studies of buildings reported to have penetrating damp problems and a questionnaire survey which was sent out to 200 practitioners (architects, building sur-
Twelve varied properties with reported damp problems were examined (Table 1). This included examination of the exterior from street level and in some instances from higher access, for any obvious defects (e.g., roofing, pointing, gutters, downpipe defects, etc.) and examination of the interior to determine the areas affected by dampness. Factors that could be contributing to dampness problems were noted. Records of previous refurbishment or remedial treatments to the buildings were examined for evidence of previous interventions and to determine the extent of current and previous damp problems. Professionals involved with current and previous interventions on the properties contributed their knowledge regarding problems and remedies. In some instances building owners were able to contribute historical information regarding the origins of damp problems. In 9 of the 12 properties damp problems affected a gable wall constructed of granite rubble. There was no indication of any preponderance of problems relating to a particular orientation with respect to prevailing wind. It was notable that the first reports of damp problems by building occupiers often occurred within a few months or years of repair interventions (Table 1). Table 2 summarises the conclusions of the case studies with respect to the most likely causes of damp problems. While a variety of defects were thought to contribute to the damp problems it is notable that problems related to pointing mortars were thought to have contributed to problems in four properties and moisture bridging across debris in the void space was though to be involved in at least five cases. Three case studies illustrate the range of problems encountered during the project. 4.1. Case study 1 A longstanding damp problem (8 years duration) in a stairwell (Fig. 3) was thought to be due to damp penetration of the solid granite wall, which was ‘‘plastered on the hard’’, i.e., there is no void space between the outer wall and the internal surface finish. Any moisture which penetrates the wall will therefore have easy access to internal surfaces. Inspection of the external wall (Fig. 4) revealed green algae and moss on the wall in line with the exit of a downpipe and overflow from the parapet gutter at the roof of the stair tower. Water was reported to ‘‘spout’’ from the overflow during heavy rain. This turned out to
Yes No No No No No No No No No No No No No No No No No No No No No Yes Yes No No No No Yes No No Yes No No No Yes October 1996 January 1989 January 2002
January 2000 January 1992
Repointing, reslating Repointing Unknown Repointing Repointing Repointing Repointing and other works
1890 1874 1900 approximately 1925 1900 approximately Early 1800s 1850 approximately 1899 1887 1903 1902 13th century 1 2 3 4 5 6 7 8 9 10 11 12
July 1992 January 1989 January 2000 None January 1991 January 1990 None January 1994 January 1992 April 1997 April 1995 None
Repointing, reslating, lead work Repointing Repointing and skews
January 1998 December 1997 December 1997
N SE SE SE NW S N NE SE S S All Squared granite rubble, no void Coursed granite rubble Coursed granite rubble Coursed granite rubble Ashlar granite Squared granite rubble Squared granite rubble Ashlar granite Squared granite rubble Squared granite rubble Granite rubble and squared rubble Granite rubble
No Yes Yes Yes No Yes Yes No Yes Yes Yes Yes
Facing direction Date of last known repairs Age of property Case study no.
Table 1 Summary of characteristics of case study properties
Nature of repairs
Earliest known reported damp problem
Wall type
Gable
Front facade
Rear wall
Stairwell
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be the clue to the source of this particular longstanding damp problem – blockage and possible damage to a parapet gutter. The downpipe from the parapet drain was blocked (it was only about 2 in. diameter) and may have been cracked. Repairs included unblocking the pipe and its replacement with a 3 in. outlet. This illustrates the difficulty of diagnosing the cause of damp problems. Previous inspections of this property over a period of four years had attributed damp to rainwater penetration through the wall. Suggested remedies included rebuilding the chimney, blocking and/or ventilating flues, dry dashing the external wall or coating it with water repellent. It was only in exceptionally damp weather that water spouting from the overflow was observed and reported by the occupant of a flat overlooking the property. Damp problems do not always manifest themselves close to the defect. Water can penetrate some distance from its point of ingress before emerging on internal surfaces. Blistering and peeling paint on the stairwell was observed from roof down to first floor level. 4.2. Case study 2 A granite tenement block in which damp problems were experienced on the gable wall at first floor level (Fig. 5). Damp patches occurred in various locations on the inner surface of the gable wall (Fig. 6). The current resident had been in the flat for 60 years or more and was never aware of any damp problems until 1989 when repointing and other repairs were carried out. Damp problems have all occurred subsequent to these repairs. The quality of granite on the gable wall was mixed and pointing was flush with the surface. Observation of cracks in the pointing mortar suggests that it may be relatively cement-rich. On tapping the sitting room wall surface it was evident that many damp patches coincided with solid areas. Five years previous to this study the wall had been opened up and bridging rubble removed from the void, but this did not solve the problem as the damp re-appeared. Previous attempts to resolve the problem have included capping chimneys and lead-lining the skews. The internal wall was taken down and rebuilt. When the void space was opened up, bedding mortar was found to be decayed and this debris formed much of the fill that was again bridging the void. In the rebuilding process, timber safe lintels over the alcoves (which were badly affected by damp), were replaced with concrete with lead caps. A breathable membrane was installed behind the newly reconstructed internal wall. This should prevent any penetrating damp from coming through although the source of the damp remains unclear. Anecdotal evidence of the flat owner points to damp problems originating at the time of repointing. Although the source of penetrating damp was not resolved in this instance, it is believed that the use of power tools in raking out of joints was a major contributory factor as this is the most likely cause of subsequent build-up of debris in the
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Table 2 Summary table showing the most likely primary causes and contributory factors relating to the damp problems investigated Case study no.
Primary cause
1 2 3 4 5 6 7 8 9 10 11 12
Defective hidden gutter Cracked pointing Porous pointing Water penetration down uncapped chimney Roof level defect Cracked pointing Defective detailing on adjacent roof Defective skews and guttering Defective pointing Unknown Poor workmanship and materials Loss of harling
Contributory factors Defective Debris in Debris in Debris in
ridge tiles, deterioration of bedding mortar, hard pointing mortar void space void space, deterioration of bedding mortar void space, deterioration of bedding mortar
Debris in void space Debris in void space, deterioration of bedding mortar
Fig. 3. Case study 1. Blistering of paint and plaster in the stairwell. The wall is solid granite with no void space, i.e., ‘‘plastered on the hard’’.
void spaces which allowed moisture bridging to internal surfaces. 4.3. Case study 3
Fig. 4. Stairwell affected by damp. Lower arrow shows location of Fig. 3. Unlike the rest of the stairwell, this granite rubble wall is not harled. Inset shows location of outlet and overflow pipes from hidden gutter.
This gable wall of this property faces south-east and is highly exposed to driving rain from that direction since there are no adjacent buildings. The gable was repointed about three years before the current study. There were some minor damp problems reported before this time, which were diagnosed as being due to deterioration of skews and local-
ised deterioration of pointing. Both these problem were resolved. More recently, serious damp problems began after repointing and other works. Exceptionally, dampness is here visible both internally and on the exterior of the gable (Fig. 7). The coincidence of darker patches with persistent
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Fig. 5. Case study 2. Arrow shows the location of damp problems illustrated in Fig. 6. The granite rubble gable wall has been repointed flush with a small overlap of pointing onto the granite.
damp (after 3 weeks of warm, sunny weather with no rainfall) was confirmed by measurements of electrical conductivity. The gable wall in the flats’ internal stairwell was affected by water penetration; damp coming through very
Fig. 7. Case study 3. South-east facing highly exposed granite rubble gable wall. Darker patches correspond to areas of persistent damp, confirmed by measurements of electrical conductivity.
Fig. 6. Case study 2. Damp patches on the internal surfaces of the gable wall. The door on the right leads into a small cupboard which was also affected by damp.
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Fig. 8. RILEM tube for measuring water absorption, applied to mortar surface.
5
Run 7 (33)
Run 4 (179)
soon after rain events implying that the wall core was saturated. Debris was found in the void space which was allowing moisture bridging across to the inner wall. Polythene linings have been installed behind new plasterboard walls of rooms adjacent to the gable and no dampness is now reported in these locations. It was clear on observing the exterior of the gable that some areas have been damp for long periods of time since patches of green algal growth could be observed, despite the south-east facing aspect of the gable. Green algae require high levels of moisture for several months to establish themselves and their presence on a south facing wall is an indication of long-standing damp. The specification for repointing was for 1:1:6 mortar. Analysis of the composition (by Sandberg Consulting Engineers) found ratios of 1:0.3:4.4. This is a cement-rich mix compared to that specified. Data on water absorption rates was obtained using RILEM tubes (Fig. 8) to measure the rate of uptake of water. Water absorption rates varied over the gable (Fig. 9). Permeability in some areas was within normal limits for 1:1:6 mortar (Table 3), but many areas had higher (or much higher) permeability, especially in areas most affected by dampness. The rapid penetration of dampness inside the flats after rainfall is consistent with observations of highly permeable pointing on some areas of the gable. In the lower flat, where the problem was investigated, this coincides with an area of porous pointing. It is likely that the damp problems in this property were caused by excessively permeable pointing mortar on a highly exposed gable. Excessive water absorption will lead Run 6 (29) Run 9 (27) Run 8 (25)
4 Water absorbed (mL)
Run 3 (15)
3
Run 5 (9)
2
1
Runs 1 & 2 (<0.33) 0 0
5
10
15
20
25
30
35
Time elapsed (√s) Fig. 9. Water absorption data from nine separate RILEM tube measurements on gable wall mortar. Numbers in brackets show the equivalent capillary water absorption coefficient (see Table 3). There is an extremely wide range in permeability; while some areas are highly impermeable (Runs 1 and 2), others are highly (probably excessively) permeable (Runs 4, 6–9).
M.E. Young / Construction and Building Materials 21 (2007) 1846–1859 Table 3 Comparative data on capillary water absorption coefficients available from other authors Substrate
pffiffiffi Capillary water absorption coefficient ðkg=m2 hÞ
Proportions of mix Portland cement
Lime mortar [12] Hydraulic lime mortar [12] Cement:lime:sand [11] Cement:lime:sand [11] Cement mortar N type [11] Cement mortar S type [11] Cement mortar [12]
Masonry cement
Hydraulic lime
Lime
Sand
1
2 3 41/2 6 3 3 3
1 1 1
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1/2 1 1 1
1
23 27 4 5 1 0.7 5
Masonry cement type N: A mix containing Portland cement and finely ground minerals. Also includes air entraining, plasticizing and set retarding agents. Regular compressive strength. Masonry cement type S: A mix containing Portland cement, finely ground minerals and additives. Produces mortars with higher mechanical resistance than masonry type N.
Cavity timber frame Don't know Never occurs Traditional cavity
Lath and plaster Very common Plastered on the hard
0
20
40
60
80
100
% respondents Fig. 10. Practitioners’ opinion regarding whether ‘‘buildings of a particular type [were] more susceptible to damp problems?’’.
to a build-up of water in the pointing and bedding mortar as the gable will absorb more water than it can easily lose by evaporation. It is clear that the wall core of some areas of the gable must have become very wet, probably saturated, allowing moisture bridging across debris in void spaces.
N NE
NW
4.4. Practitioner survey In the questionnaire survey practitioners were asked for their opinion on whether ‘‘. . . buildings of a particular type [were] more susceptible to damp problems?’’. Results (Fig. 10) show very clearly that practitioners are aware of many more damp problems with some types of building construction. Solid masonry walls plastered on the hard were most susceptible. Problems were also fairly common on solid masonry walls with lath and plaster. Substantially fewer damp problems were experienced with cavity walls. Damp problems in granite properties were most frequently thought to occur on North and north-east facing walls (Fig. 11) (note that no significant correlation of damp problems with facing direction was found in the case studies). Practitioners felt that prevailing northerly
W
E 10
20
30
SE
SW S
Fig. 11. Practitioners’ opinion regarding which facing directions of gable walls were most likely to suffer damp problems.
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Failed skews or fillets Failed downpipes or gutters Damaged slates or tiles Damaged pointing Uncapped flues Loss of harling Bedding mortar decay Gutter grading Water through pointing Water through stone 0
20
40
60
80
100
% respondents Don’t Know Never occurs
Very common
Fig. 12. Practitioners’ answers to the question: ‘‘To what extent are the following factors responsible for MOISTURE PENETRATION problems in buildings?’’. Full descriptions of each factor were as follows: failed skews or tilted fillets, overflows from blocked/damaged downpipes or gutters, lost or damaged slates/tiles, cracked or lost external pointing, rainwater penetration down uncapped chimneys, removal/loss of harling, moisture penetration due to decay of bedding mortar, improper grading of runs on gutters, moisture permeating through undamaged pointing, moisture permeating through stone.
and north-easterly winds, in combination with low levels of sun exposure and low evaporation rates contributed to damp problems. With respect to moisture penetration problems (Fig. 12), practitioners felt that the most commonly encountered problems related to defects or failures of roofing materials (slates, skews, tilted fillets) or of water shedding (downpipes and gutters). Improper grading of gutters was occasionally encountered as a contributory problem. In relation to pointing and mortar, cracked pointing was considered to be a common cause of moisture penetration, decayed bedding mortar less so, but it was cited as a contributory cause in some instances. Rainwater penetration down un-used, uncapped chimneys was noted as a relatively common problem which could be resolved by capping chimneys. Lack of harling (rendering) was found to be a relatively frequent cause of dampness on walls which were originally harled, but where the harling has subsequently been lost or removed. Water penetration through stone was seldom encountered. With respect to condensation (Fig. 13), inadequate ventilation of rooms was by far the single most important factor in causing condensation problems. Gas fires (especially portable gas heaters) were noted as a major source of water vapour. Factors that reduce the rate of ventilation were important contributory causes to condensation; these included blocking off fireplaces, poor air circulation in
voids and capping of disused flues. However, while 18% of respondents thought that condensation problems in capped flues were a moderately or very common problem, many more (31%) though rainwater penetration down uncapped flues was moderately or very common (Fig. 12). Lack of flue liners was occasionally found to cause problems though this was an uncommon defect. Factors related to the external skin of the building were not thought to be major contributory causes of condensation. These included hard, impermeable pointing, application of water repellents, painting or harling of walls. Practitioners were asked whether they thought that some ‘‘damp problems may be related to . . . characteristics of mortar or pointing?’’ (Fig. 14). Practitioners thought that incorrect specification of mortar mixes was very common – specifically, mortar mixes being too hard and cement-rich. Damaged pointing was also thought to be a very significant factor. Formation of shrinkage cracks in pointing mortar is, of course, more likely to occur where the mix is too hard. Decay of bedding mortar was probably an important influence on some damp problems and flue gases were thought occasionally to cause damage to bedding mortar through escape of moisture laden, acidic, corrosive fumes. With respect to whether penetrating damp problems are often solved by repointing, or whether they may begin after repointing – while a majority of practitioners (55%) felt that repointing could
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Poor room ventilation Condensation from gas fire Blocked off fireplaces Capped flues Poor air circulation in voids Lack of flue liners Apply water repellent Hard pointing Painting external walls Harling external walls 0
20
40 60 % respondents
80
100
Don’t know Never occurs
Very common
Fig. 13. Practitioners’ answers to the question: ‘‘To what extent are the following factors responsible for CONDENSATION problems in buildings?’’. The full descriptions of each factor were as follows: inadequate ventilation of rooms, condensation from gas fire, inadequate ventilation of blocked off fireplaces, inadequate ventilation of capped flues, lack of air circulation in void space or cavity wall, lack of flue liners, application of water repellent to external walls, excessively hard or impermeable pointing, painting external walls, harling external walls.
often solve damp problems, a number (21%) had noticed damp problems beginning after repointing and a significant minority (13%) thought that repointing normally caused more problems than it solved. The method of raking out of old pointing may be a significant issue in this respect. While very few practitioners (10%) thought that hand raking led to debris build-up in voids and cavities, rather more practitioners (34%) thought vibrations caused by mechanical raking out could lead to or exacerbate debris build-up in voids. Moisture bridging across debris in voids was one of the most commonly observed causes of damp problems. Practitioners were asked which methods they had used to tackle damp problems and for their opinion on how successful each method had been (Fig. 15). Repairs to obvious physical defects were normally successful, with reported success rates over 90% for repairs to defects in slates, guttering, skews or rebuilding a wall. Improving ventilation was a simple and effective remedy for many condensation problems. For many other methods, the reported results were quite mixed. Clearing out bridging debris from voids was reported as being successful in 94% of cases. Repairs to pointing reported mixed levels of success. Practitioners reported that replacing pointing was fully successful as a
treatment for dampness in only about 50–75% of cases. Regrouting walls to replace decayed bedding mortar was often reported to be successful although it was not commonly used. Replacing lost harling was also often successful. Treatments for rising damp, including electro-osmosis, barriers and improved drainage, were reported as being fairly successful. With respect to flues, capping flues to prevent water penetration was reported to cure the problem in about 80% of cases. Uncapping flues to prevent condensation problems was successful in only 60% of cases, and was seldom done. Installing or replacing flue liners solved condensation problems in many cases, though it was a relatively uncommon fault. The lowest success rate was reported for water repellent treatment (<50%). Water repellent treatment often (or normally) fails to address the causes of water penetration problems, it is therefore not surprising that water repellents failed more often than not. Water repellents will largely prevent moisture ingress through a stone or mortar surface, but in practice this is not likely to be a significant problem. Most moisture penetrates the building fabric through cracks and other defects which water repellents cannot seal. Once inside a wall the water repellent may inhibit drying out, exacerbating the problem.
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Mortars incorrectly specified Damaged pointing Re-pointing mortar too hard Bedding mortar decayed Problems solved by re-pointing Debris from mechanical raking Flue gases damage mortar Problems after re-pointing Debris from hand raking 0
20
40
60
80
100
% respondents No answer Definitely not
Certainly occurs
Fig. 14. Practitioners’ opinion as to whether some ‘‘damp problems may be related to . . . characteristics of mortar or pointing?’’. Factors are summarised in the graphs. The full descriptions of each factor were as follows: Do you think mortar mixes are often incorrectly specified, Does much moisture penetration occur through damaged pointing, Do you think repointing mortar mixes are often too hard, Does decay of bedding mortar lead to moisture penetration, Are many damp problems solved by repointing, Does mechanical raking out cause debris build-up in voids/cavities, Have you noticed that bedding mortar is damaged by flue gases, Have you found that damp problems begin after repointing, Does raking out by hand cause debris build-up in voids/cavities.
5. Discussion Some penetrating damp problems are relatively easy to diagnose, others are more problematic and some defy numerous attempts at a solution. Failure of slates or other roofing materials or physical defects and damage to gutters and downpipes are relatively easy to diagnose and fix. Where vegetation blocks rainwater drainage or gutters are wrongly fitted, water will overflow down the face of a building causing stone decay to be accelerated and increasing the likelihood of water penetration of joints and defects. Failures of hidden gutters may show up as excessive biological growth or soiling on lower stonework (Fig. 16). Damaged or decayed pointing allows easy ingress of water through joints, as do fissures and cracks in the stone due to settlement. Sometimes repairs use a technique which is inappropriate and potentially damaging. For instance, the practice of ribbon (or tuck) pointing (Fig. 17) where the pointing mortar stands proud of the surface is deprecated since it slows down the run-off of rainwater over the wall surface. Projecting pointing diverts water flow along the top surface of the pointing increasing the likelihood of penetration along cracks and into the joint. Retained moisture and soiling
on the upper surface encourages vegetation to colonise the joints. The case studies reported showed granite rubble gable walls were clearly implicated in a relatively large number of cases. There was no indication that damp problems were related to any particular facing direction, although with only 12 examples sample numbers were rather too small to draw any definitive conclusions in this respect. With respect to the relative timing of building owners reporting dampness problems, as far as can be ascertained, a significant proportion of reports of dampness date from within a few months or years of refurbishment or other major interventions on the property. This may imply a link between certain interventions and occurrence of dampness problems; however, although there may be no known reports of prior damp problems, it is not necessarily clear that these did not exist prior to intervention. Dampness often comes to the notice of home owners when it penetrates internal walls across dooks (wooden plugs in a wall) or accumulations of bridging debris. While this may be the route of penetration of moisture to internal surfaces, the source of that moisture it is not necessarily easily established. Root causes may be either penetration of rainwater or build-up of condensation. Even where dampness
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No experience 80 Not successful/don’t know Partially successful
40
% respondents
60
Successful
Uncapping flues
Replace hard pointing
Apply water repellent
Regrout bedding mortar
Install flue liners
Electro-osmosis
Replace soft pointing
Rebuild wall
Barrier treatment
Replace harling
Cleaning debris from voids
Capping flues
Improve ventilation
Repointing
Improve drainage
Repair skews etc
Repair guttering etc.
Repair slating etc.
20
Fig. 15. Methods used by respondents to tackle damp problems and their degree of success. ‘‘Successful’’ implies that the method has been used and that it did work in solving damp problems. ‘‘Not successful/don’t know’’ implies that the method was used but did not work. Some methods were ‘‘Partially successful’’. Respondents who did not report any results for particular methods are recorded as ‘‘No experience’’.
problems do occur following interventions, the connection between any one intervention and later dampness can seldom be unambiguously determined. Nevertheless the evidence suggests a strong link between damp problems and debris build-up in void spaces, highlighting the danger of interventions (such as the use of power tools during raking out of joints) which may dislodge bedding mortar where this is in a friable condition, as was found to be the case in many Victorian tenements. Where bedding mortar is in poor condition, the voids within it may act as reservoirs and conduits for water flow through the wall core. The low permeability and excessive hardness of some pointing mortars may make a significant contribution to damp problems by allowing moisture ingress through shrinkage cracks, and preventing adequate drying of bedding mortar and wall cavities. 6. Conclusions On granite gables the demands placed on mortar performance are particularly severe since pointing mortar often covers more than 50% of the exposed surface of rubble walls, the exposure of gables to climatic effects (e.g., wind driven rain) is often greater than that of street front facades and in granite buildings, due to the impermeability of the stone, the mortar has to do all the work in dealing with moisture movement. Many dampness problems which are
not caused by obvious physical defects appear to be related to mortar performance. Aspects of mortar performance which have been found to be problematic include the following: 1. Bedding mortar in Victorian tenements is often in a friable state and easily dislodged by vibration – especially by the severe vibration caused by the use of power tools to rake out joints. Many damp problems that appear after repointing can be attributed in part to moisture bridging across void spaces that have become clogged with dislodged debris. This problem could be greatly reduced if raking out was done by hand. 2. Some rubble buildings or gables were originally harled, especially those built in the 18th and early 19th centuries. This was done for aesthetic reasons and to reduce the danger of rainwater penetration. To modern eyes, stone is more prestigious (and more expensive) than harling so people like to see exposed stonework, but on buildings which were intended to be harled, this puts greater demands on the mortar performance – perhaps greater than it can cope with. Replacing harling should be considered as a potential solution to damp penetration problems on buildings which were originally harled. 3. Cement-rich mortars can be too hard and impermeable. They reduce the rate of water penetration through the mortar, but they do not allow moisture to evaporate
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M.E. Young / Construction and Building Materials 21 (2007) 1846–1859
Fig. 16. Failures of hidden gutters may cause excessive biological growth or soiling on lower stonework.
readily and they are prone to shrinkage cracking which provides alternative and rapid access routes for moisture into the wall core. 4. Lime-rich mortars are soft and more permeable. They allow moisture to evaporate relatively easily, but they can potentially be too permeable on highly exposed gables, allowing too much rainwater penetration relative to evaporation rates. They have the advantage of being able to self-heal by re-forming calcium carbonate in hair-line cracks thereby avoiding moisture penetration through cracks, but the reactivity of free calcium can result in build-up of calcium sulphate (gypsum) crusts in sheltered areas leading to decay of the granite surface. The mortar with the ideal combination of strength, permeability, flexibility and water soluble lime has yet to be established for granite walls. Until this is done, pointing mortars are likely to be an ongoing source of problems as conventional mixes necessarily compromise between incompatible characteristics. Acknowledgements This research was funded by Aberdeen City Council through the Aberdeen Property Repair Initiative. Case
Fig. 17. Ribbon or tuck pointing is poor practice on granite walls as it may divert rainwater run-off into joints. Retained moisture also encourages colonisation of horizontal surfaces.
study properties and maintenance history information were provided by Gilbert McCurdy, Grants Officer, Aberdeen City Council; Andy Pitblado, Aberdeen Care and Repair, Allan Cumming RIAS, Cumming and Co.; Raymond Simpson, Raymond Simpson Assoc. Ltd.; John Carroll, JV Carroll Building Consultants; and Dave Gauld, Ron Shanks Assoc. References [1] Scottish House Condition Survey; 2002. Available from: http:// www.shcs.gov.uk/ [accessed May 2005]. [2] Groot CJWP, Gunneweg J. Water permeance problems in single wythe masonry walls: the case of wind mills. Constr Build Mater 2004;18:325–9. [3] SPAB. Repointing stone and brick walling. SPAB Technical Pamphlet No. 5; 2002. [4] Allen G, Allen J, Elton M, Farey M, Holmes S, Livesey P, et al. Hydraulic lime mortar for stone, brick and block masonry. Shaftesbury: Donhead; 2003. [5] Duffy AP, Cooper TP, Perry SH. Repointing mortars for conservation of a historic stone building in Trinity College, Dublin. Mater Struct 1993;26:302–6. [6] Duffy AP, Perry SH. The effects of mortars on granite decay. In: Bell E, Cooper TP, editors. Granite Weathering and Conservation. Trinity College, Dublin, September 1993. Dublin: Director of Buildings’ Office; 1994. p. 1–9.
M.E. Young / Construction and Building Materials 21 (2007) 1846–1859 [7] O’Brien PF, Bell E, Pavia Santamaria S, Boyland P, Cooper TP. Role of mortars in the decay of granite. Sci Total Environ 1995;167:103–10. [8] Perry SH, Duffy AP. The short-term effects of mortar joints on salt movement in stone. Atmos Environ 1997;31(9):1297–305. [9] Livesey P. Succeeding with hydraulic lime mortars. J Arch Conser 2002;8(2):23–37. [10] Mosquera MJ, Benı´tez D, Perry SH. Pore structure in mortars applied on restoration: effect on properties relevant to decay of granite buildings. Cement Concr Res 2002;32:1883–8.
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[11] Kumaran K, Lackey J, Normandin N, van Reenen D, Tariku F. Summary Report from Task 3 of MEWS Project at the Institute for Research in Construction. Hygrothermal properties of several building materials. IRC Research Report 110. Ottawa: National Research Council Canada; 2002. [12] Moropoulou A, Avdelidis NP, Koui M. Compatibility assessment of building materials using Infrared Thermography. In: Proceedings of the 15th world conference on nondestructive testing, Roma, Italy, 15– 21 October 2000.
MATERIALS
Construction and Building Materials 21 (2007) 1846–1859
www.elsevier.com/locate/conbuildmat
Dampness penetration problems in granite buildings in Aberdeen, UK: Causes and remedies Maureen E. Young
*
Scott Sutherland School, Robert Gordon University, Garthdee Road, Aberdeen AB10 7QB, United Kingdom Received 23 June 2005; received in revised form 13 January 2006; accepted 31 May 2006 Available online 10 October 2006
Abstract Many of the buildings in Aberdeen constructed prior to the mid 20th century are constructed of granite. The impermeable nature of this stone puts heavy demands on the performance of bedding and pointing mortars and affects the nature of penetrating damp problems. The particular dampness problems associated with granite buildings, especially with gable walls was investigated by case studies and a questionnaire survey of building professionals. While a variety of defects contributed to investigated damp problems, a high proportion were found to be involve mortar performance issues and decayed mortar debris blocking void spaces. A relatively high proportion of practitioners reported problems caused by incorrect specification of mortars and had encountered problems that originated at the time of repointing of buildings. A significant minority were of the opinion that the use of power tools for raking out joints contributed to debris build-up in voids. On granite gables the demands placed on mortar performance are particularly severe since the mortar has to do all the work in dealing with moisture movement. Many of the penetrating damp problems encountered in granite buildings can be attributed to debris dislodged by the use of power tools in raking out of joints, loss or removal of harling (rendering) and to incorrect specification of pointing mortars. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Damp; Penetration; Granite; Masonry; Mortar
1. Introduction Many of the older dwellings in Aberdeen (pre-1945) are of solid granite construction rather than sandstone which is commonly used elsewhere in Scotland. Granite buildings are no more likely to suffer from penetrating damp problems than sandstone buildings – the rates for both construction types are 10% [1]. But, despite the similarity in numbers, the nature of problems encountered in granite buildings is different to those in sandstone buildings. Rising damp affects a significant number of sandstone properties (of those with penetrating dampness, approximately 40% are affected by rising damp [1]) – this is much less of a problem in granite buildings – implying that a greater proportion of the problems in granite buildings are from non*
Tel.: +44 0 1224 263710; fax: +44 0 1224 263777. E-mail address: [email protected]
0950-0618/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.05.027
rising damp sources. The extent of damp penetration problems in Aberdeen City is at about the Scottish national average (6%) [1]. The city is below the national average for problems with condensation at 8% compared to a national average of 11% [1]. Due to changes in construction methods, date of construction has a very significant effect on subsequent damp problems. Data clearly indicates that the older the building, the greater the likelihood of penetrating damp problems [1]. This backs up anecdotal information from Aberdeen City Council that many notifications of damp problems involve Victorian granite tenements. These buildings are constructed of a solid masonry wall (granite ashlar and/ or rubble), with lime-based bedding mortar, variable pointing composition and a void space of a few centimetres width between the external wall and internal lath and plaster. On ashlar walls, pointing mortar constitutes only a small percentage of the surface area, but on rubble walls
M.E. Young / Construction and Building Materials 21 (2007) 1846–1859
more than 50% of the exposed surface may be pointing mortar (Fig. 1). The performance of pointing and bedding mortar is therefore critically important in relation to moisture penetration. Concern at the number of such buildings affected by penetrating damp problems which were proving difficult to resolve, prompted Aberdeen City Council to establish the research programme outlined here. 2. Previous research Unless it is significantly weathered, granite is impermeable – it does not absorb rainwater and moisture cannot flow through it. All moisture movement in granite walls therefore occurs in the mortar joints. Similar problems have also been noted in brick structures [2] where low water absorption rates in brick concentrated moisture movement in mortar joints. In such structures cement-rich mortars have caused problems due to water access through shrinkage cracks [2]. Although it is the general rule that harder mortars are used for harder masonry, this rule does not apply where the stone is impermeable, as is the case with granite. The Society for the Protection of Ancient Buildings (SPAB) [3] recommend using the softest, most permeable mortar possible. A relatively high fines or lime content may be necessary to ensure adequate bonding between the mortar and
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granite [4]. In permeable masonry, moisture transfer from the mortar draws fine binding agents to the interface zone to contribute to a good mechanical bond [2] – this does not happen with impermeable stones. This advice must however be tempered by consideration of the degree of exposure of a wall since a very soft, permeable mortar may be unsuitable for such a situation. Lime-rich mortars are relatively soft, permeable and are able to accommodate more strain than cement-rich mortars. They can also self-heal: effectively resetting after movement occurs – an important property with respect to preventing moisture ingress. However, lime-rich mixes can be a significant source of calcium salts which have been shown to be harmful to stone by contributing to salt decay processes [5–7]. Gypsum is a known cause of granite decay and black, gypsum-rich soiling crusts and associated blistering, scaling and other forms of decay are commonly observed in association with pointing mortars (Fig. 2). While lime-rich mortars may be preferred for their permeability and self-healing properties, lime–sand mortars are more difficult to use in practice than mixes which incorporate cement and the free calcium from lime-rich pointing mortars is a concern with respect to soluble salts [8]. Natural hydraulic lime mortars, like non-hydraulic limes, are able to accommodate movement in a structure [4,9]. Hydraulic lime contains reactive silica and alumina compounds which allow a more rapid early gain in strength
Fig. 1. On ashlar walls (left), pointing mortar constitutes only a small percentage of the surface area, but on rubble walls (right) more than 50% of the exposed surface may be pointing mortar.
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veyors, building consultants, chartered surveyors and individuals and companies involved in architectural services and damp control) in the Grampian area. The aim of the questionnaire survey was to establish the degree of knowledge among correspondents regarding current damp problems (both penetrating damp and condensation) and solutions to these problems. Sixty-one completed questionnaires were returned. 4. Results
Fig. 2. Build-up of calcium sulphate (gypsum) crusts on granite surfaces lead to decay of the granite surface by blistering and granulation of the substrate.
than occurs with non-hydraulic lime mortars and their use may be preferred where there is concern about the possible harmful effects of free lime [3]. Hydraulic limes also have good vapour permeability [4]. Cement-rich mortar mixes have relatively high strength. They can be prone to development of shrinkage cracks which can be routes for rapid ingress of rainwater to the core of a wall [3]. The characteristics and workability of cement-rich mortars can be altered by the addition of lime, pozzolan, plasticizers (which improve air entrainment) or other additives and by altering the proportion of cement or water in the mix. Some authors [5,10] have investigated variants of cement-rich mixes to overcome their known disadvantages while retaining their low soluble calcium content. 3. Research methodology The Masonry Conservation Research Group was commissioned by the Aberdeen Property Repair Initiative to examine the particular problems associated with granite buildings, especially with gable walls. Investigations consisted of 12 case studies of buildings reported to have penetrating damp problems and a questionnaire survey which was sent out to 200 practitioners (architects, building sur-
Twelve varied properties with reported damp problems were examined (Table 1). This included examination of the exterior from street level and in some instances from higher access, for any obvious defects (e.g., roofing, pointing, gutters, downpipe defects, etc.) and examination of the interior to determine the areas affected by dampness. Factors that could be contributing to dampness problems were noted. Records of previous refurbishment or remedial treatments to the buildings were examined for evidence of previous interventions and to determine the extent of current and previous damp problems. Professionals involved with current and previous interventions on the properties contributed their knowledge regarding problems and remedies. In some instances building owners were able to contribute historical information regarding the origins of damp problems. In 9 of the 12 properties damp problems affected a gable wall constructed of granite rubble. There was no indication of any preponderance of problems relating to a particular orientation with respect to prevailing wind. It was notable that the first reports of damp problems by building occupiers often occurred within a few months or years of repair interventions (Table 1). Table 2 summarises the conclusions of the case studies with respect to the most likely causes of damp problems. While a variety of defects were thought to contribute to the damp problems it is notable that problems related to pointing mortars were thought to have contributed to problems in four properties and moisture bridging across debris in the void space was though to be involved in at least five cases. Three case studies illustrate the range of problems encountered during the project. 4.1. Case study 1 A longstanding damp problem (8 years duration) in a stairwell (Fig. 3) was thought to be due to damp penetration of the solid granite wall, which was ‘‘plastered on the hard’’, i.e., there is no void space between the outer wall and the internal surface finish. Any moisture which penetrates the wall will therefore have easy access to internal surfaces. Inspection of the external wall (Fig. 4) revealed green algae and moss on the wall in line with the exit of a downpipe and overflow from the parapet gutter at the roof of the stair tower. Water was reported to ‘‘spout’’ from the overflow during heavy rain. This turned out to
Yes No No No No No No No No No No No No No No No No No No No No No Yes Yes No No No No Yes No No Yes No No No Yes October 1996 January 1989 January 2002
January 2000 January 1992
Repointing, reslating Repointing Unknown Repointing Repointing Repointing Repointing and other works
1890 1874 1900 approximately 1925 1900 approximately Early 1800s 1850 approximately 1899 1887 1903 1902 13th century 1 2 3 4 5 6 7 8 9 10 11 12
July 1992 January 1989 January 2000 None January 1991 January 1990 None January 1994 January 1992 April 1997 April 1995 None
Repointing, reslating, lead work Repointing Repointing and skews
January 1998 December 1997 December 1997
N SE SE SE NW S N NE SE S S All Squared granite rubble, no void Coursed granite rubble Coursed granite rubble Coursed granite rubble Ashlar granite Squared granite rubble Squared granite rubble Ashlar granite Squared granite rubble Squared granite rubble Granite rubble and squared rubble Granite rubble
No Yes Yes Yes No Yes Yes No Yes Yes Yes Yes
Facing direction Date of last known repairs Age of property Case study no.
Table 1 Summary of characteristics of case study properties
Nature of repairs
Earliest known reported damp problem
Wall type
Gable
Front facade
Rear wall
Stairwell
M.E. Young / Construction and Building Materials 21 (2007) 1846–1859
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be the clue to the source of this particular longstanding damp problem – blockage and possible damage to a parapet gutter. The downpipe from the parapet drain was blocked (it was only about 2 in. diameter) and may have been cracked. Repairs included unblocking the pipe and its replacement with a 3 in. outlet. This illustrates the difficulty of diagnosing the cause of damp problems. Previous inspections of this property over a period of four years had attributed damp to rainwater penetration through the wall. Suggested remedies included rebuilding the chimney, blocking and/or ventilating flues, dry dashing the external wall or coating it with water repellent. It was only in exceptionally damp weather that water spouting from the overflow was observed and reported by the occupant of a flat overlooking the property. Damp problems do not always manifest themselves close to the defect. Water can penetrate some distance from its point of ingress before emerging on internal surfaces. Blistering and peeling paint on the stairwell was observed from roof down to first floor level. 4.2. Case study 2 A granite tenement block in which damp problems were experienced on the gable wall at first floor level (Fig. 5). Damp patches occurred in various locations on the inner surface of the gable wall (Fig. 6). The current resident had been in the flat for 60 years or more and was never aware of any damp problems until 1989 when repointing and other repairs were carried out. Damp problems have all occurred subsequent to these repairs. The quality of granite on the gable wall was mixed and pointing was flush with the surface. Observation of cracks in the pointing mortar suggests that it may be relatively cement-rich. On tapping the sitting room wall surface it was evident that many damp patches coincided with solid areas. Five years previous to this study the wall had been opened up and bridging rubble removed from the void, but this did not solve the problem as the damp re-appeared. Previous attempts to resolve the problem have included capping chimneys and lead-lining the skews. The internal wall was taken down and rebuilt. When the void space was opened up, bedding mortar was found to be decayed and this debris formed much of the fill that was again bridging the void. In the rebuilding process, timber safe lintels over the alcoves (which were badly affected by damp), were replaced with concrete with lead caps. A breathable membrane was installed behind the newly reconstructed internal wall. This should prevent any penetrating damp from coming through although the source of the damp remains unclear. Anecdotal evidence of the flat owner points to damp problems originating at the time of repointing. Although the source of penetrating damp was not resolved in this instance, it is believed that the use of power tools in raking out of joints was a major contributory factor as this is the most likely cause of subsequent build-up of debris in the
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Table 2 Summary table showing the most likely primary causes and contributory factors relating to the damp problems investigated Case study no.
Primary cause
1 2 3 4 5 6 7 8 9 10 11 12
Defective hidden gutter Cracked pointing Porous pointing Water penetration down uncapped chimney Roof level defect Cracked pointing Defective detailing on adjacent roof Defective skews and guttering Defective pointing Unknown Poor workmanship and materials Loss of harling
Contributory factors Defective Debris in Debris in Debris in
ridge tiles, deterioration of bedding mortar, hard pointing mortar void space void space, deterioration of bedding mortar void space, deterioration of bedding mortar
Debris in void space Debris in void space, deterioration of bedding mortar
Fig. 3. Case study 1. Blistering of paint and plaster in the stairwell. The wall is solid granite with no void space, i.e., ‘‘plastered on the hard’’.
void spaces which allowed moisture bridging to internal surfaces. 4.3. Case study 3
Fig. 4. Stairwell affected by damp. Lower arrow shows location of Fig. 3. Unlike the rest of the stairwell, this granite rubble wall is not harled. Inset shows location of outlet and overflow pipes from hidden gutter.
This gable wall of this property faces south-east and is highly exposed to driving rain from that direction since there are no adjacent buildings. The gable was repointed about three years before the current study. There were some minor damp problems reported before this time, which were diagnosed as being due to deterioration of skews and local-
ised deterioration of pointing. Both these problem were resolved. More recently, serious damp problems began after repointing and other works. Exceptionally, dampness is here visible both internally and on the exterior of the gable (Fig. 7). The coincidence of darker patches with persistent
M.E. Young / Construction and Building Materials 21 (2007) 1846–1859
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Fig. 5. Case study 2. Arrow shows the location of damp problems illustrated in Fig. 6. The granite rubble gable wall has been repointed flush with a small overlap of pointing onto the granite.
damp (after 3 weeks of warm, sunny weather with no rainfall) was confirmed by measurements of electrical conductivity. The gable wall in the flats’ internal stairwell was affected by water penetration; damp coming through very
Fig. 7. Case study 3. South-east facing highly exposed granite rubble gable wall. Darker patches correspond to areas of persistent damp, confirmed by measurements of electrical conductivity.
Fig. 6. Case study 2. Damp patches on the internal surfaces of the gable wall. The door on the right leads into a small cupboard which was also affected by damp.
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Fig. 8. RILEM tube for measuring water absorption, applied to mortar surface.
5
Run 7 (33)
Run 4 (179)
soon after rain events implying that the wall core was saturated. Debris was found in the void space which was allowing moisture bridging across to the inner wall. Polythene linings have been installed behind new plasterboard walls of rooms adjacent to the gable and no dampness is now reported in these locations. It was clear on observing the exterior of the gable that some areas have been damp for long periods of time since patches of green algal growth could be observed, despite the south-east facing aspect of the gable. Green algae require high levels of moisture for several months to establish themselves and their presence on a south facing wall is an indication of long-standing damp. The specification for repointing was for 1:1:6 mortar. Analysis of the composition (by Sandberg Consulting Engineers) found ratios of 1:0.3:4.4. This is a cement-rich mix compared to that specified. Data on water absorption rates was obtained using RILEM tubes (Fig. 8) to measure the rate of uptake of water. Water absorption rates varied over the gable (Fig. 9). Permeability in some areas was within normal limits for 1:1:6 mortar (Table 3), but many areas had higher (or much higher) permeability, especially in areas most affected by dampness. The rapid penetration of dampness inside the flats after rainfall is consistent with observations of highly permeable pointing on some areas of the gable. In the lower flat, where the problem was investigated, this coincides with an area of porous pointing. It is likely that the damp problems in this property were caused by excessively permeable pointing mortar on a highly exposed gable. Excessive water absorption will lead Run 6 (29) Run 9 (27) Run 8 (25)
4 Water absorbed (mL)
Run 3 (15)
3
Run 5 (9)
2
1
Runs 1 & 2 (<0.33) 0 0
5
10
15
20
25
30
35
Time elapsed (√s) Fig. 9. Water absorption data from nine separate RILEM tube measurements on gable wall mortar. Numbers in brackets show the equivalent capillary water absorption coefficient (see Table 3). There is an extremely wide range in permeability; while some areas are highly impermeable (Runs 1 and 2), others are highly (probably excessively) permeable (Runs 4, 6–9).
M.E. Young / Construction and Building Materials 21 (2007) 1846–1859 Table 3 Comparative data on capillary water absorption coefficients available from other authors Substrate
pffiffiffi Capillary water absorption coefficient ðkg=m2 hÞ
Proportions of mix Portland cement
Lime mortar [12] Hydraulic lime mortar [12] Cement:lime:sand [11] Cement:lime:sand [11] Cement mortar N type [11] Cement mortar S type [11] Cement mortar [12]
Masonry cement
Hydraulic lime
Lime
Sand
1
2 3 41/2 6 3 3 3
1 1 1
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1/2 1 1 1
1
23 27 4 5 1 0.7 5
Masonry cement type N: A mix containing Portland cement and finely ground minerals. Also includes air entraining, plasticizing and set retarding agents. Regular compressive strength. Masonry cement type S: A mix containing Portland cement, finely ground minerals and additives. Produces mortars with higher mechanical resistance than masonry type N.
Cavity timber frame Don't know Never occurs Traditional cavity
Lath and plaster Very common Plastered on the hard
0
20
40
60
80
100
% respondents Fig. 10. Practitioners’ opinion regarding whether ‘‘buildings of a particular type [were] more susceptible to damp problems?’’.
to a build-up of water in the pointing and bedding mortar as the gable will absorb more water than it can easily lose by evaporation. It is clear that the wall core of some areas of the gable must have become very wet, probably saturated, allowing moisture bridging across debris in void spaces.
N NE
NW
4.4. Practitioner survey In the questionnaire survey practitioners were asked for their opinion on whether ‘‘. . . buildings of a particular type [were] more susceptible to damp problems?’’. Results (Fig. 10) show very clearly that practitioners are aware of many more damp problems with some types of building construction. Solid masonry walls plastered on the hard were most susceptible. Problems were also fairly common on solid masonry walls with lath and plaster. Substantially fewer damp problems were experienced with cavity walls. Damp problems in granite properties were most frequently thought to occur on North and north-east facing walls (Fig. 11) (note that no significant correlation of damp problems with facing direction was found in the case studies). Practitioners felt that prevailing northerly
W
E 10
20
30
SE
SW S
Fig. 11. Practitioners’ opinion regarding which facing directions of gable walls were most likely to suffer damp problems.
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M.E. Young / Construction and Building Materials 21 (2007) 1846–1859
Failed skews or fillets Failed downpipes or gutters Damaged slates or tiles Damaged pointing Uncapped flues Loss of harling Bedding mortar decay Gutter grading Water through pointing Water through stone 0
20
40
60
80
100
% respondents Don’t Know Never occurs
Very common
Fig. 12. Practitioners’ answers to the question: ‘‘To what extent are the following factors responsible for MOISTURE PENETRATION problems in buildings?’’. Full descriptions of each factor were as follows: failed skews or tilted fillets, overflows from blocked/damaged downpipes or gutters, lost or damaged slates/tiles, cracked or lost external pointing, rainwater penetration down uncapped chimneys, removal/loss of harling, moisture penetration due to decay of bedding mortar, improper grading of runs on gutters, moisture permeating through undamaged pointing, moisture permeating through stone.
and north-easterly winds, in combination with low levels of sun exposure and low evaporation rates contributed to damp problems. With respect to moisture penetration problems (Fig. 12), practitioners felt that the most commonly encountered problems related to defects or failures of roofing materials (slates, skews, tilted fillets) or of water shedding (downpipes and gutters). Improper grading of gutters was occasionally encountered as a contributory problem. In relation to pointing and mortar, cracked pointing was considered to be a common cause of moisture penetration, decayed bedding mortar less so, but it was cited as a contributory cause in some instances. Rainwater penetration down un-used, uncapped chimneys was noted as a relatively common problem which could be resolved by capping chimneys. Lack of harling (rendering) was found to be a relatively frequent cause of dampness on walls which were originally harled, but where the harling has subsequently been lost or removed. Water penetration through stone was seldom encountered. With respect to condensation (Fig. 13), inadequate ventilation of rooms was by far the single most important factor in causing condensation problems. Gas fires (especially portable gas heaters) were noted as a major source of water vapour. Factors that reduce the rate of ventilation were important contributory causes to condensation; these included blocking off fireplaces, poor air circulation in
voids and capping of disused flues. However, while 18% of respondents thought that condensation problems in capped flues were a moderately or very common problem, many more (31%) though rainwater penetration down uncapped flues was moderately or very common (Fig. 12). Lack of flue liners was occasionally found to cause problems though this was an uncommon defect. Factors related to the external skin of the building were not thought to be major contributory causes of condensation. These included hard, impermeable pointing, application of water repellents, painting or harling of walls. Practitioners were asked whether they thought that some ‘‘damp problems may be related to . . . characteristics of mortar or pointing?’’ (Fig. 14). Practitioners thought that incorrect specification of mortar mixes was very common – specifically, mortar mixes being too hard and cement-rich. Damaged pointing was also thought to be a very significant factor. Formation of shrinkage cracks in pointing mortar is, of course, more likely to occur where the mix is too hard. Decay of bedding mortar was probably an important influence on some damp problems and flue gases were thought occasionally to cause damage to bedding mortar through escape of moisture laden, acidic, corrosive fumes. With respect to whether penetrating damp problems are often solved by repointing, or whether they may begin after repointing – while a majority of practitioners (55%) felt that repointing could
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Poor room ventilation Condensation from gas fire Blocked off fireplaces Capped flues Poor air circulation in voids Lack of flue liners Apply water repellent Hard pointing Painting external walls Harling external walls 0
20
40 60 % respondents
80
100
Don’t know Never occurs
Very common
Fig. 13. Practitioners’ answers to the question: ‘‘To what extent are the following factors responsible for CONDENSATION problems in buildings?’’. The full descriptions of each factor were as follows: inadequate ventilation of rooms, condensation from gas fire, inadequate ventilation of blocked off fireplaces, inadequate ventilation of capped flues, lack of air circulation in void space or cavity wall, lack of flue liners, application of water repellent to external walls, excessively hard or impermeable pointing, painting external walls, harling external walls.
often solve damp problems, a number (21%) had noticed damp problems beginning after repointing and a significant minority (13%) thought that repointing normally caused more problems than it solved. The method of raking out of old pointing may be a significant issue in this respect. While very few practitioners (10%) thought that hand raking led to debris build-up in voids and cavities, rather more practitioners (34%) thought vibrations caused by mechanical raking out could lead to or exacerbate debris build-up in voids. Moisture bridging across debris in voids was one of the most commonly observed causes of damp problems. Practitioners were asked which methods they had used to tackle damp problems and for their opinion on how successful each method had been (Fig. 15). Repairs to obvious physical defects were normally successful, with reported success rates over 90% for repairs to defects in slates, guttering, skews or rebuilding a wall. Improving ventilation was a simple and effective remedy for many condensation problems. For many other methods, the reported results were quite mixed. Clearing out bridging debris from voids was reported as being successful in 94% of cases. Repairs to pointing reported mixed levels of success. Practitioners reported that replacing pointing was fully successful as a
treatment for dampness in only about 50–75% of cases. Regrouting walls to replace decayed bedding mortar was often reported to be successful although it was not commonly used. Replacing lost harling was also often successful. Treatments for rising damp, including electro-osmosis, barriers and improved drainage, were reported as being fairly successful. With respect to flues, capping flues to prevent water penetration was reported to cure the problem in about 80% of cases. Uncapping flues to prevent condensation problems was successful in only 60% of cases, and was seldom done. Installing or replacing flue liners solved condensation problems in many cases, though it was a relatively uncommon fault. The lowest success rate was reported for water repellent treatment (<50%). Water repellent treatment often (or normally) fails to address the causes of water penetration problems, it is therefore not surprising that water repellents failed more often than not. Water repellents will largely prevent moisture ingress through a stone or mortar surface, but in practice this is not likely to be a significant problem. Most moisture penetrates the building fabric through cracks and other defects which water repellents cannot seal. Once inside a wall the water repellent may inhibit drying out, exacerbating the problem.
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Mortars incorrectly specified Damaged pointing Re-pointing mortar too hard Bedding mortar decayed Problems solved by re-pointing Debris from mechanical raking Flue gases damage mortar Problems after re-pointing Debris from hand raking 0
20
40
60
80
100
% respondents No answer Definitely not
Certainly occurs
Fig. 14. Practitioners’ opinion as to whether some ‘‘damp problems may be related to . . . characteristics of mortar or pointing?’’. Factors are summarised in the graphs. The full descriptions of each factor were as follows: Do you think mortar mixes are often incorrectly specified, Does much moisture penetration occur through damaged pointing, Do you think repointing mortar mixes are often too hard, Does decay of bedding mortar lead to moisture penetration, Are many damp problems solved by repointing, Does mechanical raking out cause debris build-up in voids/cavities, Have you noticed that bedding mortar is damaged by flue gases, Have you found that damp problems begin after repointing, Does raking out by hand cause debris build-up in voids/cavities.
5. Discussion Some penetrating damp problems are relatively easy to diagnose, others are more problematic and some defy numerous attempts at a solution. Failure of slates or other roofing materials or physical defects and damage to gutters and downpipes are relatively easy to diagnose and fix. Where vegetation blocks rainwater drainage or gutters are wrongly fitted, water will overflow down the face of a building causing stone decay to be accelerated and increasing the likelihood of water penetration of joints and defects. Failures of hidden gutters may show up as excessive biological growth or soiling on lower stonework (Fig. 16). Damaged or decayed pointing allows easy ingress of water through joints, as do fissures and cracks in the stone due to settlement. Sometimes repairs use a technique which is inappropriate and potentially damaging. For instance, the practice of ribbon (or tuck) pointing (Fig. 17) where the pointing mortar stands proud of the surface is deprecated since it slows down the run-off of rainwater over the wall surface. Projecting pointing diverts water flow along the top surface of the pointing increasing the likelihood of penetration along cracks and into the joint. Retained moisture and soiling
on the upper surface encourages vegetation to colonise the joints. The case studies reported showed granite rubble gable walls were clearly implicated in a relatively large number of cases. There was no indication that damp problems were related to any particular facing direction, although with only 12 examples sample numbers were rather too small to draw any definitive conclusions in this respect. With respect to the relative timing of building owners reporting dampness problems, as far as can be ascertained, a significant proportion of reports of dampness date from within a few months or years of refurbishment or other major interventions on the property. This may imply a link between certain interventions and occurrence of dampness problems; however, although there may be no known reports of prior damp problems, it is not necessarily clear that these did not exist prior to intervention. Dampness often comes to the notice of home owners when it penetrates internal walls across dooks (wooden plugs in a wall) or accumulations of bridging debris. While this may be the route of penetration of moisture to internal surfaces, the source of that moisture it is not necessarily easily established. Root causes may be either penetration of rainwater or build-up of condensation. Even where dampness
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No experience 80 Not successful/don’t know Partially successful
40
% respondents
60
Successful
Uncapping flues
Replace hard pointing
Apply water repellent
Regrout bedding mortar
Install flue liners
Electro-osmosis
Replace soft pointing
Rebuild wall
Barrier treatment
Replace harling
Cleaning debris from voids
Capping flues
Improve ventilation
Repointing
Improve drainage
Repair skews etc
Repair guttering etc.
Repair slating etc.
20
Fig. 15. Methods used by respondents to tackle damp problems and their degree of success. ‘‘Successful’’ implies that the method has been used and that it did work in solving damp problems. ‘‘Not successful/don’t know’’ implies that the method was used but did not work. Some methods were ‘‘Partially successful’’. Respondents who did not report any results for particular methods are recorded as ‘‘No experience’’.
problems do occur following interventions, the connection between any one intervention and later dampness can seldom be unambiguously determined. Nevertheless the evidence suggests a strong link between damp problems and debris build-up in void spaces, highlighting the danger of interventions (such as the use of power tools during raking out of joints) which may dislodge bedding mortar where this is in a friable condition, as was found to be the case in many Victorian tenements. Where bedding mortar is in poor condition, the voids within it may act as reservoirs and conduits for water flow through the wall core. The low permeability and excessive hardness of some pointing mortars may make a significant contribution to damp problems by allowing moisture ingress through shrinkage cracks, and preventing adequate drying of bedding mortar and wall cavities. 6. Conclusions On granite gables the demands placed on mortar performance are particularly severe since pointing mortar often covers more than 50% of the exposed surface of rubble walls, the exposure of gables to climatic effects (e.g., wind driven rain) is often greater than that of street front facades and in granite buildings, due to the impermeability of the stone, the mortar has to do all the work in dealing with moisture movement. Many dampness problems which are
not caused by obvious physical defects appear to be related to mortar performance. Aspects of mortar performance which have been found to be problematic include the following: 1. Bedding mortar in Victorian tenements is often in a friable state and easily dislodged by vibration – especially by the severe vibration caused by the use of power tools to rake out joints. Many damp problems that appear after repointing can be attributed in part to moisture bridging across void spaces that have become clogged with dislodged debris. This problem could be greatly reduced if raking out was done by hand. 2. Some rubble buildings or gables were originally harled, especially those built in the 18th and early 19th centuries. This was done for aesthetic reasons and to reduce the danger of rainwater penetration. To modern eyes, stone is more prestigious (and more expensive) than harling so people like to see exposed stonework, but on buildings which were intended to be harled, this puts greater demands on the mortar performance – perhaps greater than it can cope with. Replacing harling should be considered as a potential solution to damp penetration problems on buildings which were originally harled. 3. Cement-rich mortars can be too hard and impermeable. They reduce the rate of water penetration through the mortar, but they do not allow moisture to evaporate
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Fig. 16. Failures of hidden gutters may cause excessive biological growth or soiling on lower stonework.
readily and they are prone to shrinkage cracking which provides alternative and rapid access routes for moisture into the wall core. 4. Lime-rich mortars are soft and more permeable. They allow moisture to evaporate relatively easily, but they can potentially be too permeable on highly exposed gables, allowing too much rainwater penetration relative to evaporation rates. They have the advantage of being able to self-heal by re-forming calcium carbonate in hair-line cracks thereby avoiding moisture penetration through cracks, but the reactivity of free calcium can result in build-up of calcium sulphate (gypsum) crusts in sheltered areas leading to decay of the granite surface. The mortar with the ideal combination of strength, permeability, flexibility and water soluble lime has yet to be established for granite walls. Until this is done, pointing mortars are likely to be an ongoing source of problems as conventional mixes necessarily compromise between incompatible characteristics. Acknowledgements This research was funded by Aberdeen City Council through the Aberdeen Property Repair Initiative. Case
Fig. 17. Ribbon or tuck pointing is poor practice on granite walls as it may divert rainwater run-off into joints. Retained moisture also encourages colonisation of horizontal surfaces.
study properties and maintenance history information were provided by Gilbert McCurdy, Grants Officer, Aberdeen City Council; Andy Pitblado, Aberdeen Care and Repair, Allan Cumming RIAS, Cumming and Co.; Raymond Simpson, Raymond Simpson Assoc. Ltd.; John Carroll, JV Carroll Building Consultants; and Dave Gauld, Ron Shanks Assoc. References [1] Scottish House Condition Survey; 2002. Available from: http:// www.shcs.gov.uk/ [accessed May 2005]. [2] Groot CJWP, Gunneweg J. Water permeance problems in single wythe masonry walls: the case of wind mills. Constr Build Mater 2004;18:325–9. [3] SPAB. Repointing stone and brick walling. SPAB Technical Pamphlet No. 5; 2002. [4] Allen G, Allen J, Elton M, Farey M, Holmes S, Livesey P, et al. Hydraulic lime mortar for stone, brick and block masonry. Shaftesbury: Donhead; 2003. [5] Duffy AP, Cooper TP, Perry SH. Repointing mortars for conservation of a historic stone building in Trinity College, Dublin. Mater Struct 1993;26:302–6. [6] Duffy AP, Perry SH. The effects of mortars on granite decay. In: Bell E, Cooper TP, editors. Granite Weathering and Conservation. Trinity College, Dublin, September 1993. Dublin: Director of Buildings’ Office; 1994. p. 1–9.
M.E. Young / Construction and Building Materials 21 (2007) 1846–1859 [7] O’Brien PF, Bell E, Pavia Santamaria S, Boyland P, Cooper TP. Role of mortars in the decay of granite. Sci Total Environ 1995;167:103–10. [8] Perry SH, Duffy AP. The short-term effects of mortar joints on salt movement in stone. Atmos Environ 1997;31(9):1297–305. [9] Livesey P. Succeeding with hydraulic lime mortars. J Arch Conser 2002;8(2):23–37. [10] Mosquera MJ, Benı´tez D, Perry SH. Pore structure in mortars applied on restoration: effect on properties relevant to decay of granite buildings. Cement Concr Res 2002;32:1883–8.
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[11] Kumaran K, Lackey J, Normandin N, van Reenen D, Tariku F. Summary Report from Task 3 of MEWS Project at the Institute for Research in Construction. Hygrothermal properties of several building materials. IRC Research Report 110. Ottawa: National Research Council Canada; 2002. [12] Moropoulou A, Avdelidis NP, Koui M. Compatibility assessment of building materials using Infrared Thermography. In: Proceedings of the 15th world conference on nondestructive testing, Roma, Italy, 15– 21 October 2000.