There are a number of factors to be taken into account when considering chemical damp proofing systems and replastering works. The following information is intended to provide an objective insight into chemical damp proofing.
It should be fully appreciated that free water in building materials is not desirable, it can lead to decorative spoiling and rot and in some cases it can lead to collapse of the material itself, eg, cob. Indeed, a law was passed at the end of the 1800's to stipulate the introduction of damp-proof courses as a whole and this practice is clearly an essential part of all properties constructed since that time.
If a damp proof course was of no consequence it would not be necessary or be part of the 'building Regulations' etc, thus damp proof courses are obviously essential to the occupants and structure of a property.
Rising damp tends to rise higher in thick walls than thin walls. This is due to the lower surface to volume ratio of thicker walls and evaporation being mostly subjected to the surface area. This is an important feature to consider when dealing with properties with larger dimensioned walls and simply the so-called 'allowing walls to breathe' syndrome to stop the rising water may prove of little effect in such cases. Inserting tubes in walls will not prevent rising damp as some claim.
Chemical Damp Proof Course ( Dpc)
Lifecote Chemical damp proof courses are inserted to control the vertical passage of moisture from the ground and are almost all installed in properties where no damp proof course exists or it has broken down with age.
Care should be taken to ensure that the damp proof course is not bridged by high external ground levels, blocked cavities or debris piled against the wall. Ground levels should be lowered, cavities cleaned out or the area below the inserted damp-proof course might be 'tanked' internally if deemed necessary. Lifecote now has the policy to tank party walls due to possible ground level problems with the adjacent property.
Rising ground water should be reduced to such a level that, in association with specialist replastering, it should no longer cause decorative spoiling or damage.
The efficiency of the water repellent damp proofing systems can be affected where there are detergents (surfactants) impregnated into the wall by, for example, past leakage from sink waste pipes. A similar problem may occur when walls are sterilized against dry rot infection by biocide formulations containing surfactants.
The overall effectiveness of a remedial damp proof course can be investigated by examining the relationship between the distribution of free moisture (water due to rising dampness or other source of active water ingress) and contaminant salts (chloride and nitrate). Where rising dampness is still active, capillary moisture will be found to the full height of salts. In the absence of capillary moisture in the presence of salts, arising from rising dampness, this indicates that drying back has occurred and that the damp-proof course is effective. Intermediary stages are also found which demonstrate different degrees of control of the rising dampness.
If chloride and nitrate are not detected in a sampled profile it is possible that the ingress of moisture is due to recently developed rising dampness or more likely through rainwater penetration, condensation, plumbing defect or other sources.
When evaluating the efficiency of remedial damp proof courses care must be taken not to misinterpret electrical moisture meter readings. High readings might not indicate that the damp-proof course itself has failed. They may reflect a number of possibilities including contaminated or inadequate plasterwork. Thus, an accurate assessment of the efficiency of a damp proof course can only be undertaken by determining full moisture profiles linked with analysis for contaminant salts. Lifecote can conduct on site salt analysis if required.
It is identified in BS 6576:1985 that where timber suspended floors are encountered the damp proof course must be injected, where possible, below joist level; this is to protect the embedded timbers from dampness and the risk of fungal decay. However, given the likely efficacy of injection systems the embedded joist ends could still remain in contact with damp masonry even if above the injected damp proof course and may therefore remain at risk to fungal decay. It would be considered prudent that in all cases where a damp-proof course is installed in relation to a timber suspended floor, action is taken to protect any embedded timbers just above and certainly below the injected damp-proof course to prevent potential decay.
A long-term rising damp complex brings with it certain soluble ground water salts into the wall, these are left behind as the water evaporates, and become concentrated at such sites. A proportion of these salts are hygroscopic, that is they are capable of absorbing water from the surrounding environment. As a result affected plasters and masonry may remain damp even though the source of moisture which lead to the build up of the salts has been eliminated.
It therefore stands to reason that any property, which has been subject to a long-term rising damp complex, must have some degree of salt contamination in the plaster and the underlying masonry. These salts can, on their own, cause spoiling to certain types of decoration, even in relatively low quantities.
Following the insertion of a remedial damp-proof course a damp wall can take many months to dry out (Building Research Establishment Digest 163).
Where hygroscopic salt contamination is very heavy, the wall may never dry out adequately. This is due to continued moisture absorption from the surrounding environment. Under conditions of very high humidity some of these salts can become deliquescent, ie, they can absorb so much moisture that they become liquid. This in itself can lead to wet masonry.
Removal of the old contaminated decorations and plasterwork are essential because:
It removes the contaminated surface which could cause spoiling of any new decorations.
In order to prevent the new surfaces from becoming contaminated and damp the new plasterwork has a most important and specific function. This is to prevent the passage of residual moisture and contaminant salts from diffusing from the underlying wall to the new surface thus preventing future spoiling. These functions and the importance of the new plasterwork are described in Building Research Establishment Defect Action Sheet 86.
It must be considered as important as the injected damp proof course, indeed, if not more important. Many disputes are centered upon whether a dampness problem is due to the failure of a remedial damp proof course or inadequate new plasterwork. In such cases plasterwork should be examined as well as the efficiency of the damp proof course since it is usually the more expensive of the two to put right.
In order to obtain the above system it is essential to use either:
A dense sand/cement mix, preferably incorporating a 'waterproofer' or 'salt inhibitor'. The 'integral waterproofer' and 'salt inhibitors' specified to be incorporated in cement based internal renders both perform the same function--they are used to help restrict the passage of moisture to the decorative surface. Salts can only move or diffuse in solution therefore restriction of water flow also restricts the passage of salts.
Alternatively, one of the special premixed 'renovating plasters' designed for use in such conditions and which preferably carry an Agrement Certificate could be used.
Limited bridging by plasterwork (but not the gypsum finish) should not cause the complete failure of a damp proof course (see above). It is certainly advisable to keep the new plasterwork cut well short of any solid floor and this reduces some of risks of spoiling which are greatest at the base of the wall for the reasons described above. Under no circumstances must lightweight premix gypsum based backing and bonding plasters or other highly porous plasters be used.
Replastering must be carried out to a height in excess of the maximum rise of the dampness and the salt contamination. Dampness can frequently rise in excess of 1 metre, the height being governed by numerous factors including pore structure and rates of evaporation. For example, restriction of evaporative processes causes dampness to rise higher than if the wall surface was well ventilated. This is well illustrated in thick walls where moisture tends to rise higher than in thinner walls due to the lower surface area to volume ratio.
Sometimes, where a remedial damp proof course is ineffective the moisture can rise above the new plasterwork as the result of its low permeability retarding evaporation of water from the underlying masonry. This tends to 'drive' the active rising dampness higher. Similarly, new plasterwork may not have been removed to sufficient height so leaving the old salt contaminated plasterwork above. Both cases may give a similar pattern of readings on an electrical moisture meter, ie, very high readings just above the new plasterwork line, but analysis for moisture and salt distributions may be necessary to properly distinguish between the two causes of the problem. Where the problems only occur above the new plaster line it does demonstrate the efficacy of good plasterwork in performing its required design functions.
Perhaps the most common defect encountered in replastering is the use of weak sand/cement mixes. Building Research Establishment Defect Action Sheet No. 86 identifies that where cement/sand mixes are used these should be 1 : 3 cement to sand, or alternatively, use a specialist premixed render designed for the purpose; these latter materials are especially useful on 'weak' backgrounds.
The use of much weaker mixes, ie, often weaker than 1 : 6 cement to sand or when lime has been added (eg, 1 : 1 : 6 cement to lime to sand), are more likely to lead to more porous plasterwork which are unlikely to achieve their required design function. Lime/sand mixes will also suffer from the same problem (see above).
The use of poorly graded sand containing a high proportion of fines, especially in cement weak mixes, also exacerbates problems. The figure below shows the result of grading 2 sands to BS882:1992: the lower sand contains far to much 'fines' and would be unsuitable for use following damp-proofing works. Insufficient thickness of plasterwork can also add tothe apparent failure in the required design function. In the above cases the cement weak mixes will not prevent the diffusion of salts and residual moisture from the underlying masonry which can potentially damage the new decorative surface.
Porous cement/sand mixes may also become contaminated with soluble sulphate diffusing from the underlying masonry. While still alkaline sulphate attack can occur which causes serious disintegration of the cement render. Similar damage also occurs where cement renders have been applied over gypsum (calcium sulphate) plasters or where a proportion of gypsum plaster has been added to a cement mix to obtain a rapid set. In the latter case small flakes of exfoliated mica (vermiculite), part of some lightweight gypsum plasters, can sometimes be observed on close examination of a sample so identifying the probable addition of such lightweight gypsum materials. Pieces of grey or pink gypsum plaster might also be seen in the mix if a sample is closely examined. Where gypsum plaster has been used to fix metal angles, severe expansion and disintegration may occur to overlying/adjacent cement render; this, again, is caused by sulphate attack.
A common problem associated with building practice is taking new plasterwork, including the gypsum finishing coat, behind and below the damp-proof membrane and floor screed. This occurs when replastering has been completed before laying of a new solid floor. Frequently, the edge of the damp-proof membrane is cut very short or rolled under during the laying of the floor. This not only fails to comply with the recommendations described in BS CP102:1973 but also serves to cause other problems. Such cases usually result in the dampness being restricted close to floor level or just above the skirtings and also around the perimeter of the solid floor.
Finally, in situations where light coloured wallpapers have been used, especially relatively impervious papers, diffuse dark areas can appear. On examination, these are shown to be caused by black mould growth on the back of the wallpaper. The cause is due moisture in the wall leading to high humidities/dampness behind the paper so leading to mould growth. It is sometimes encountered following damp-proofing works but in can appear in almost any situation where moisture/high humidities are present in the underlying masonry. Care must therefore be taken in selecting new decorative finishes; initial decoration should be regarded to be of a temporary nature whilst the drying processes take place.