Chemical Damp-Proof Courses and Replastering


The article below is from Remedial Technical Services


The facts about chemical damp-proof courses and replastering


There are a number of factors to be taken into account when considering chemical damp-proofing systems and attendant replastering works. The following guide is intended to provide an objective insight into chemical damp-proofing, its performance and the importance of the replastering works.


It should be fully appreciated that free water in building materials is not desirable, it can lead to decorative spoiling and rot: in some cases it can lead to collapse of the material itself, eg, cob. Indeed, someone saw fit 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; it certainly is now. If a damp-proof course was of no consequence it would not be necessary or be part of the ‘Regulations’ etc, and would not still be part of all new properties. Thus damp-proof courses are obviously beneficial to both the building and the occupants alike.


Another factor to consider is that, all things being equal, 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, evaporation being mostly subject to surface area. This is an important feature to consider when dealing with properties with larger dimensioned walls – simply the so-called ‘allowing walls to breathe’ syndrome to stop the rising water may prove of little effect in such cases.


So there may be a case for chemical damp-proofing.

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.


Chemical damp-proof courses should be installed in a position in accordance with good practice as described in BS 6576:1985, “Code of Practice for the Installation of Chemical Damp-proof Courses”. They are installed in walls by various methods depending on the particular system being employed but the ultimate objective is to provide a water repellent or pore blocking material in a continuous horizontal band in the masonry thereby to provide a ‘barrier’ to water rising from the ground. Moisture paths




The only continuous pathways through which water can rise through a wall are the mortar beds: for water to pass, say, from brick to brick it must still cross a mortar bed (bottom of figure, right). It is therefore essential that the damp-proofing material impregnates the mortar courses since these form the major pathway for the rise of water within walls. Damp-proofing the masonry units (eg, bricks) alone is of very little value! Porous mortar and impervious/water repellent brickwork will still allow rising dampness to occur. However, if the pores in the mortar line are made water repellent or blocked then the water cannot rise since it cannot traverse the mortar beds to do so (top figure, right).


In the majority of older properties the mortar is not alkaline so that the water repellent formulations based either silicone resins, aluminium stearate, or methyl siliconate (sodium or potassium methyl siliconate) can be used. Occasionally, however, the mortar may highly alkaline such as in a recently constructed wall (eg. where the physical damp-proof course has been omitted). This will exclude the use of the methyl siliconates since the highly alkaline conditions found, for example, in new mortars prevent the formation of the water repellent resin.


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.


NOTE: Should minor bridging of an effective damp-proof course occur, for example by moderately porous plasterwork, then it is highly unlikely that the dampness would continue to rise to its original height. If the damp-proof course is effective the pathway for moisture should be limited within the wall itself. Any porous plasterwork is at the surface where evaporation would serve to restrict the flow of rising water through it. Therefore, moisture is unlikely to pass through this relatively narrow pathway at a rate sufficient for it to reach the height of rise prior to the insertion of the damp-proof course assuming, of course, that the injected damp-proof course is moderately effective. Problems of this type together with defects in construction of the floor/wall junction usually manifest themselves at the base of the wall.



Unlike a physical damp-proof course these injected damp-proof courses do not form a ‘discrete impermeable plane’, but more of a ‘diffuse band’.




When fluids are injected into a heterogeneous substrate such as brick/mortar they do not totally Viscous fingeringfill up the porous structures and neither do they completely push out the water in front of the advancing injection fluid as is so often claimed. Instead, the fluid tends to ‘finger’ within the substrate, a process known as ‘viscous fingering’ (figure , left). The fingers of the injected material form when the fluid takes the lines of least resistance such as the larger pores and cracks. Unfortunately, such pathways are not the most important elements in the conductance of water up the wall. Furthermore, the damper the substrate the greater this fingering is likely to be, especially with solvent based systems since these are not miscible with the resident moisture. Fingering is also increased by injection at high pressure. Reduction of the phenomenon is obtained by low pressure injection or, better still, by gravity diffusion of the dpc fluid.


The result of the damp-proofing fluids forming fingers give rise to non-impregnated ‘pools’ within the wall through which water can continue to rise. In the case of pressure injection damp-proof courses, this suggests that it is unlikely that the diffuse band of the damp-proofing agent will be totally complete. The resultant chemical damp-proof course may therefore not stop rising dampness by causing an immediate cut-off of rising water above the damp-proof course like that effected by a physical damp-proof course. Instead, a relatively rapid decline in the moisture gradient should occur above the inserted chemical damp-proof course due to the ‘control’ exerted. Thus, in practice, the 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 efficacy 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 rising damp figuresother 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 (figure, left). The absence of capillary moisture in the presence of salts arising from rising dampness indicates that drying back has occurred (Figure centre) 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 efficacy 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 efficacy of a damp-proof course can only be undertaken by determining full moisture profiles linked with analysis for contaminant salts. It is also important to give consideration to the expected performance and limitations of the installed system as described above.


NOTE: 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). Furthermore, due to the limitations of chemical damp-proof courses the wall is always likely to remain damp at the base (this is an important consideration when evaluating the efficacy of a remedial damp-proof course).


Where hygroscopic salt contamination is very heavy the wall may never dry out adequately 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, AND there is the limitation in the performance of the damp-proof course itself which is likely to leave the lower part of a wall permanently damp.


These functions and the importance of the new plasterwork are described in Building Research Establishment Defect Action Sheet 86.


The importance of the replastering works cannot be over-emphasised. 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 efficacy of the damp-proof course since it is usually the more expensive of the two to put right.


In order to obtain the above design function 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; 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.


Note on historic/listed buildings:
Whilst the insertion of a remedial damp-proof course will control the rising dampness it is unlikely to stop it; nevertheless it will do no harm in that it will at least reduce the flow of moisture into the material (NB. Certain considerations however are necessary for injection damp-proofing cob construction {Trotman , P.M. “Dampness in Cob Walls” BRE, 1995}).


The main problem is where old lime plasters still remain. This may be badly stained, heavily salt contaminated and deteriorated. In this state it clearly shows that there has certainly been a problem, and replacing it with a similar material isn’t likely to do much better in the longer term! So to remove these material and apply a new lime plaster is highly likely to lead to similar problems – lime plasters are very permeable (although apparently no more vapour permeable than sand/cement mixes of the order of 1 : 6) and as such are highly likely to let through the dampness/colouration/salts already in the substrate. As such, spoiling may occur again in a relatively short time and lime plaster is a very expensive sacrificial coating! Also note that hygroscopic salt contamination alone can pass into new permeable material from the substrate without any ‘free’ water being present; this can occur when such salts become ‘deliquescent’ (ie, they become liquid and therefore mobile) under conditions of high humidities.


There is no problem with the above provided that the owner is prepared to accept that this may occur, but as stated above, this will make lime plastering a very expensive ‘sacrificial’ material, ie, as it spoils it will have to be periodically replaced if the owner requires a clean decorative surface. There is always an argument that lime plastering should be used but if it allowed the walls to ‘breath'(?) then there shouldn’t have been a problem in the first place!


There may also be a problem with the use of the stronger sand/cement mixes as described above; to meet their required design functions mixes of the type necessary may be far too strong for the background. It may be possible to use expanded metal lath to aid bonding but perhaps the best practice, should it be acceptable, would be to use a dry lining technique to provide a non-spoiling decorative surface. However, it may be possible to use one of the specialist ‘renovating’ plasters which are usually less ‘strong’ than the traditional dense sand/cement mixes (see below).


The answer to damp-roofing and replastering such properties is clearly with the owner – what are they expecting and what are they willing to accept. If they expect clean non-spoiling decorative surface then some kind of action with reference to replastering/finishing will need to be taken, but if they are quite happy with some degree of staining/spoiling then consideration can be given to leaving the old material but being aware what has actually caused it to deteriorate in the first place. BUT REMEMBER – make sure no wood or other biodegradable material is left in contact with any dampness – it will be at a high risk to rot developing!!


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 tosand grades the 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.


The importance of the replastering works associated with the insertion of chemical damp-proof courses cannot be over-emphasised. Chemical damp-proof coursing must be regarded as an integrated system, the damp-proof course and the replastering. The chemical damp-proof course will control the rising dampness and the new plasterwork will complete the system by preventing residual moisture, especially at the base of the wall, and contaminant salts in the underlying masonry from passing to the new decorative surface. Where one is dealing with the considerations of historic/listed buildings there are clearly other factors to be taken into account in relation to finishes (see above).


Because chemical damp-proofing is a system it is prudent to avoid problems of ‘split responsibility’ – that is where the installation of the damp-proof course is undertaken by one contractor and the replastering is carried out by another. The problem of split responsibility is the most common cause of system failures and the associated disputes which develop, ie, the plasterer blames the damp-proofing installer for any developing problem and the damp-proofer holds the plasterer responsible. The simple answer is to avoid ‘split responsibility’, so frequently the origin of expensive problems and disputes. This is easily achieved by allowing the specialist installer to undertake both the damp-proof course installation and the replastering. Experience has shown that this approach significantly reduces problems; it also has the advantage to the client of only one single chain of responsibility.


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