Specialist Plastering by Damp Aid North Wales.
Specialist Plastering in Association with the Installation of a Remedial Damp proof course
A damp-proof course is solely designed to stop moisture rising from the ground into the walls, directly above ground level. Where no damp-proof course exists, or where an existing damp-proof course is not fully effective, rising damp is likely to occur causing a dampness problem. In such situations the installation of a damp-proof course into the walls at the correct level will prevent further rising damp.
Irrespective of the type of damp-proof course used, e.g. chemical or physical, dampness is likely to continue to be a problem due to:-
a) Residual moisture in the walls.
b) The action of hygroscopic salts.
In time residual moisture will dry out, but dampness caused by the presence of hygroscopic salts within the wall and wall plaster may continue to give problems even though all other sources of moisture ingress have been rectified.
As moisture rises from the ground into a wall it carries with it soluble ground salts which are deposited in the wall fabric and wall plaster as the moisture evaporates. Certain of the soluble salts carried up from the ground by rising damp and in particular chlorides and nitrates are hygroscopic, that is they will attract moisture from the atmosphere. Where dampness continues to rise into a wall for a substantial period the deposit of hygroscopic salts built up may be sufficient to cause a significant dampness problem, particularly at periods when the relative humidity of the atmosphere is high.
Note: Hygroscopic Salts should not be confused with the deposits of efflorescent salts which may appear on a wall surface as the wall dries out. Efflorescence on wall surfaces is usually caused by sulphate salts which are soluble but not hygroscopic.
Rectification of Dampness caused by Salt Contamination
Wall plaster affected by hygroscopic salts should be removed and the wall replastered using a plastering system that does not readily allow any salts within the wall fabric to migrate to the surface of the new plaster. The majority of specialist companies engaged in the installation of damp-proof courses and in particular members of the Property Care Association (a division of the British Wood Preserving and Damp-proofing Association), issue specific recommendations regarding replastering with any report and/or quotation that they submit. It is obviously advisable to follow any such recommendations made, paying particular attention to the ratio of the mix and the quality of the components. Should there be any doubts as to the extent of replastering or the specification to be followed; further guidance should be sought from the damp-proof coursing contractor prior to the commencement of any replastering.
When to Re-plaster
Both the BWPDA Code of Practice for Installation of chemical damp proof courses and BS6576 (2006) recommend that replastering should be delayed for as long as possible. By delaying replastering, time is allowed for soluble salts to move from the brickwork into the old plaster. In addition, if replastering can be delayed until after residual moisture has dried out, the true extent of dampness caused by hygroscopic salts may be more easily established and it may be found possible to restrict the extent of the replastering. Where early redecoration is required following damp-proof coursing replastering can, in the majority of cases, be carried out shortly after the completion of the damp coursing work. In such cases the plaster work must be able to hold back hygroscopic salts during the drying out period. Any additives used must be of a type to permit the passage of water vapour. It is therefore advisable to check with the damp-proof coursing contractor to ensure that the plaster specification recommended is suitable for this purpose.
Wall plaster should be allowed to dry out thoroughly before any redecoration is undertaken. Temporary redecoration such as one coat of water based plaster primer paint may then be carried out but any permanent decoration should be delayed until such time as all residual moisture has dried out from the walls. A guide is approximately 1 month per inch thickness of wall, therefore a 9″ wall will take approximately 9 months to dry out, but this will depend on temperature and ventilation.
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