Why Important
What to look for
Staining of the plaster quite visible. If stained by rising damp, the insertion of a dpc may not solve the problem immediately, as the drying-out process can take several months. Evaporation will lead to hygroscopic salts forming a damp patch on the plaster
When to look
The drying-out process of damp patches can take several months. The hygroscopic salts which form a damp patch on the plaster, which may result in the plaster needing to be replaced to a height of 450 mm above the affected area.
How to fix
The new plaster should provide a dry, salt-free surface, able to resist efflorescence, residual moisture and hygroscopic salts; it should be able to ‘breathe’, and an alkaline base will enable it to resist mould growth.
The choice is between
- A traditional 1:6 cement/sand mix with or without an additive (salt inhibitor, waterproofer),
- A lightweight cementitious ‘renovating’ plaster, used in accordance with BS 5492. Renovating plaster should be applied to a thickness of approximately 11 mm, scratched and left to set, during which time slight shrinkage may occur.
The resulting crazing should be hidden by the finishing coat.
A minimum of three days should be allowed before plastering walls which have been treated with fungicide.
Avoiding creating problems
Renovating plaster should never be applied to backgrounds containing traces of gypsum, or over plasterboard.