Plastering - Damp walls

In existing houses with damp walls, or where walls are to be re-plastered following the installation of a DPC, gypsum plasters are not suitable. They readily absorb water and will eventually break away from the background. Furthermore, gypsum plaster cannot stop the passage of salts caused by migration as water evaporates from the wall. Some of these salts occur naturally in building materials, others are drawn from the wet ground. These salts, depending on their chemical make-up, can either appear as efflorescence on the surface of the wall (a white fluffy powder) or as well-defined damp patches. Efflorescent salts are nearly always present in building materials and their presence merely indicates that moisture is evaporating from the structure. Much more serious is the presence of chlorides or nitrates; chemicals which will have been drawn into the wall through failure of the DPC. These materials are hygroscopic. Hygroscopic salts continually absorb moisture from the air, resulting in damp patches on the walls. This can be confused with failure of a replacement DPC.

Following DPC replacement it is usual to use render and floating coats of sand and cement containing a waterproofer or salt retarder to keep the salts away from the face of the plaster. The thickness depends on the nature of the wall, but will be in the region of 8–10mm per coat. The sand should be clean, ‘sharp’ and well graded. The finish or setting coat is usually a gypsum plaster applied in a very thin layer, about 2mm thick. This gives a smooth finish ready for decoration.

In wet conditions a chemical reaction can occur between cement and calcium sulfate. It is therefore important to ensure that the floating coat is reasonably dry before applying the finish. A dry floating coat will help to ensure that the effect of any shrinkage in the finish coat is minimised. Sand/cement plasters are very brittle and are not always suitable for application to walls which may have been laid in lime or weak cement mortars. The dense plasters cannot readily accommodate minor movement of the wall and cracking will occur. Lime or plasticisers can be added and the strength of the mix should take into account the key and the suction of the background.

Alternatively, other specialist plasters, such as Limelite can be specified. Limelite is the trade name for pre-mixed plaster with cement and lime, rather than gypsum, as the binding agent. It shares the same advantages as gypsum lightweight plaster, i.e. for use after the installation of a DPC in an existing building. Unlike some of the gypsum plasters, Limelite is not suitable for use on plasterboard.

Where there is no risk of dampness there seems little point in using site-mixed, cement- based plasters. They are slightly more expensive, offer less thermal insulation, are prone to drying shrinkage and are more likely to be contaminated as mixing with sand is required on site. However, they are sometimes required by some Building Regulations Approved Document design solutions or accredited construction details schemes for party walls, as they help to improve sound insulation by blocking any small air paths or gaps in the wall. See the two images above: both are party walls and have been plastered using a sand/cement plaster to reduce incidence of noise transfer. The walls will subsequently be dry lined.