Treating damp walls insternally

Various damp treatments are available for internal walls in the home; they do not cure the dampness in the wall, but they will prevent the effects of damp ruining inside decoration. Although suitable for any room, they are especially useful in cellars and basements where access for external treatment is a problem. Before carrying out treatment, always check the manufacturer’s instructions carefully.

Bituminous emulsion

This rubber-rich substance adheres strongly to a smooth surface to leave a black, waterproof, elastic film. The method of treatment depends on the extent of dampness in the wall; in moderately damp conditions with reasonably sound plaster, you will fust have to strip oil’all loose distemper, paint and dirt and repair any damaged plaster.

Before use. and occasionally during use. stir the emulsion thoroughly. Put it on with a brush, working in one direction only to give an even coating. Before starting work, dip the brush in water and shake off the moisture. When you are working it is best to immerse the bristles only halfway into the emulsion. The type of brush you should use depends on how the wall is to be finished after treatment. Where a lining paper is to be used as a base for normal decorating, you will need soft bristle brushes; where a plaster finish is required, use coconut fibre brushes. You will find you need two brushes to prevent hold-ups during the work because after five to ten minutes’ use a brush should be placed in water and left to soften.

When applying the treatment, you must first dampen the affected surface with water. Put on three coats of emulsion, allowing each to dry thoroughly before applying the next. For each coat of emulsion, allow 5 litres per 1 lsq m. You can speed up the drying process by using a fan type electric heater. At the end of the day. suspend the brushes in creosote and next day wipe them dry and clean them off in turpentine or a proprietary equivalent.

Warning The fumes from this substance can be harmful, so make sure the room in which you are working is very well ventilated.

If the wall is very damp, a more rigorous preparation is needed. Use a cold chisel and a club hammer to hack the existing plaster back to the brickwork and spread a thin layer of cement mortar over the brickwork. When this is dry. dampen the surface with water and apply three coats of emulsion. While the final coat is still tacky you should sand-blind it by throwing clean sharp sand at the surface with a shovel wear protective spectacles or some other form of eye protection when carrying out this process. The sand-blinding provides a key for a plaster finish which you should apply in a layer at least 6mm thick.

Preparing for decoration Leave new plaster to dry out thoroughly before decorating, then prepare the surface to suit the kind of material you are using for decoration. For emulsion paint, allow the wall to dry then hang lining paper with heavy duty cellulose paste.

Emulsion paint can be applied direct to a plaster finish as long as the plaster is at least 6mm thick. You should apply a coat of plaster sealer followed by two coats of emulsion. If you intend using oil paint, a plaster finish at least 15mm thick over a sand-blinding is required. Apply two layers of plaster and allow it to dry – this can take up to six months. You can test for the degree of dryness by using a damp meter; repair the holes made by the prongs of the damp meter with cellulose filler. For wallpaper, hang lining paper horizontally over the bituminous emulsion with heavy duty cellulose paste; use the same paste to Ix the wallpaper over it. For vinyl or washable paper, a 15mm thick plaster surface is needed. 2b


This material conies with a special adhesive and produces a long-lasting barrier which stops both damp penetration and the appearance of efflorescent salts. It will adhere to moist surfaces.

Remove existing loose wallpaper, paint or distemper. repair damaged plaster, sand down any high spots in the wall and remove the skirling boards. Brush on a diluted coat of adhesive to prime the wall and allow this to dry for an hour. The laminate is hung in horizontal lengths, allowing a little excess for trimming. It has a white and a brown side: brush water onto the brown side of each length and fold the ends loosely to the middle with the brown surface inside. Wait an hour, then brush adhesive onto the wall to cover an area to be occupied by one length of laminate: hang the laminate immediately with the brown surface against the wall. Brush outwards over it to remove air bubbles. Even after this there may be a slightly bubbly appearance, but this will disappear in time. Hang successive lengths of laminate so the edges overlap each preceding length by 13mm. Use a sharp knife to trim off excess laminate at the wall sides and allow 24 hours before carrying out decoration and refitting skirting boards.

Pitch-impregnated fibre base Any condition of damp wall can be treated using this material, since it forms an entirely new wall surface. It is available in 5xlm rolls and is corrugated with a dovetail key to provide for plaster on one side and insulating cavities on the other.

Remove the skirting and any cove and use a cold chisel and club hammer to hack damp plaster back to the brickwork of the wall and for about Im along adjoining party or partition walls in case damp has crept into these walls as well. Using a sharp knife, cut the rolls into lengths according to the width of bare wall. You may have to trim the width of the final strip of material to fit the remaining gap at the top of the wall; where there is a solid floor, the material should stop 25 50mm above lloor level.. On a suspended timber floor the material should be laid between the wall and the floorboards.

Place each length in position against the wall with the corrugations vertical and the metal strips facing into the room. Nail through the corrugations at intervals of 200-300mm with galvanized clout nails or hardened masonry nails, depending on the wall surface. Alternatively, on a very hard surface, use a cartridge gun to shoot nails through special rectangular washers which fix into the corrugations. Any polythene damp proof membrane on a newly scrceded floor should rise 150mm on to the wall and be placed behind the material.

The material is flexible and therefore can be bent round corners. Try to avoid joining lengths at corners: if this is unavoidable, place a 100mm wide strip of bitumen felt behind the joint as reinforcement to prevent the plaster finish being pushed through to contact the damp wall. Vertical joints must be lapped, while horizontal joints are butted up and lined with 100mm wide strips of bitumen felt. If the material has to be cut to fit round pipework, seal the gaps with waterproof mastic.

When the wall is fully lined a normal three-coat plastering can be applied. If necessary, you can use a render and setting coat to leave a joint flush with any existing plaster or adjacent areas. Drying out will take longer than usual, since the damp treatment material does not absorb water; so make sure of good ventilation to speed up the process.

Alternatively you can use panel boards to finish over the material. The boards can be nailed through into the wall or fixed by applying blobs of special adhesive at 350-450mm intervals to the back of the board. This adhesive can be obtained with the damp-treatment material; a 5kg tub of adhesive is sufficient for 12sq m. Rest the boards on a level 50 x 25mm timber plinth to ensure a neat joint between boards and to give the required air circulation gap at the base. The adhesive will set in two or three days and temporary support can be provided with one or two hardened fixing pins in the middle of the panel. These can be removed or driven home later. Use brown paper tape, masking tape or cellulose filler to seal the joints between boards. You can, if you wish, apply a skim coat of plaster over plasterboard panels.

Finally replace the skirting, having checked there are no signs of rot or decay, and cover the gap between the top of the lining material and the ceiling with cove. Fix the cove to the wall and flush with the top of the lining, or to the ceiling. leaving a slight gap between the cove and the wall to provide ventilation.

Levelling wall Sometimes it may be necessary to level an uneven wall before applying the damp treatment material. You can do this by lining the wall with battens – before fixing, treat these with a wood preservative. Use a spirit level to find true horizontals and fill any low areas behind the battens with timber packing. Arrange the battens so all the edges of the damp treatment material will be supported – one or two intermediate horizontal batten supports should also be provided. Space the battens at approximately 330mm intervals and place the corrugations of the material at right-angles to the battens.

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