Repairing plaster

Few homes escape damage to plaster caused by wear and tear, settlement of the main structure or through damp or excessive heat. Not only does it look unsightly, but it can become a major problem. So tackle the job early and save yourself time and trouble.

The thought of having to repair damaged or flaking plaster may be fairly daunting to some people, but tackled in the right way with the right materials it is a relatively straightforward operation. Obviously it is quite a different matter if you have to replaster an entire wall or ceiling, in which case this is probably best left to the professional. But most of the small repairs normally required in the home are well within the capabilities of the DIY enthusiast.

Ceiling cracks

These are caused by the movement in roofing and flooring joists, leading to the plasterboard (where fitted) parting at the joints. Hairline cracks can easily be filled. With larger gaps you should seek professional advice.

Wall cracks

These are more likely to occur in new houses and are caused by the settlement of the main structure. The area most likely to be affected is the angle between the wall and the ceiling. Apart from filling in these cracks, which are likely to reopen later, one of the most effective methods is to cover over the gaps round the room. This can be done simply by fitting cove. Unsightly cracks across the wall may also be caused by settlement and normally only affect the plaster. If, however, you get a wide diagonal crack appearing not only in the plaster but also in the wall, this could be a major problem and professional advice should be sought immediately. When this happens it is usually on external walls and is clearly visible from the outside as well.

Filling cracks

Common hairline cracks can be repaired simply and quickly. Rake out the affected area with a sharp knife or the edge of a paint scraper. Cut a V shape into the wall along the crack so that it is widest at the deepest point of the crack. This allows you to push the filler into the cavity dovetail fashion to prevent it falling out on drying. Apply cellulose filler with a flexible steel filling knife. Use either a 75 or 100mm filling knife, the larger size being preferred since you can work quickly over large areas. Dont confuse this knife with a paint stripping knife, which looks similar, but must not be used as a substitute. The blade, which will bend about 90 degrees, should be perfectly straight and undamaged. Correctly used the knife can be used to give a smooth finish and make the job of rubbing down later unnecessary. Otherwise rub down with medium fine, then fine, glasspaper when the filler has completely dried, before redecorating.


Replacing loose plaster

Plaster often comes away from the surface around fireplaces. It can work loose due to vibration such as excessive hammering near the affected area possibly when fitting a door or window frame. One simple test for loose plaster is to tap the suspect surface with the handle of a knife or a small blunt instrument. A hollow sound indicates poor adhesion between the plaster and substrate. Lift all the loose pieces with a broad knife and clean the surface beneath with a soft brush.

If you are dealing with only a small cavity you will probably get away with filling the area with fresh finishing plaster. In the case of a deep cavity. first apply a plaster undercoat. Wet the wall thoroughly, then roughly fill the cavity to within about 3mm of the original plaster surface, applying the undercoat with a plasterers trowel. The undercoat will dry with a rough texture which will provide a key for the finishing plaster. You will find a small hawk useful to carry the plaster to the wall area after mixing it; make one by nailing a square of plywood to a short length of broom handle. When the undercoat is quite dry, mix up enough finishing plaster to a creamy consistency to complete the job. In powder form it does not keep that long and old plaster will often set too quickly to enable you to spread it properly; in this case the application will just crack and fall away.

If you find the plaster is hardening before you have a chance to use it, take it back to your supplier for replacement. To complete filling, go over the undercoat surface with a dampened brush, put a generous amount of plaster onto the bottom of a wood float or plasterers trowel and apply it into the remaining cavity. When the cavity is filled you can rule off the plaster. Using a timber straight-edge, which must be longer than the area being repaired, start from the bottom and work upwards over the new plaster with a sawing action, making sure both ends of the timber keep in contact with the surface of the existing plaster. This method ensures high spots are removed and low spots are built up as excess plaster is pushed up the wall, giving a level finish. When the plaster has almost set, rub a plasterers trowel over the new surface to give a smooth, polished finish. Lift the front edge of the trowel away from the wall so only the back edge is in contact; this will prevent the trowel cutting into the new plaster. Alternatively wait until the plaster has set completely and apply a layer of cellulose filler over the fresh plaster using a filling knife.

Make sure you fix the batten well clear of the affected area or you may cause further damage. Build up the level by applying the undercoat plaster with a trowel or float, always working away from the corner. On one side, plaster the area to within about 3mm of the original surface, then move the batten to the other wall to complete the undercoating. When this is dry, complete the repair with finishing plaster, using the batten on each wall as before. If you nailed or screwed the batten to the wall, fill the holes with any plaster you have left over or with cellulose filler. Before the plaster sets hard, round off the corner by rubbing your fingers over the plaster to form an edge to match that on the rest of the corner. Use glasspaper if the plaster has set really hard.

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