Preparation of a wall for rendering (advice)

Preparing a wall to be re-rendered

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For stone and brick any hollow or decayed render should be hacked off and any loose pointing should be raked out and replaced prior to rendering. Brush the wall to remove loose material. Do not rake out pointing to provide a key.

Do not use plastering bead on corners as this will give a modern appearance. Do not use chicken wire or metal lath to form a key as it can cause stress in the render due to differential thermal movements and can lead to large-scale failure, especially when it rusts.

For plastering onto existing wooden laths check that they are firmly fixed and free of lumps of old plaster. New laths should ideally be riven oak or chestnut. Sawn laths are inferior as they are smoother and weaker than those split along the natural grain of the wood. Laths should be fixed so that the distance between them is approximately 8-10mm. This allows the right amount of space for the plaster to be pushed between the laths and flop over to form a key. Do not apply preservative treatments to either old or new laths as they can introduce harmful salts into the plasterwork.

Metal lath is sometimes used internally instead of timber laths as it is quicker to fix and cheaper, but it is harder to plaster onto as it is slippery and the sharp edges may cut into and weaken the plaster key. Plenty of hair in the mix is essential.

For masonry, thoroughly wet the wall with clean water using a hose-pipe or sprayer. The more porous the background the more water will be required. Allow the water to soak in a bit then spray again, and repeat until the surface layers of the wall are thoroughly damp. When the render is applied the wall should be damp but without water glistening on the surface. Timber laths should be thoroughly dampened before plastering.