Original plaster and render finishes need to be treated with respect. They add texture and character to a building and help it function as a ‘breathing’ structure, avoiding problems with damp and condensation because moisture is able to escape through outside surfaces. Knowing how to maintain and care for these plasters and renders is an important part of owning a traditional home.
Sadly, the wrong techniques and materials are all too frequently used for repairs so moisture is sealed in, resulting in damage and decay, often with disastrous consequences to walls and floors.
Look out for…
- Cement renders and other coatings that may be preventing a wall from breathing.
- Renders that have bridged damp proof courses and are causing moisture to rise up a wall.
- Ceilings that are sagging. This can be due to the plaster key having failed but may also be caused by problems with the joists.
- Bulges or cracks in walls that are unrelated to the render or plaster. These might result from structural problems, so it may be advisable to consult a structural engineer.
- Wall paintings in pre-Georgian houses. Early plaster layers may be finished with decorative wall paintings, now often hidden behind multiple layers of paint and later plaster.
Unlike modern cement based products and gypsum plasters which are hard, inflexible and non-breathable, traditional lime and clay plasters and renders have an attractive, soft appearance, offer a degree of flexibility and are breathable. Traditional plasters therefore suit pre-1919 building construction techniques, which rely on moisture being able to evaporate through the walls.
The materials themselves are relatively simple, consisting of lime and sand, and sometimes clay, often with animal hair added as a binder to provide extra strength and reduce shrinkage and cracking.
Left-right: ‘Scratching’ the surface of the plaster so the next coat will bond properly; a pigmented clay plaster by Clayworks gives a matt, mellow feel to walls
When making repairs ensure any underlying structural or damp problems have been rectified and that the wall has been allowed to dry out fully before applying a plaster or render. Beware of using lime-based materials when frost is imminent as they are likely to fail. Good suppliers should be able to give appropriate advice on the most suitable materials and application methods but remember plastering is a highly skilled job and achieving an acceptable finish is far from easy.
Where inappropriate cement render has been used and is trapping moisture it may be sensible to remove it. The downside to this is that further damage can be caused to the wall, so always trial a small area initially. If removal is difficult, it is sometimes better only to remove loose sections or areas where damp is most problematic.
Left-right: Lime render being applied to a cottage. The hessian shelters the ‘scratch coat’ of the new render from the sun; the finished cottage rendered in lime plaster coloured with a creamy pigment
When re-rendering use a lime render. After it has been applied protect it from direct sun and drying winds by keeping it damp with a mist spray and covering it with damp hessian to prevent the moisture evaporating too quickly. Where a wall is to be painted, a traditional limewash is the most appropriate finish as this will allow the wall to breathe. Avoid all sealants, paints or finishes that may hinder this process.
The facades on many buildings have a plain render finish, but it is important to respect the local tradition. For example, roughcast, known in Scotland and northern England as ‘harling’, is a lime render mixed with pea gravel thrown on to the face of the wall. It is commonly used in exposed locations as its increased surface area serves to maximise evaporation and protect against driving rain.
Old plaster finishes on masonry can sometimes sound hollow when tapped but need not necessarily be replaced as traditional haired plaster is likely to be relatively strong since the hair binds it together, even if a section is detached from the wall.
In the case of stud walls and ceilings, timber laths are fixed between the studs or joists and the plaster is pushed through the gaps between the laths. Problems occur when these plaster keys fail over time or, in more extreme cases, when the laths have rotted or suffered from beetle attack. Sometimes it is possible to screw the lath and plasterwork back into place.
In the case of ceilings, where access is available from above, it may be possible to add reinforcement on top by using fresh plaster poured on and strengthened with a jute scrim set into this and secured to the joists.
Dampness may result in salts appearing on internal plaster. Where the problem is severe the best solution may be to replace the plaster. Soot staining from flues can often be seen on the plaster surface above a fireplace.
Left-right: Mixing animal hair into the plaster mix helps bind it together and make it stronger; failing cement render on a traditional stone wall
If you decide to replaster such areas, when the problem is not severe it is sometimes suggested to use a slurry of cow dung painted on to the masonry before applying new lime plaster. If this fails to hold back the staining, the only option may be to ‘dry line’ by fixing battens to the wall and applying laths and lime plaster.
Always try to avoid the use of plasterboard. Unlike traditional plaster finishes it is flat and can look out of place in a period building. This is especially true with old ceilings. Often, when a ceiling is damaged, perhaps by a leaking pipe above, builders recommend taking it down entirely and replacing it with plasterboard.
Generally ceilings are relatively easy to repair. Where necessary laths can be replaced — these are best fixed with screws to avoid vibrations. Lime plaster is then applied over them and matched in with the surrounding surface.
Where decorative plasterwork, such as cornicing, is damaged repairs can usually be undertaken by a skilled plasterer. Early plasterwork was ‘run’ in situ using a template and running it around the top of the wall over wet plaster to create the profile of the cornice. Later details were formed using fibrous plasterwork. These incorporated a hessian scrim that strengthened the back and were formed in flexible moulds.
Plasterwork detail is often obscured under paint layers but take great care when attempting to strip these areas. Warm water, steam, chemical removers, poultice systems and proprietary wallpaper stripper may all be successful in removing paint from plasterwork but always undertake a test first on a small unobtrusive area. A toothbrush and a sharpened stick are useful tools, but be careful to avoid any accidental digging with scrapers.
Left-right: For complex decorative render – or ‘pargeting’ – designs as seen here, far greater skill was needed and wet lime-based plaster was cast or built up layer by layer; major repairs underway to a lath and plaster wall; plastering laths during a cottage renovation
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