Original plaster contributes immeasurably to the qualities of an old building. Unlike modern cement based products and gypsum plasters, which are hard, inflexible and non-breathable, traditional lime and clay formulas have a soft, characterful appearance, offer a degree of flexibility and are breathable.
The breathability of wall and ceiling finishes is a very important issue in period homes. Old brick, stone and ancient wattle and daub walls rely on moisture being able to evaporate through them. This helps to prevent damp and condensation.
Sadly, the wrong techniques and inappropriate modern materials are all too frequently used for repairs, so moisture is sealed in, resulting in damage and decay.
What type of plaster does my home have?
- Lime plaster is the traditional finish for houses pre-1919, but may have been used up until the 1950s when plasterboard and gypsum took over.
- A pinkish colour is likely to indicate a plaster bound with gypsum.
- An off-white colour is typical of a lime plaster.
- An earthy colour suggests an earth binder.
- If your old house has been replastered with modern materials, or if you have damp walls, it might be worth replacing with lime plaster.
Look out for historic markings
Original plasterwork may contain fascinating evidence relating to the building’s history, such as traces of historic decorative schemes, as well as superstitious candle smoke marks and apotropaic symbols intended to ward off evil spirits.
Look out for 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.
How traditional plaster is made
Old plasters are generally made with lime and sand, with animal hair added as a binder to provide extra strength and reduce shrinking and cracking. Sometimes in high-status work it is gauged with gypsum. Plaster may also be clay based, with animal dung and a lime binder.
Traditional plasters are applied directly to solid backings, such as masonry or cob (unfired earth), or flexible supports, including timber laths or reed.
The aggregates found in old finishes vary according to the materials that were available locally. There might, therefore, be a high proportion of very coarse or very fine particles, the use of which would not comply with modern standards for sands and aggregates.
Most aggregates are ‘as raised’ (i.e. dug out of the ground on or near the site). They include silica sand, other mineral-based sands and crushed stone.
When to replaster walls and ceilings
Plasterwork should last indefinitely, but when deterioration does occur it can take various forms:
- loose plaster or delaminating coats
- crumbling or flaking
- mechanical damage and missing areas
Deterioration often looks worse than it is and repair rather than replacement is often possible.
Any replacement, where justified, should normally comprise the same material and number of coats. Adequate hair or other reinforcement is crucial on flexible backgrounds.
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.
Generally ceilings are relatively easy to replaster. 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.
How much does plastering cost?
- Hiring a specialist plasterer with experience in dealing with traditional finishes on period homes is more expensive than standard plasterings.
- For a standard job using gypsum-based finishes, expect to pay between £450-£750 for walls in an average room, and £200-£350 for the ceiling.
- The price can double for lime or specialist clay coatings.
How to plaster with lime
Employ an experienced lime plasterer. Lots of plasterers will say they can do it, but most of them can’t. Request a reference from a recent job. Alternatively, ask an on-site trainer to work alongside a team of plasterers who are keen to learn. Your lime supplier should be able to recommend a local trainer.
Take advantage of technical advice from lime suppliers when you order your plaster.
- You will need to apply two to three coats, depending on how flat you want the finished work. Finest quality uses three coats, but generally two coats will suffice.
- If you’re on a strict budget use a reed mat instead of riven laths on the ceiling – that is, if you decide to lime plaster that too. This will save several thousands of pounds. Fix it to the bottom of the joists using laths and screws.
- The mix for the first coat, or ‘scratch coat’, should be one part lime putty to three parts sharp, well-graded sand, mixed with plenty of animal hair. This is called ‘haired coarse stuff’.
- For a smooth finish, apply a setting coat of one part lime to two parts fine sand, called ‘fine stuff’. This can also be bought ready mixed.
- Unless your plasterer advises you otherwise, buy your plaster ready mixed by the tonne and delivered to site.
- Don’t leave bags of haired plaster sitting around for more than four weeks – the hair will start to rot due to the alkalinity of the lime.
Assessing problems with plasterwork
Loose plaster, bulges and cracks
- Old 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, 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.
- Ceilings that are sagging 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 plaster might result from structural problems. It is advisable to consult a structural engineer.
Dampness may result in salts appearing on internal plaster. Where the problem is severe the best solution may be to replace it.
Soot staining from flues can often be seen on the surface above a fireplace.
If you decide to replaster stained 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.
Removing a loose section of old lime plaster
Conservators use specialist methods to reattach loose plaster or delaminating layers, especially where of historic value, so if in doubt bring in an expert. 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.
- Beware of using lime-based materials when frost is imminent as they are likely to fail.
- Plaster that matches the existing material is preferable where repairs will be limewashed.
- 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 plastering.
Small cracks and patches
Cracks and pockmarks may be filled using matching plaster, or narrow ones with proprietary interior filler gauged with whiting.
To patch repair plasterwork, slightly treat the exposed edges of the existing plaster with water or diluted PVA (one part to 10 of water) to control suction and prevent cracking. Alternatively, gypsum may sometimes be added to a lime plaster mix for the same purpose.
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.
Repairing decorative plasterwork
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.
Words Roger Hunt, Douglas Kent, Marianne Suhr