Lime Rendering Doncaster: Limestone and Older Solid Walls

Why cement on an old solid wall goes wrong

SPAB explains that old buildings rely on being permeable, so water absorbed by the wall can evaporate back out. A cement render is more rigid than the wall behind it. Hairline cracks open, draw in rain and trap it behind a skin that will not let it dry. Cement renders tend to fail in patches and come away, while lime renders erode back slowly and evenly. Damp patches inside, salt staining and hollow areas near the base are typical signs on a cement-rendered stone wall.

That does not mean every cement render must come off. SPAB's advice is to remove a small trial area first, because removal can damage the stone. Where cement still adheres well, leaving it to age may be wiser, or taking off only the base of the wall and hollow-sounding patches. Removal is done with a hammer and, where needed, a chisel; levering off large sheets can tear away soft stone faces, and power tools belong only in very experienced hands.

How a lime render is built up

Lime render goes on in several thin coats rather than one thick one. Mike Wye's guide for its own mortars starts by filling deep holes with a haired lime mortar and spraying the wall lightly with water half an hour before each coat, so the stone does not suck the water out of the mix. A 3mm harl coat is thrown on for grip and left at least 24 hours, then a 15mm scratch coat of haired lime mortar is applied and combed to give a key. After about five to seven days a 10mm float coat follows, and a final harled coat can give a roughcast look.

Each coat must be firm before the next goes on. The guide's test is that the surface resists a fingernail, which it calls green hard. SPAB adds that a mix with a faster, harder set can come from adding a pozzolan or using a natural hydraulic lime, but strongly hydraulic limes should be avoided on soft walls.

1. 1

   Remove failed cement render with care, starting with a trial area.

2. 2

   Let a saturated wall dry, then make decayed stone or joints sound.

3. 3

   Dampen the wall and throw on a thin harl coat for grip.

4. 4

   Apply the haired scratch coat and comb a key into it.

5. 5

   Apply the float coat once the scratch coat is green hard.

6. 6

   Keep each coat shaded and damp under hessian while it cures, then limewash.

Curing, protection and timing

Lime hardens by taking in carbon dioxide from the air over many months, so the early weeks matter. Mike Wye's guide asks for each coat to be protected from hot drying wind, strong sun and rain, covered with hessian for as long as needed, and not applied below 5°C where there is a risk of frost. That makes lime a job for the warmer months, with time allowed between coats. SPAB also warns that a wall which wore cement for years may carry salts, which a new lime render can draw out; some local re-rendering after a couple of years may be needed.

Finishes, conservation areas and listing

Limewash is the traditional finish. Mike Wye suggests four coats and recoating every three to seven years on average, with a silicate paint as a harder-wearing breathable alternative; both it and SPAB advise against ordinary modern paints on lime. Doncaster's conservation areas include places such as Bawtry and Conisbrough, and in a conservation area national guidance says rendering a house is not permitted development and needs planning permission. A listed building needs listed building consent for alterations that affect its special architectural or historic interest, and the council keeps a separate local heritage list, so check the address before work starts.