Lime render, lime plaster and limewash

A coat that lets the wall dry

Before the mid-19th century, mortars, plasters, renders and washes were mostly made with lime ². SPAB explains that old walls stay dry by letting moisture that soaks in evaporate out again ¹. Outside, lime render was absorbent, and a rough, open-textured finish gave more surface for water to evaporate from ⁴. Inside and out, walls were decorated almost entirely with limewash, which is also porous ⁴.

Lime render or cement render?

SPAB gives some ways to tell ¹:

• Age. If the building is older than about 1800, its original render was probably lime.
• Look. Cement render tends to look more uniform, with sharper corners and details.
• How it fails. Cement render often fails in patches and comes away from the wall. Lime render wears back gradually and evenly.

Why cement render causes trouble

SPAB calls a modern cement render incompatible with most old buildings ¹. It is impervious, so water in the wall can’t evaporate. It is also more rigid than the wall, so fine cracks form. Rain is drawn into those cracks and trapped behind the render ¹⁴. Timber-framed and earth buildings are at particular risk of serious structural damage ¹.

Should cement render come off?

Not always. SPAB’s advice is ¹:

• Remove a small trial area first, because taking render off can damage the wall behind.
• Where cement render is still well stuck, it is probably best left to age.
• A compromise can be to remove only the bottom of the wall or hollow-sounding patches.
• Take it off in small pieces. Levering off large sheets can badly damage soft material underneath.

Once it is off, SPAB says the wall should generally be re-rendered, because it was probably rendered originally or needed the protection ¹. If the wall is saturated, wait a short time for it to dry first ¹. The new render should generally be a weaker, more permeable lime render without cement, applied by someone who knows lime ¹. A wall that has been under cement for years may hold a lot of salt, and SPAB says some patch re-rendering may be needed after a couple of years as the salts come out ¹.

Lime plaster inside

The same logic applies indoors. SPAB explains that dense modern plasters stop moisture evaporating from the inside face, so dampness in the wall builds up. Salts then crystallise behind the plaster and can push it off or make it bulge ⁴. Where dense plaster is used to cover a wall suffering from rising damp, the effect may be only to drive the moisture higher up the wall, which can affect old plaster that had been sound ⁴.

SPAB says the slow set and workability of non-hydraulic lime make it ideal for inside plasterwork, and that lime putty suits it well ². Its damp guidance recommends lime-based plasters and vapour-permeable paints, rather than modern substitutes, on traditional solid walls ⁵.

Limewash

Limewash is a simple matt paint of lime and water, coloured with lime-fast pigments ³. SPAB lists its advantages: it is much more breathable than most modern paints, it can consolidate surfaces, and it is inexpensive ³. It suits lime plaster and render, earth walls, limestone and timber. It takes less well on cement render, plasterboard or emulsion, and it isn’t suitable for impervious materials such as flint or hard brick ³.

The catches, in SPAB’s words, are that it needs careful work, coloured batches are hard to match, and it struggles in very fast-drying conditions ³. SPAB says it typically needs renewing every five years and goes on in several thin coats over a dampened surface ³. Wear gloves and eye protection ³.

For new lime render SPAB says traditional limewash will normally be the most appropriate finish. It advises against colourless water-repellent treatments because their long-term effects are unknown ¹.

Listed buildings

Changing a render, stripping it or replacing plaster on a listed building can need consent. SPAB says listed building consent may be required for work that exceeds like-for-like repair and that carrying out work that needs consent without getting it beforehand is a criminal offence ⁵. If in doubt, consult the conservation officer first ⁵.