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Guide

Lime plaster walls.

How to tell if you have them, and why it matters more than it sounds.

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01 Why it matters

The wrong repair causes the damp

If your house was built before about 1919, the walls are probably lime, and they were designed to let moisture pass through and dry out again.

Gypsum plaster and modern vinyl paints do the opposite: they hold moisture. Patch a lime wall with gypsum and the water that used to leave through the plaster now goes around it instead, and reappears somewhere else as a damp patch, blown plaster or a tide mark.

This is why so many period houses develop damp shortly after being replastered. Nothing started leaking. The wall simply lost its way of drying out.

If you are being sold a damp course

In an older house, get the plaster question answered before anyone injects anything. Damp in a solid-walled property is very often a breathability problem rather than rising damp, and the fix is different and usually cheaper.

02 Identifying it

How to tell lime plaster from gypsum

No tools needed for the first four. Do them somewhere hidden — behind a radiator, inside a cupboard, low down behind furniture.

  1. Look at the colour of a breakChip a fragment from a hidden edge. Lime is off-white, cream or buff all the way through, and often has a sandy, gritty look. Modern gypsum browning is pink or grey, and the finish skim over it is a distinct pale pink.
  2. Look for hairOlder lime plaster was reinforced with animal hair, usually horse. Fine dark fibres in a broken edge are as close to proof as you will get without a lab.
  3. Feel how hard it isLime is softer. A fingernail or a coin will mark it and leave a chalky powder behind. Gypsum skim resists a nail and feels glassy by comparison.
  4. Try water on a bare patchLime drinks it in and darkens, then dries back with no mark. Gypsum darkens too but much more slowly, and a sealed or painted surface simply beads.
  5. Tap along the wallLime over timber laths sounds hollow between the laths and solid on them, in a regular rhythm about every centimetre. That rhythm is the giveaway, and it is the most common arrangement in a pre-war upstairs wall or ceiling.
  6. Check the age of the housePre-1919 is almost certainly lime originally, though it may have been skimmed over since. Interwar houses are mixed. Post-war is gypsum unless someone has deliberately gone back.
03 Safety

Is lime plaster dangerous?

Set lime plaster on a wall is not hazardous. It is a mineral surface, and living with it is exactly as safe as living with any other plaster.

Wet lime is caustic while it is being mixed and applied, which is a matter for whoever is doing the work: eye protection, gloves and a mask against the dust. It is not a reason to avoid the material.

The real caution in an older house is not the lime. It is what may have been put over it since. Textured coatings and some plaster and board products from the middle of the last century until 1999 can contain asbestos, and the original lime beneath them does not.

So the rule is about disturbance rather than about lime: do not sand, drill, scrape or break out any unidentified coating in a house of that age. If there is textured or artex-style covering on the wall or ceiling, have it tested before anyone touches it. Testing is inexpensive and settles it.

Short version

Lime plaster itself: no asbestos, not hazardous in place.

Unknown textured coatings in a pre-2000 house: get them tested before disturbing them.

Any plaster at all: wear a mask when you sand it. Fine dust is fine dust.

04 Living with it

What to do with a lime wall

Repair it like for like

Lime patches go into lime walls. A gypsum patch is harder than what surrounds it and it cracks around the edges as the wall moves, as well as blocking the moisture.

Use a breathable paint

Limewash, silicate or a mineral emulsion. Vinyl and most modern washable emulsions seal the surface, which is the same mistake as gypsum in a thinner layer.

Leave the small cracks alone

Fine hairline cracking is what lime does. It is flexible by design and it moves with the building rather than resisting it. Filling every one with a rigid filler makes it worse.

Get damp diagnosed before it is plastered over

A wall that is wet for a reason will be wet under a new finish too. Find the reason first — ground levels, a blocked gutter, a failed pointing joint, a bridged cavity — then plaster.

Decorative lime is an option, not just a repair

Marmorino, Tadelakt and limewash are lime plasters. In a period house they are both the historically correct material and a finish worth looking at, which is a rare combination.

05 Questions

What people ask

What does lime plaster look like?
Off-white, cream or buff through the full thickness, often sandy in texture and frequently with fine animal hair visible in a broken edge. On the wall it looks softer and less perfectly flat than a modern skim, with gentle undulation across it.
Is lime plaster better than gypsum?
In a solid-walled older building, yes, because it lets the wall dry out and it moves with it. In a modern cavity-walled house with a damp-proof course, gypsum is cheaper, faster, flatter and perfectly correct. It is about the building, not about the material.
Can you skim over lime plaster with gypsum?
Physically yes, and it is done constantly. In a period house it is usually a mistake, for the reason above. If the wall is sound, a lime skim keeps the property working the way it was built to.
What paint can you use on lime plaster?
Limewash, silicate or mineral paint. Avoid vinyl emulsion and anything sold as wipeable or washable, because the same property that makes it wipeable is what stops the wall breathing.
How long does lime plaster take to dry?
Touch dry in days, but it carbonates for months — it is still turning back into stone long after it looks finished. Do not paint it early, and do not put heaters on it to speed it up, which causes it to crack.
Does lime plaster contain asbestos?
No. Lime plaster is lime, sand and often hair. The concern in older houses is textured coatings and some mid-century products applied over the top, which can contain it. Anything unidentified in a pre-2000 property should be tested before it is disturbed.
Can lime plaster be used in a bathroom?
Yes — Tadelakt is lime, and it is used in showers. Plain lime plaster in a normal bathroom is fine and copes with humidity better than gypsum does, because it can take moisture up and release it again.
06 Start

Period property, and not sure what you have?

Send a photograph of a broken edge or a hidden corner and we will tell you what we think it is. We work on listed and period buildings and we will say honestly when a job wants a heritage specialist rather than us.

Ask us Limewash finishes

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