Crumbling Lime Plaster In An Old House

What's happening?

Lime plaster is a different material doing a different job. It is softer and more porous than gypsum, it lets a solid wall absorb and release moisture, and it moves a little rather than cracking. That is why it survived two centuries on walls with no damp proof course. It fails when that system is interrupted: a gypsum or cement patch is harder and less porous than the lime around it, so moisture is pushed into the surrounding lime and the salts crystallise there instead, and the old plaster goes sandy in a halo around the new repair. The pattern is diagnostic. Failure concentrated around a modern patch, behind a cement render, or under a sealed paint is a compatibility problem, not old age, and repairing it with the same material repeats it.

What usually causes it?

• Gypsum or cement patch repairs in a lime wall, concentrating moisture and salts into the lime beside them
• Cement render or hard pointing outside stopping the wall drying outward, so the moisture leaves through the inside face
• Modern impermeable paints or a vinyl wall covering sealing the internal face of a breathable wall
• A raised external ground level, failed rainwater goods or a leak keeping that part of the wall wet
• Hygroscopic salts in the plaster from long-term damp, holding moisture out of the room air
• Loss of the original hair reinforcement or the plaster's key on riven laths as it dries and ages
• Injected or tanked damp treatments applied to a solid breathable wall, moving the problem rather than removing it

When should you call a plasterer?

Call a plasterer who works in lime where a whole wall or ceiling is affected, where salts keep coming through, or where earlier gypsum or cement repairs need reversing. Take advice before any work if the building is listed or in a conservation area. Where the cause is outside — cement pointing, hard render, ground levels or rainwater goods — that is fixed first, because a correct lime repair on a wall that is still being wetted will still salt and soften.

What will the plasterer actually do?

• Identify the original plaster and the background, so the repair matches rather than outperforms them
• Cut out incompatible gypsum or cement repairs that are concentrating moisture into the surrounding lime
• Rake back to sound material with dust control, keeping the sound lime and any hair reinforcement intact
• Rebuild in haired lime coats to match the original thickness, key and finish, with proper curing between coats
• Protect the repair while it carbonates, and advise on a breathable decoration rather than a sealing paint
• Refer external cement render, hard pointing or ground level faults to the trade that owns them