Should I repair an old wall in lime plaster or gypsum?

This is not a heritage argument, it is a moisture argument. A solid masonry wall built before damp proof courses handles water by absorbing it and letting it evaporate back out. Lime plaster is porous and lets that happen. Gypsum plaster is not, and it also dissolves slowly in the presence of persistent moisture and soluble salts. Put gypsum on a wall that periodically gets wet and it will blow off, sometimes within a couple of winters, taking the paint with it.

Safety

Step by step

1. Establish what is there now.Scratch an inconspicuous patch with a key. Lime is softer and gives a sandy, granular dust, often with visible animal hair, and the finish coat is thin over a much thicker coarse base. Gypsum is harder, breaks in brittle flakes, and gives a fine white or pinkish powder.
2. Establish what the wall behind it is.Solid brick or stone with lime mortar, no cavity, no damp proof course, is the case where the material choice genuinely matters. A cavity wall or a modern block wall in a dry house is not.
3. Establish whether the wall gets wet.Tide marks, salting, seasonal patches, plaster that has failed at low level before, a solid floor without a membrane. If the plaster has already blown once in the same place, the wall is telling you the answer.
4. Choose lime where the wall needs to breathe.Non-hydraulic lime putty or a natural hydraulic lime, in coats, with the finish coat matched to the existing. It stays porous, tolerates movement, and lets the wall dry through the surface rather than trapping water behind it.
5. Choose gypsum where the wall is dry and always has been.On an ordinary dry internal partition or a modern wall, gypsum is faster, harder, cheaper, easier to finish and entirely appropriate. Traditional materials are not automatically better; they are better in specific conditions.
6. Never mix the two in the same repair.Gypsum patched into lime, or a gypsum skim over a lime backing on a damp wall, fails at the join. If you are repairing lime, repair it in lime; if the whole wall is gypsum and dry, keep it gypsum.
7. Allow for the timescale.Lime is slow. Coats need days rather than hours between them, it must be protected from drying too fast and from frost, and it gains strength over weeks. If the deadline cannot accommodate that, the deadline is the constraint, not the material.
8. Get a specialist for a listed or historic building.On a listed building, replacing lime plaster with gypsum can be a consent issue as well as a technical mistake. A lime specialist will also match the aggregate and the hair so the repair is not visible in raking light.

Common mistakes

• Re-skimming a repeatedly blown wall in gypsum — The blowing is being caused by moisture and salts at the surface. A harder, less porous material concentrates the problem instead of relieving it, and the repair fails in the same place.
• Assuming lime is only for period correctness — The reason for using it is physical. If the wall is dry and modern, lime brings cost and delay for no benefit.
• Sealing a lime wall with modern vinyl emulsion — It undoes the point of the plaster. Breathable mineral paints, limewash or clay paints keep the surface open.

Alternatives

• Non-hydraulic lime putty plaster: The traditional material, sold as a matured putty mixed with sand and hair. Highly porous and flexible, very slow to set by carbonation, and the correct choice on old solid walls where breathability matters most.
• Natural hydraulic lime (NHL): Sets partly by hydration so it is faster and harder than putty lime, and available in grades. A common compromise on backing coats where a putty mix would take too long or is exposed to more water.
• Gypsum backing and finish: Bonding or hardwall with a multi-finish skim. Fast, hard, smooth and inexpensive. Correct on dry walls; a mistake on a wall that gets damp.
• Clay plaster: Breathable, reversible and increasingly available. Suits dry interiors of old buildings well, but it has no water resistance at all and cannot go anywhere that gets splashed.

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