Lime, Clay and Gypsum: Matching a Finish to Older Backgrounds

Gypsum plaster sets by chemical reaction, comparatively fast, and ends up hard and far less open to water vapour. Lime plaster sets slowly, partly by drying and, for non-hydraulic limes, by reacting with air over a long period, and ends up softer, more flexible and considerably more open. Clay plaster does not set chemically at all: it simply dries, and the clay particles hold the mix together by their own cohesion, which means it stays re-wettable.

Those three mechanisms are the whole basis of the decision. A slow, flexible set accommodates small movements in a background instead of resisting them; a hard set resists them and shows them; and a finish that never chemically sets handles humidity in the air well and liquid water badly.

Solid masonry was built on the assumption that moisture would enter the wall and leave again through the finish. Timber lath is an organic, moving background whose plaster is held by nibs squeezed through the gaps, not by adhesion to the timber. A reed mat base is a plant material bound with wire that keys lime and clay plasters. An earth infill panel within a timber frame is the wall itself rather than a finish over one.

Then there is what previous work added: existing lime plaster in varying condition, cement renders, dense patches, and repointing in cement. Old backgrounds are rarely uniform, so one wall may well need more than one treatment, and every edge between two of them is a line where materials of different hardness meet.

Compatibility is the whole subject. Lime on a lime background behaves as intended. A hard modern finish over soft old fabric concentrates stress and moisture at the interface, and that is the failure the lime entry exists to describe. A lime repair patched with gypsum, or the reverse, creates a permanent weak line.

The same asymmetry appears on lath. A background built around a soft and flexible coat does not accept a hard modern one in the same way, and replacing lath with a rigid board changes how the whole surface moves, making the junction with retained original work the new weak point.

With lath, the plaster is held by nibs hidden behind the strips, so the visible face tells you very little about the real condition. Old nails often fail before the timber does, and a sound-looking lath can still be loose on the joist. Any history of water reaching the void changes what can sensibly be done to the surface.

That is why this decision starts with an inspection rather than with a product. Condition of a buried background can only be established by someone qualified looking at it, and a sagging or springy ceiling is a matter for immediate assessment rather than for a finish selection.

Coatings applied over lime affect how it behaves, and sealing a lime wall with a dense paint undoes the main reason for choosing it. Clay plaster finished with a conventional film-forming paint loses both the texture and the buffering behaviour it was chosen for.

So the finish coat is chosen with the plaster rather than after it. A permeable decoration over a sealed render or a tanked backing does not restore anything either, which is the same point made from the other direction.

Lime is worked and conditioned over a much longer period than gypsum, and the timetable is set by the material and the weather rather than by the construction programme. Programme pressure is the usual cause of problems, because lime does not respond to being rushed. Lime materials are also caustic when wet, and site protection in an occupied house is a real consideration.

An insulating coat brings its own surprises: it is applied considerably thicker, built up in successive passes, and completed with a separate compatible finishing coat, which changes reveals, sills, skirtings and door linings before anybody has thought about them.

Work to historic timber-framed buildings and to earth infill panels belongs with a professional experienced in traditional construction. Historic panels are normally repaired in like materials rather than replaced, because what survives carries evidence about the building, and how much intervention is appropriate is a conservation judgement.

Consents for work to listed buildings and to buildings in conservation areas take time to establish and are the owner's responsibility. Settling them early avoids a programme built on assumptions.