Heritage Lime Plaster Over Wood Lath: Assessing a Veneer Plaster Upgrade
You are contracted to restore plaster in a 1908 two-storey house. The walls are wood lath with three-coat lime plaster. The parlour ceiling is 4.3 m x 5.5 m and shows a fan of cracks radiating from one corner, plus a 1.5 square metre area that sounds hollow when tapped. The homeowner wants a modern smooth finish and has already bought 20 kg bags of gypsum veneer plaster and a pail of acrylic bonding agent. The baseboard has been removed and the original wood grounds are exposed: one sits about 20 mm proud of the plaster plane, and another is soft and rotted at its lower end. Two weeks ago a plumber replaced a radiator in the room above and the wall below was wetted. The house is heated to 20 degrees C.
1. Assess the delamination and the proposed gypsum veneer plaster over the existing lime. What are the risks?
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The hollow 1.5 square metre area is detached from its keys; the plaster is no longer mechanically held to the lath, and tapping tells you the extent. The radiating crack pattern from a corner is not ordinary shrinkage; it points to movement, often settlement, a concentrated load, or a leak. The recent radiator work and the wetting of the wall are a strong hint that moisture has been introduced into the assembly.
Applying gypsum veneer over this is the wrong repair. Veneer plaster is dense, hard, and much less vapour-permeable than the lime base, and it needs a sound, rigid, well-keyed substrate. Over a detached lime coat it will crack along the same lines and then delaminate under its own stiffness once the base moves. The acrylic bonding agent does not fix a hollow base; on friable lime it can form a film that traps moisture and fails at the interface. Gypsum and lime also behave differently with moisture and thermal movement, so a hard skin over a soft, moving base is a crack waiting to happen.
Before any repair, the wet area must be dried and the moisture source confirmed, and the corner crack investigated for structural or roof causes. A moisture meter reading and a look above the ceiling are part of the scope.
2. What repair sequence would you specify for the hollow area and the cracks, and what materials would you use?
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Sound the whole ceiling with a hammer or knuckles and mark the boundaries. Cut the unsound plaster back to sound material with square, slightly undercut edges so the patch can be feathered; do not leave a thin, crumbling rim.
Where the lath is sound but the keys have failed, re-key with high-calcium lime putty and hair, or use plaster washers screwed through the plaster into the lath to re-secure the field before patching. Where lath is broken or missing, cut it out and fit metal lath furred off the framing, then apply a lime scratch and brown coat. Fill in thin coats with lime putty, or a compatible lime-gypsum mix, allowing each coat to set and dry; do not build out a deep patch in one pass.
For the cracks, widen them slightly, dampen, and fill with lime putty, embedding a fibreglass mesh in the repair zone to bridge movement. Keep the repair material matched to the original in strength and permeability, so the patch moves with the field rather than fighting it. The homeowner's gypsum veneer can be used only after the base is fully sound, and only with the manufacturer's specified base coat system, not directly over friable lime. Final decoration should wait until the plaster has cured and dried.
3. How do you deal with the two grounds before the baseboard goes back on?
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The grounds are the fixing surface for the baseboard, so they have to be straight, sound, and set to the finished plaster plane before any patching is closed in.
The proud ground is reset or replaced. If it is sound, plane or cut it back so its face sits flush with the intended plaster finish, then re-fix it plumb and level. The rotted ground is removed entirely and replaced with treated lumber of the same dimensions, fixed to the framing and set to the finished plane, with the bottom end kept clear of the floor so it cannot wick moisture.
Set the new ground with a straightedge and level against the adjoining grounds so the baseboard has a continuous nailing surface and lands flat against the plaster without gaps or a wavy top edge. Check that the plaster patch around the ground is filled tight to the ground face, not built over it, or the baseboard will stand off the wall.
Prime the new ground on all faces, and photograph or record the ground positions before the baseboard goes back, since the baseboard is the last item and the grounds will be hidden.