Repairing Decorative Plasterwork and Cornices in Old Homes

Know Your Plaster Before You Touch It

Before you reach for a filler, work out what you are repairing. In most Georgian, Victorian and Edwardian homes, decorative plasterwork is lime-based: lime putty mixed with sharp sand and often animal hair, applied over timber lath, brick or stone. Cornices, ceiling roses and enriched mouldings were usually run in a gauged mix — lime putty accelerated with plaster of Paris so it would set quickly enough to be worked.

That matters because lime is soft, breathable and slightly flexible. Modern gypsum fillers, polymer repair compounds and ready-mixed lightweight fillers set hard and dense. Used on old plaster they can shrink away from the edges, crack along the joint, and trap moisture that would otherwise evaporate. Test a hidden area with a fingernail: if it crumbles easily, looks off-white and you can see hair fibres, it is lime. Match it.

Reading the Damage — Cracks, Blown Plaster and Missing Sections

Damage rarely tells you the whole story on its own. Learn to interpret it:

• Hairline cracks along a cornice arris or ceiling rose are usually seasonal movement. They will return if you simply fill them, so rake out and fill with a lime mix that moves with the background.
• Diagonal or stepped cracks, especially with brown staining, usually point to water. Check gutters, valleys, flashing and roof tiles above before you do any cosmetic work.
• Blown plaster gives a hollow, papery tap. The plaster has delaminated from the lath or masonry behind it. Press gently — if the whole area flexes, it needs securing or replacing, not filling.
• Missing sections are often the legacy of a previous plumber, electrician or a clumsy removal of a picture rail. Photograph them before you start.

Securing Loose Plaster and Filling Cracks

Cut back any crumbly material to a firm edge, then dampen the exposed plaster with a fine water spray. Dry lime will suck the moisture out of your repair and leave it weak.

For a small blown patch, drill a few 3mm holes through the sound surface into the void and inject a thin lime grout — lime putty thinned with water — using a syringe or mastic gun. Work from the centre outwards, then brace the area with battens and props lined with polythene, leaving it two or three days. Larger blown areas are better cut out and rebuilt in two coats, with a coarse lime-and-sand backing and a finer finishing coat.

For cracks, open them slightly into a shallow V with a craft knife or small tool, dust out, dampen, and fill in thin layers rather than one thick bead. Deep cracks over lath benefit from a strip of scrim or fine fibreglass mesh pressed into the first coat. Let each layer firm up before adding the next, and finish flush with a damp sponge rather than sandpaper.

Recreating Mouldings: Casting from What Survives

On any cornice run there is almost always an undamaged section elsewhere — a corner, a short return, or the same profile in another room. That is your master pattern. Clean it gently with a soft brush and water, then take a mould using a condensation-cure silicone rubber, supported by a plaster or fibreglass jacket built in sections so it can be removed without distortion.

Cast replacements in a gauged mix — roughly two parts lime putty to one part plaster of Paris by volume, with a little fine silver sand for body. Pour in thin layers, working it into the detail with a small brush to avoid air pockets. Once set and thoroughly dry, bed the casting into place with a lime-based adhesive, align it carefully, and fill the joints with the same mix, tooling them while still green.

Running New Mouldings by Hand

For straight runs, running in situ is the traditional method and still the neatest. Cut a profile in zinc or steel to the exact reverse of the moulding, fix it to a wooden running mould or box with a shaped horn to hold the mix, and set a straight guide rule along the wall.

Dampen the background, then apply a coarse core coat of lime and sand, followed by a finer floating coat. When that has stiffened, run the mould through a gauged cream mix in two or three steady passes, letting each stiffen slightly. Keep the rule clean, keep your pace even, and resist the urge to go back over a pass that is already setting.

Tools, Materials and Finishing

• Tools: a plasterer's small tool, leaf and square, joint knife, gauging trowel, running mould, fine spray bottle, soft brushes, and a good light angled across the work to reveal hollows.
• Materials: lime putty, hydrated lime, plaster of Paris, casting plaster, fine sand, horsehair for backing coats, scrim or mesh, and clean buckets.
• Finishing: lime needs time to carbonate — weeks, not days. Decorate with a breathable finish such as distemper, clay paint or a breathable emulsion. Avoid vinyl silk and other sealed coatings, which trap moisture and can blow the plaster beneath.

Work in small, manageable areas, keep samples of the original mix and profile, and photograph everything before you begin. Repaired patiently and in keeping with the original, decorative plasterwork will outlast most of what is sold as a modern alternative.