How to Restore Original Cornicing in a Period Home

Read the room before you reach for filler

Original cornicing is often the last surviving trace of a period room's decorative scheme, and it deserves a proper assessment before any repair. Stand back and study the run with a torch held at a raking angle — side-light reveals hairline cracks, bulges and old repairs that flat daylight hides. Georgian and Victorian cornices were built up in layers: a scratch coat on wall and ceiling, the running mould drawn along a timber rule while the plaster was still green, then enrichment — dentils, modillions, acanthus leaves — applied by hand or cast in a workshop and bedded in.

Tap gently along the underside with your knuckles. A dull thud means the plaster is still keyed to the lath or masonry. A hollow ring means it has come away and is being held only by paint, and it will eventually drop. Check the ceiling above for sagging or a dipping line, and look for rust streaks beneath the cornice, which usually point to a corroding nail or wall tie. Before you start, treat anything pre-1960 as potentially lead-painted: damp it down, wear a mask and avoid dry sanding.

Sort out the cause, not just the symptom

Cracks along the join between cornice and ceiling rarely mean the cornice itself has failed; they usually signal movement in the ceiling above. Diagnose properly before you fill anything, because patching a symptom simply buys a few years.

• Fine, even crazing across lime plaster is normal age and is almost always cosmetic.
• A long horizontal crack beneath the run often follows a rusting wall tie or a failed lintel.
• Crumbling, sandy patches suggest damp, a blocked gutter or a failed damp-proof course.
• A corner that has dropped while the rest of the run is level points to a failed fixing rather than failed plaster.
• Staining and tidemarks mean water is getting in — solve that first or the repair will not last.

Repairing in situ with traditional methods

Where the original plaster is sound, repair it rather than replace it. Brush out dust and wet the area thoroughly with clean water — both lime and gypsum need a damp surface to bond, and dry plaster will suck the moisture straight out of your mix. Build up in thin layers rather than one generous dollop, and use a lime-based filler or casting plaster mixed to a stiff, buttery paste. A small pointing trowel followed by a damp sponge will blend edges far better than sandpaper.

For detached runs, consolidate rather than dismantle. Drill small holes along the top of the run, then inject a lime grout or a proprietary adhesive, working in short lengths and propping with padded battens until set. Where a section is missing but the profile survives nearby, take a squeeze mould of the original, cast a replacement in casting plaster reinforced with scrim, and bed it in with plaster of Paris and more scrim, feathering the edges so the join disappears beneath paint. Never run gypsum-rich filler hard against bare lime — it sets harder than its surroundings and will crack along the junction.

Sourcing a matching profile

Matching a cornice is a question of profile, projection and proportion, not just pattern name. Measure the drop from ceiling to the underside of the run, and the projection out from the wall, to the nearest millimetre. Photograph the section with a steel rule lying alongside it.

• Reclamation yards sometimes hold lengths of genuine Victorian and Georgian cornice, though rarely the exact profile you need.
• Specialist plaster workshops hold pattern books of thousands of historic sections and can often identify a match from a photograph.
• Having a new mould taken from your own surviving cornice is the most accurate route, and often the cheapest if you need several metres.
• Standard modern profiles are sold by the length, but check the scale carefully — a 150 mm run looks very different from a 225 mm one in a room of modest height.

Running new cornicing the traditional way

For a complete replacement, running in situ gives the crispest, most seamless result. A zinc or steel profile is cut to the exact reverse of the cornice section, fixed to a running mould, and drawn along timber rules fixed to wall and ceiling. Three coats are usual: a scratch coat rich in lime and hair, a finer floating coat, then the running coat itself, each allowed to stiffen but not dry before the next. Keep the mould and the rule damp, work in manageable lengths, and mix small batches so nothing goes off in the bucket.

Walls are rarely true, so expect to build out hollows in layers before the final run. Where a wall leans, the running rule is packed out to suit, which is exactly how the original plasterers coped with the same problem two hundred years ago.

Finishing, decoration and long-term care

Let new plaster dry slowly and naturally — gentle ventilation, not a hot room, or it will shrink and crack. Paint choice matters more here than almost anywhere else, because lime plaster needs to breathe. Traditional distemper and limewash are the most sympathetic finishes; modern clay paints and breathable emulsions are a sensible compromise. Avoid vinyl silk and oil-based gloss over lime, which trap moisture and blister.

Once decorated, dust cornices with a soft brush rather than washing them, and keep an eye on gutters and roof flashings above the room. With careful assessment, sympathetic repairs and a breathable finish, original cornicing will carry on doing exactly what it was made for: softening the join between wall and ceiling and giving a room its quiet, generous sense of proportion.