How do I repair a chipped or missing section of plaster cornice?

Original run-in-situ or fibrous plaster cornice is one of the things that makes a period room, and it is routinely written off as beyond repair when the real problem is fifteen coats of emulsion. Repair happens in three tiers: paint removal, hand patching, and mould-and-cast for a genuinely missing section — and the first tier alone transforms most cornices.

What you'll need

• Fine surface filler or casting plaster
• Card or thin plastic for a profile template
• Small tools — dental tools, a spatula, fine brushes
• Paint softener or a steamer (optional)
• Silicone moulding rubber, for casting (optional)
• P3 mask and goggles
• Access platform

Step by step

1. Establish what the cornice is made of.Run-in-situ plaster is formed on site and is continuous with the wall and ceiling plaster. Fibrous plaster is cast in a workshop, fixed in lengths and usually has hessian scrim on the back. The repair method is similar; the fixing and the removal are not.
2. Take the paint off before assessing the damage.Layers of distemper and emulsion fill the detail and make a good cornice look ruined. Strip a small area first with a suitable softener or careful steaming to see what is actually under there. Very often nothing needs repairing at all.
3. Work gently and never with abrasives or power tools.The detail is only a few millimetres deep. Dental tools, a stiff brush and patience. A wire brush or a sander destroys in seconds what took a craftsman a day.
4. Make a profile template from a sound section.Press a piece of card or thin plastic against an undamaged length, scribe the profile, and cut it out. This template is what you will run the filler against, and its accuracy is the accuracy of the repair.
5. Fill small losses in layers.Fine surface filler or casting plaster, built up in thin passes so it does not slump or shrink, each one allowed to firm before the next. Deep single fills crack.
6. Run the template through the fill while it is soft.Drag the template along the cornice through the fresh material to form the profile. Multiple light passes give a better result than one heavy one, and the template needs to bear on sound cornice at both ends.
7. Take a mould where a length is missing.For anything longer than about 100mm of complex profile, take a silicone mould from an intact section of the same run, cast a replacement in plaster, and fix it in place. This is far more accurate than trying to model it freehand and is within reach of a careful amateur.
8. Fix a cast section properly.Bedded in plaster or a suitable adhesive, mechanically supported while it sets, and with the joints filled and run through with the template so the transition disappears.
9. Repair the background as well as the surface.Cornice that has cracked away from the ceiling or wall has usually moved or lost its fixing. Filling the crack alone leaves it loose. Screw it back through into timber where you can, then fill.
10. Know when to call a fibrous plasterer.A whole missing run, a complex enriched cornice, a ceiling rose, or anything in a listed building. They will take a mould, cast lengths and fit them, and on a good original cornice it is money well spent.

Tips

• Photograph and measure the intact profile before starting anything. If a piece falls off, you want a record.
• Keep any fragment that comes away. A broken piece is the best possible template and can often be re-fixed rather than replaced.
• Paint removal on cornice is slow and drips. Sheet everything, work in small sections, and use a softener designed for the job rather than a hot air gun, which can scorch and can be hazardous with old lead-based paints.
• Old paint in a pre-1970s property may contain lead. Do not dry-sand it; use a chemical softener and wet methods, and keep children away from the work.

Common mistakes

• Filling a cornice with decorator's caulk — It shrinks, it never takes a crisp profile, and it reads as a soft blur under paint where the rest of the cornice is sharp.
• Trying to model the profile freehand — The eye is very good at spotting an inconsistent run. A template costs nothing and is the difference between an invisible repair and a lump.
• Sanding to shape — Sanding rounds the arrises that give a cornice its shadow lines, so the repair reads as flat and dull next to the original.
• Painting over the detail again — If you have gone to the trouble of stripping it, use a thinner, well-brushed-out paint and resist the temptation to roll it.

Questions people ask