Cracked plaster cornice: repair or replace?

Cracks along a cornice are one of the most common things we are asked about in Glasgow's tenements and period houses. They look alarming, but in most cases the original plasterwork can be saved — and even where a length is missing, it can usually be matched and re-run rather than torn out. The important thing is to work out why the damage has happened before deciding how to fix it.

First, find out why it cracked

Plaster cornice rarely fails on its own. A crack is usually a symptom of something behind it, and repairing the surface without addressing the cause simply means it returns. The usual reasons are:

• Normal seasonal movement — fine hairline cracks where the cornice meets the ceiling or wall are extremely common and often cosmetic.
• Old, failed fixings — Victorian and Edwardian cornices were often fixed with nails or screws into timber grounds that have since loosened, letting a run sag.
• Damp or a past water leak — water softens plaster and rusts fixings. The plaster may be sound now but weakened.
• Structural movement — settlement, a removed wall, or work in the flat above can open up wider cracks.

Where damp or movement is involved, that comes first. Plaster repair does not cure a leak or stop a wall moving, so we would want the cause dealt with — by you, a surveyor or a builder — before the decorative work is reinstated.

When a cornice can be repaired in place

The majority of cornices we see can be repaired without replacing whole lengths. A cornice is usually a good candidate for in-situ repair when:

• the cracks are hairline or localised rather than running the length of the room;
• the plaster is still firmly attached and does not sound hollow when tapped;
• the detail — the enrichments, beads and runs — is intact and only the joints or flat sections have opened up.

In those cases the work is careful rather than dramatic: raking out and re-filling cracks with a compatible material, re-securing any loose runs, and making good so the repair disappears under paint. Original plaster is worth keeping — it is denser and crisper than most modern replacements, and once repaired it will usually outlast a wholesale replacement.

When a section needs replacing

Some damage is beyond a surface repair. A new section is the right answer when:

• a length of cornice is missing altogether, or has been lost to a previous alteration;
• the plaster has been softened by water to the point where it crumbles;
• a run has detached from the ceiling and cannot safely be re-fixed.

Replacing does not mean settling for a generic off-the-shelf profile that does not match the rest of the room. Where an original run survives, we take a mould from a sound section and cast new lengths to the same profile, so the new work sits naturally alongside the old. Matching the profile is the part that makes or breaks a replacement — a cornice that is the right size but the wrong section will always look wrong.

How we decide — and how to get an answer quickly

Most of the time we can give you a clear steer from photographs before ever visiting. The most useful things to send are:

• a photo of the whole run so we can see the room and the extent of the damage;
• a close-up of the worst area;
• a photo of an intact section that shows the original profile;
• a rough idea of the length affected.

From that we can usually tell you whether it is a repair or a replacement, and what is involved. For listed buildings and conservation work we can also provide a photographic record of the repair for your own or your architect's files.

If you have a cracked or damaged cornice in a Glasgow period property, send us a few photographs and we will advise on the right approach — repair wherever we can, and a properly matched replacement only where it is genuinely needed.