The instinctive response to a cornice crack is to fill it and paint over it. For a genuinely stable, seasonal crack that's actually the correct repair — provided the right material is used. For a crack that's still moving because of joist deflection or a loading change upstairs, filling it with a rigid material is the repair that guarantees the crack comes straight back, often as a wider or messier line than before.
A lasting repair starts by establishing which category the crack falls into, because the two situations need genuinely different treatment, not the same filler used more carefully.
For a stable, seasonal crack
This is a genuinely lasting repair for a genuinely stable crack — it isn't a lesser version of the fix, it's the correct one, because the joint is expected to keep moving slightly and a flexible material is designed to move with it.
- Rake out any old, rigid filler along the crack line so nothing brittle is left bridging the joint.
- Fill with a flexible decorator's caulk designed to accommodate ongoing minor movement, rather than a hard filler.
- Allow the caulk to cure fully before painting, following the product's stated timescale rather than rushing to redecorate the same day.
- On period cornice profiles, take care that the caulk line follows the moulding cleanly rather than smearing across the detail.
For a crack caused by joist deflection or new loading
Here the repair has to start above the ceiling, not at the crack itself. That means investigating what's actually loading the joists — checking whether a loft floor has been boarded and is being used for storage beyond what the joists were designed to carry, or whether the joists themselves show signs of genuine sag.
Where the loading is the issue, the practical fix is reducing or properly supporting that load — for example, not storing heavy items directly on ceiling joists that were only ever intended to support a plasterboard ceiling. Where the deflection is more significant or its cause isn't obvious, a structural engineer needs to assess the joists before any cosmetic repair is carried out, because filling the crack without addressing the load simply hides a problem that's still active.
Once the loading issue is resolved or the structural position is confirmed as sound, the cornice can be reinstated. On period properties with an ornate profile, this sometimes means casting a matching section rather than trying to patch a damaged run invisibly — a patched joint in a decorative moulding rarely disappears the way a flat plaster repair does.
Why a hard filler over a live crack always fails
Rigid fillers have no give in them. Applied over a joint that's still moving — whether from seasonal cycling or ongoing structural deflection — the filler simply cracks again along the same line as soon as the joint moves through its usual range, often within one season. It's the single most common reason a cornice crack seems to 'come back' after being repaired: it was never actually repaired with the right material for a joint that moves.
How you know the repair has worked
For a flexibly filled, stable crack, success looks like the repair simply disappearing under paint and not reopening through the following heating season. For a repair that followed structural investigation, success means the crack doesn't return once the loading issue has been addressed — if it does reopen despite that, it's a sign the structural cause needs a further look rather than another round of filling.
Common questions
Do I need scaffolding or access equipment to fix a cornice crack?+
For most domestic ceiling heights, a stepladder and standard access is enough. Taller stairwells or double-height ceilings may need proper access equipment, which is worth factoring into the job.
Why did my last cornice repair crack again within months?+
Almost always because a rigid filler was used over a joint that's still moving, or because the underlying cause — such as joist loading — was never addressed before the crack was filled.
Can cornice mouldings be matched if a section needs replacing?+
Yes, on most period profiles a matching run can be cast or sourced to reinstate a damaged section, so the repair doesn't leave a visibly different profile along the run.
Get a written quote for the repair
We establish whether the crack is stable or progressive first, then quote for the repair that actually matches what's going on.