Why do the joints between my coving lengths keep cracking?
Butt joints between coving lengths crack because the joint is rigid and the wall is not. Bonding the joint properly and filling it flexibly is what keeps it closed.
- Difficulty
- intermediate
- Time
- 45 min
- Read
- 3 min
Short answer
Most cracked coving joints were dry-butted, filled with rigid filler and painted. The fix is to open the joint out, bond the two ends to each other with adhesive rather than just to the wall, and finish with a flexible filler. Joints also crack when the coving was fitted onto a surface that was not sound.
Coving spans two surfaces that move relative to each other — a ceiling that flexes with the floor above and a wall that does not. That is why the joints, which are the weakest points along the run, are where the movement appears. The corners get all the attention because mitres are fiddly, but butt joints between lengths are what usually opens first.
Step by step
- Check the coving is still stuck before repairing the joint.Press either side of the crack. If the coving moves at all, the joint is a symptom rather than the problem, and GuideHQ's guide on refixing loose coving is the place to start.
- Rake the cracked joint open.Run a filling knife or a scraper along it to remove the old failed filler and paint. You want a clean open joint with two solid faces, not a hairline you are trying to fill.
- Bond the two coving ends to each other.This is the step most often skipped at first fit. Adhesive between the mating faces makes the run continuous. Coving stuck only to the wall and ceiling has a joint that is free to open.
- Fill with a flexible filler, not a rigid one.Decorator's caulk or a flexible acrylic filler tolerates the small seasonal movement that opens these joints. Rigid powder filler in a moving joint cracks again, usually within a year.
- Work it in and smooth it with a wet finger.Press the filler into the joint rather than laying it on the surface, then smooth along the profile so the moulding runs through uninterrupted.
- Fill the wall and ceiling lines at the same time.The thin gaps along the top and bottom edges of the coving crack for the same reason. Caulking them at the same time gives a continuous, uncracked appearance.
- Let it dry fully before painting.Caulk shrinks as it cures. Painting too soon means the paint film cracks along the joint as the caulk pulls back.
Tips
- On long runs, a scarf joint — both ends cut at a shallow matching angle — presents more gluing area than a square butt and opens less visibly if it does move.
- Cracks along the ceiling line specifically are often the ceiling moving, not the coving. Caulk is the right answer and may need redoing every few years.
Common mistakes
- Filling a cracked joint without checking the coving is still bonded. — If the length has come away, the joint will crack again as soon as the filler cures.
- Using rigid filler at joints and edges. — There is genuine movement at both. Rigid material has only one way to respond to movement, which is to crack.
Questions people ask
Should the joints between lengths be glued?
Yes. Adhesive on the mating ends as well as on the wall and ceiling faces is what makes a run behave as one piece.
Why do my mitres crack but not my butt joints?
Mitres concentrate movement at a corner and are often less well bonded because the surfaces are angled. GuideHQ has a separate guide on cutting coving mitres that close.