How do I cut coving mitres that actually close?

Coving bridges the wall and the ceiling at an angle, so its mitre is a compound cut rather than a simple 45. Holding it in the position it will occupy, but inverted, makes the mitre box do that geometry for you.

What you'll need

• A mitre box or a mitre saw
• A fine-toothed saw
• Coving adhesive and a filling knife
• A pencil and a tape measure
• A sharp knife for scribing
• Fine filler

Step by step

1. Establish the coving's sprung angle.Most domestic coving sits at 45 degrees to both surfaces. Some profiles differ, and the packaging states it. The mitre box method assumes 45.
2. Hold it upside down and back to front.The edge that will sit against the ceiling goes on the base of the box; the edge that will sit against the wall goes against the fence. This is the whole technique.
3. Keep it pressed into the corner of the box.Any movement during the cut changes the angle. Hold it firmly or clamp it — the cut is only as good as the grip.
4. Understand internal versus external.For an internal corner the long point of the mitre is on the wall face. For an external corner the long point is on the ceiling face. They are mirror images.
5. Cut both halves of a corner from the same setting.Cut one piece in the left slot and its partner in the right slot. Cutting a pair this way guarantees they are complementary.
6. Measure to the internal corner, not the mitre.The length you measure on the wall is the length of the coving at the wall edge. Mark that point and cut the mitre from it.
7. Dry fit before any adhesive.Hold both pieces up together and look at the joint. Adjusting a mitre is trivial before the adhesive goes on and impossible afterwards.
8. Deal with corners that are not square.Almost none are. Either split the difference by trimming both mitres slightly, or cut a paper template of the actual angle and halve it.
9. Use adhesive as gap filler at the corner.Coving adhesive is a plaster-based filler as well as a glue. A slightly open mitre is filled with adhesive and smoothed, which is normal practice rather than a bodge.
10. Fix external corners with a slight overlap.Cut them a fraction long so the joint is under compression, then trim the point back and fill.
11. Support it while the adhesive grabs.Panel pins into the wall below the coving, or a couple of props, removed once it has set.
12. Fill and sand the joints last.Once everything is fixed, run adhesive or fine filler into the mitres, wipe with a damp brush, and sand lightly when dry.

Tips

• Cut a spare set of mitres in scrap coving to check your orientation before touching a full length. Getting it wrong costs a length; the practice costs nothing.
• Write 'ceiling edge' on a piece of masking tape and stick it to the base of the mitre box. It eliminates the one error that ruins every cut.
• A coving mitre jig or a purpose-made corner block avoids the geometry entirely and is worth considering for a whole house.

Common mistakes

• Cutting coving flat in the mitre box — Laid flat, the box produces a simple 45 degree mitre. Coving sits at an angle to both surfaces, so a flat 45 leaves a joint that gapes at the front and crosses at the back, and no amount of filler makes it look right.
• Assuming the corners are 90 degrees — Almost no domestic corner is exactly square. Cuts made on the assumption of 90 leave a wedge gap on one face. Dry fitting and trimming to the actual angle is what makes a mitre close.

If it doesn't work

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