Why have my mitre joints opened up?
A mitre that was tight when you made it and has a gap a year later is usually wood movement, not bad cutting — and the two look completely different.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 3 min
Short answer
Look at where the gap is. A gap that opens at the inside of the corner and closes at the outside was cut over 45 degrees. A gap that has appeared evenly along the whole joint after months is wood movement or glue failure at end grain. A gap only in one season is humidity and will close again. Reinforce and re-glue rather than filling.
Two completely different problems present as the same symptom. One is geometry — the angle was wrong, or the pieces were not the same length — and is visible the day you make it. The other is time: the mitre is largely end grain, glue holds end grain badly, and the wood is changing dimension with the seasons while the joint is trying not to.
Step by step
- Establish whether the gap was there from the start.If the corner was tight when clamped and opened later, it is not a cutting error. If it never closed properly, the angle or the length is wrong.
- Check where along the joint the gap sits.Open at the inside corner, tight at the outside: the cut was over 45 degrees. Open at the outside, tight inside: under 45. Open along the whole line: the pieces separated.
- Rule out the frame being out of square.Measure the diagonals. Four perfect 45 degree cuts assembled into a frame that is not square will show gaps at two opposite corners.
- For a joint that has separated over time, accept the real cause.End grain glue joints fail. Add heat, dry indoor air in winter and damp in summer, and a plain glued mitre works itself apart. The fix is reinforcement, not more glue.
- Take the corner apart if you can.Gentle heat and moisture will release PVA on a light frame. Clean the old glue off both faces down to bare wood — new glue will not stick to old glue.
- Re-glue with sizing and reinforce.Size the end grain with thinned glue first, then glue and clamp square, and add corner keys or a spline once it is set. That converts it to a joint that will not do this again.
- If it cannot be dismantled, key it in place.Work glue into the open joint with a thin blade, clamp it closed and square, and once cured, saw key slots across the corner and glue keys in.
- Only fill as a last resort, and match the movement.A filled mitre reopens with the next seasonal cycle unless the joint itself has been fixed. Flexible filler on a painted frame will hold; hard filler on a clear-finished one will crack and show.
Tips
- A mitre in a wide board is the worst case, because the two pieces are trying to change width in directions 90 degrees apart at the corner. Narrow mitres are far more stable.
- Fitted mitres on skirting and architrave open for the same reason and are usually caulked rather than repaired, because they are painted.
- Kitchen and joinery mitres cut a whisker under 45 close tight on the show face and hide the discrepancy on the inside.
If it doesn't work
Gap at the inside of the corner only
Cause: Cut slightly over 45 degrees — Fix: Trim a fraction off the mitre with a shooting board or a fine saw and re-fit. On an assembled frame, close the gap with a corner clamp and pin it.
All four corners open slightly in winter, close in summer
Cause: Seasonal moisture movement in a solid timber frame — Fix: Normal. Reinforce the joints so they do not fail, and expect the hairline to come and go. A more stable timber or a quarter-sawn stock reduces it.
The corner has come apart completely
Cause: End-grain glue failure — Fix: Dismantle, clean off old glue, size the end grain, re-glue and add keys or a spline.
One corner gaps and the opposite one is tight
Cause: The frame is out of square, or one piece is a different length — Fix: Check diagonals. Re-cut the offending piece rather than trying to force the frame square.
Questions people ask
Will filler fix an open mitre permanently?
Not on its own. If the joint is still moving, the filler cracks out at the next humidity swing. Fix or reinforce the joint first, then fill the remaining hairline if it needs it.
Why are corners never exactly 90 degrees?
Walls, boxes and frames are all slightly out. That is why good practice is to copy the actual angle with a bevel and halve it, rather than trusting the 45 detent on a saw.