Where this starts
Joinery moves because timber moves, and it moves unevenly. Across the width of a board, shrinkage on drying is substantial; along its length it is almost nothing. Every joint in a run of trim is a decision about which of those two directions meets which.
A MITRE puts two cut faces together at forty-five degrees, so each piece's width is partly across the joint. As both dry, they shrink in different directions relative to the joint line, and the two faces slide apart — the mitre opens at the outside of the corner, which is exactly where it is most visible.
A SCRIBE does not. One piece runs straight into the corner and the second is cut to the first one's moulded profile, so it sits against a face rather than against another cut end. When the timber moves, the scribed piece moves along the face it is pressed against and the joint stays closed. That is why internal corners on skirting and coving are scribed by a joiner and mitred by somebody with a mitre saw and no reason to know better.
TIMING IS THE OTHER HALF, and it is the one nobody controls. Trim fixed in a building that is still drying out — new plaster, a new screed, a wet winter with no heating on yet — is fixed at a moisture content it will not keep. Everything then shrinks together, and every joint in the house opens in the first heating season. That is normal, it stops, and it is the reason second-fix joinery has a specified moisture content and a sequence it is supposed to follow.
The practical distinction for what to do: joints that opened once and stopped can be filled and will stay filled. Joints that open every winter and close every summer will reject filler every year, and the answer is a flexible sealant or a re-cut, not more filler.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
The building drying out after the trim was fixed
Most commonJoints across the whole house opening in the first heating season after installation, then stabilising, in new or recently plastered work.
How to check: When it was fitted and whether the heating had run. Everything opening at once, once, is the building drying.
Mitred internal corners instead of scribed
Most commonInternal angles opening while external ones and the long joints stay closed, often with a clean forty-five degree cut visible in the gap.
How to check: Look into the gap at an internal corner. Two cut faces is a mitre; a shaped profile against a flat face is a scribe.
Seasonal movement that never settles
CommonGaps that open every winter and close every summer, on solid timber rather than MDF, with filler cracking out annually.
How to check: Whether it closes again in humid months. Movement that reverses is the material breathing and will not accept rigid filler.
The joint was only held by paint
CommonJoints opening with no timber movement elsewhere, on work where the mitres were not glued or pinned through, often with paint bridging the gap and torn.
How to check: Whether the joint was glued and pinned. A mitre held by decoration alone opens on the first movement of any kind.
The substrate behind has moved
Less commonOne corner or one run affected while the rest of the house is fine, sometimes with a crack in the plaster running from the same corner, or a door nearby binding.
How to check: Whether anything else nearby has moved. A joint opening alone, with cracking above it, is reporting the wall rather than the timber.
Wet timber delivered and fixed
Less commonSevere, rapid opening within weeks rather than a season, sometimes with cupping or twisting of the trim itself and split ends at fixings.
How to check: How fast it happened. Weeks rather than a season means the material arrived far wetter than the building.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It is the difference between a joint that was always going to open and one that has been moved by something, and it decides whether re-cutting is worth doing.
- 2
Once and stopped takes filler and stays filled. Seasonal movement rejects rigid filler every year and needs a flexible joint or a different detail.
- 3
Everywhere at once is the building drying after installation, which is normal and self-limiting. One place alone is that place, and may be the wall rather than the timber.
- 4
MDF barely moves across its width and holds a mitre well; solid timber moves a great deal, which is why the same detail behaves differently in the two materials.
- 5
A joint opening with a crack above it is not a joinery problem, and filling the joint leaves the thing that moved unexamined.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where a joint has opened together with cracking in the plaster or a door that has started to bind, since the trim is reporting movement in the structure rather than in itself.
- Where opening is severe and rapid on newly fitted work, which points at the material's moisture content on delivery and is a supply question rather than a workmanship one.
- Where the affected trim is original to a period building and has a profile that would have to be matched, because a re-cut is then joinery rather than decorating.
- Where the same joints have been filled repeatedly and keep failing, which means the movement has never been diagnosed and more filler will not change that.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.