Wood moves. It takes on moisture in humid months and gives it back in dry ones, and it moves more across its width than along its length. Good trim carpentry does not try to stop that — it puts the joints where the movement will not show.
The gap above the door frame
A crack along the top of the casing, or at the mitre where the head casing meets the leg, is almost always seasonal movement plus a joint that was fastened but not glued. The mitre opens a hair, filler cracks out of it, and it reappears every year at the same time.
The fix is to re-cut or re-close the joint, glue it, fasten it from both faces so it cannot open, then fill and finish. Filling alone puts the same crack back next season.
The gap along the top of the skirting
This one is usually not the wood at all — it is the wall. Walls are rarely dead flat, and skirting is straight. Push a straight board against a slightly wavy wall and you get a gap that varies along its length.
That is what caulk is for, and it is the correct fix rather than a bodge: a bead along the top edge, tooled in, painted over. It flexes with seasonal movement instead of cracking. What caulk is not for is a gap you could lose a coin in — that one needs the trim scribed to the wall or the wall made flat.
Mitres that open on outside corners
Outside corners take knocks and take the most movement. A mitre held with nothing but a couple of pins will open. Glue plus fasteners from both directions holds it closed, and on a corner that gets regular traffic it is worth the extra minute.
When it is not the trim
Most trim gaps are cosmetic. A few are not, and they are worth telling apart:
- A door that has started catching or dragging at one corner, alongside new gaps in the casing.
- Gaps that keep widening rather than opening and closing with the seasons.
- Cracks in the wall above a door or window running diagonally, appearing at the same time as the trim moves.
- Skirting pulling away at the same spot as a floor that has started to slope or feel soft.
Why the same gaps keep coming back
Because filler is not a joint. Filler in an open mitre has nothing to hold onto and no ability to flex, so it cracks out the first time the wood moves. Caulk in a joint that should have been glued does the same. Closing a gap permanently means dealing with why it opened — which is usually a joint that was never mechanically closed in the first place.
If gaps are appearing in more than one room at once, or a door has started sticking at the same time, mention that when you call — it changes what is worth looking at first.