Quick answer
A composite door that's 'not square' has a frame or slab that's become a parallelogram rather than a true rectangle, usually because the frame was racked during installation or the surrounding structure has moved. Measuring the two diagonals is the quickest diagnostic — equal diagonals mean square; a difference of more than a few millimetres confirms racking.
Overview
'Not square' means the frame or door is a parallelogram instead of a rectangle. Frame racking during install is the usual cause - measures as differing diagonals.
Symptoms
- Reveal gap tapers from top to bottom.
- Door only meets frame on two corners.
- Lock catches at one corner but not the other.
Common causes
- Frame racked during install.
- Building has moved.
- Door slab warped (rare).
What 'square' actually means for a door
A square frame has four right angles and, critically, two diagonal measurements — corner to opposite corner — that are equal. When a frame is racked, it's been pushed slightly out of true so it becomes a parallelogram: still four straight sides, but the angles are no longer 90 degrees. This is surprisingly easy to do by accident during installation if the frame isn't checked and corrected before final fixing.
The practical effect is that the door only ever contacts the frame properly at two opposite corners, however carefully the hinges and lock are subsequently adjusted, because the frame it's hanging in simply isn't the right shape.
How to spot it without any special tools
Measure from the top-left corner of the frame to the bottom-right, and then from the top-right to the bottom-left, using a tape measure held as straight as possible. On a square frame these two diagonals will be equal, or within a couple of millimetres of each other on a well-installed door. A difference beyond about 3mm is a reasonably clear sign of racking, and beyond 5-6mm it's unmistakable.
Visually, a racked frame often shows as a reveal gap that tapers noticeably from wide at the top to narrow at the bottom (or vice versa) on one side, while the other side does the opposite. The lock may only catch cleanly at one corner of the door, with the opposite corner needing extra pressure to seat.
Why it happens
The most common cause by far is the frame simply not being checked for square before the final fixings were driven — a fitter working quickly can get a frame looking plumb and level without confirming the diagonals match, and once it's screwed in place at that angle, it stays that way. This is a pure installation error and entirely avoidable with a two-minute check.
The second cause is building movement after a correct installation — settlement, particularly in older Cornish properties with historic movement in cob or rubble-stone walls, can gradually rack even a well-fitted frame over years. This is much less common than an installation fault but does happen, especially in properties with a history of subsidence or seasonal ground movement.
Distinguishing installation fault from building movement
If the door has never closed properly since it was fitted, that's almost certainly an installation fault — the frame went in racked and has stayed racked. If the door used to close fine and has gradually got worse over months or years, particularly alongside other signs like new cracks in nearby plaster or render, building movement is the more likely explanation, and that changes the repair approach because you may need to address the movement, not just the door.
A rare third possibility is a warped door slab rather than a racked frame — this shows up as diagonals on the frame matching but the door itself not sitting flush, and is a manufacturing or material issue rather than an installation one.
What can and can't be corrected without a full refit
Minor racking, up to a few millimetres of diagonal difference, can sometimes be corrected by carefully re-shimming and re-fixing the frame at key points to nudge it back towards true, without removing it from the opening. This is a delicate job that needs care — over-correcting or fixing at the wrong points can make things worse rather than better.
More significant racking generally can't be reliably corrected in place. The frame has to come out, the opening re-checked, and the frame re-fitted with square confirmed by diagonal measurement before final fixing. It's more disruptive but it's the only way to guarantee a lasting fix rather than a temporary improvement.
Living with it in the meantime
A door that's not square but still locks, even with some extra effort, isn't an emergency, but it will get harder to use over time as hinges and locking mechanisms wear unevenly to compensate for the misalignment. If you're waiting for a repair slot, avoid slamming the door and try to lift slightly on the handle as you close it to reduce strain on the parts taking the uneven load.
What a remedial survey looks for on an out-of-square door
Diagnosing 'not square' properly means measuring, not guessing. We check both diagonals of the frame corner to corner — equal measurements confirm a true square, while a difference of more than a few millimetres confirms racking. We also check the reveal gap at four points per side against the wall opening, and test the door leaf itself against the frame with the door open, since sometimes it's the leaf that's warped rather than the frame that's racked.
This distinction matters for the fix: a racked frame needs re-fixing or a refit, while a warped leaf, much rarer with a genuine composite slab, may point to a manufacturing fault covered separately under the door manufacturer's own warranty rather than the installer's workmanship guarantee.
What it costs to correct an out-of-square frame
Minor racking that's caught early — where the frame is still within a few millimetres of square — can often be corrected by adjusting packers and re-fixing without removing the frame, at £120 to £220. Where racking has progressed to the point the door binds against the frame or the lock won't engage cleanly, a full refit is the only durable fix, at £450 to £850.
Left unaddressed, a racked frame accelerates seal wear on whichever side is under the most tension, which is often what brings people to us in the first place — not the squareness itself, but a draught or water ingress that turns out to be a symptom of it.
Second homes: why racking is often discovered late
An out-of-square door frame doesn't announce itself the way a leak does — the door still closes, still locks, and looks fine to anyone not checking diagonals or push-testing. On a second home visited a handful of times a year, mild racking can progress for a long time before anyone notices the lock's got stiffer or a draught's appeared.
If you manage a Cornwall property remotely, it's worth asking whoever checks the property between visits to specifically test that the door closes and locks with normal, light pressure — not just that it looks shut — since a stiffening lock is often the first practical sign of a frame going out of square.
Exposure and how often to check for movement
Doors on exposed coastal elevations, where wind loading racks a frame faster than sheltered locations, benefit from an annual push-test and diagonal check even without obvious symptoms. Sheltered or inland doors can usually go 18 to 24 months between the same checks. This is a five-minute check, not a service — worth doing yourself if you're confident, or asking any tradesperson visiting the property to glance at.
Deciding between re-fix and refit
As a working rule: diagonal difference under 6mm with no binding — re-fix with additional packers. Diagonal difference over 10mm, or the door binds or the lock needs extra force — full refit. Anything in between is a judgement call best made in person, since the condition of the existing fixing points and the opening itself often decides it more than the diagonal measurement alone.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Diagnosis with diagonal measurement | £45–£75 | Confirms racking vs. other causes |
| Minor re-shim correction (frame stays in place) | £120–£220 | For small diagonal differences |
| Full frame removal, re-square and refit | £450–£850 | For significant racking |
| Structural assessment referral | Quoted separately | Where ongoing building movement suspected |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Frame refit needed.
Prevention
- Only accept a squared install.
