Quick answer
Composite door slabs expand by 2 to 4mm across the year with temperature, with dark, south-facing doors expanding the most. This is normal, but if a door was already close to the edge of correct alignment, summer heat can push it into catching or stiff locking. The fix is a proper compression and reveal adjustment for the season, not forcing the door.
Overview
Composite door slabs expand and contract by 2-4mm across the year with temperature. Dark colours in south-facing installations expand most. If the door was set on the edge of alignment, summer heat pushes it over.
Symptoms
- Door hard to close on hot days.
- Sticky lock only in summer.
- Fine the rest of the year.
Common causes
- Normal thermal expansion.
- Dark colour + south-facing = worst case.
Why composite slabs move with temperature at all
A composite door slab is a layered construction — a GRP outer skin, an insulating foam or timber core, and often a timber sub-frame around the edges — and each of those materials responds slightly differently to heat. The combined effect is a slab that measurably expands in warm weather and contracts in cold, typically by 2 to 4mm across a full seasonal range, concentrated mostly at the edges and corners rather than uniformly across the whole slab.
This is completely normal and expected behaviour, and manufacturers design the hinge and lock tolerances with this movement in mind. The problems we're called out for arise specifically when a door was already set close to one edge of its tolerance — meaning the seasonal shift is enough to tip it from 'slightly tight' to 'won't lock' or 'catching the frame'.
Why colour and orientation matter more than people expect
Dark-coloured doors — anthracite grey, black, dark blue — absorb significantly more solar radiation than lighter colours like white or cream, and a dark door on a south or south-west facing elevation in direct Cornish summer sun can reach a surface temperature well above the surrounding air temperature. This means two identical doors on the same property, one dark and south-facing, one light and north-facing, can experience meaningfully different amounts of expansion across the same summer.
This is worth knowing before choosing a colour for a replacement door on an exposed, sun-facing Cornish elevation — it's not a reason to avoid dark colours, which remain very popular and durable, but it does mean slightly more attentive seasonal maintenance is worthwhile for a dark door in full sun than for a lighter one in shade.
The pattern that tells you it's genuinely seasonal
The clearest sign that a problem is expansion-related rather than a hinge fault is a seasonal pattern: fine in winter and spring, then developing a stiff lock or slight catching specifically during the hottest weeks of summer, and easing again as autumn cools things down. A hinge-wear problem, by contrast, tends to be present year-round and gets gradually worse over months rather than switching on and off with the weather.
Keeping a simple mental note — or even a quick phone photo of the reveal gap — at different times of year is genuinely useful if you're trying to work out which category your door falls into before booking a repair.
- Problem appears only in the hottest weeks and eases in cooler weather: likely seasonal expansion.
- Problem is present all year, slowly worsening: likely hinge wear, not expansion.
- Problem appeared suddenly after an impact or storm: likely hardware damage, not expansion.
Why forcing a summer-tight door causes lasting damage
The instinctive response to a door that's tight in summer heat is to shove it harder, but this is exactly how a marginal seasonal issue turns into genuine hardware damage. Forcing hooks that are slightly misaligned due to expansion into their keeps bends them permanently; forcing a stiff handle repeatedly strains the gearbox internals. Damage caused this way doesn't reverse when the weather cools — you're left with a hardware fault on top of the original seasonal tightness.
If a door is noticeably tighter in hot weather, the correct response is a professional check of the underlying alignment rather than repeated forcing through the summer months.
Setting a door correctly to accommodate seasonal movement
A skilled adjustment doesn't just fix the door for the day it's visited — it aims to set the reveal and compression with enough margin either side to absorb normal seasonal movement without tipping into catching or poor sealing at either extreme. This is part of why the four-point reveal measurement and small-increment approach matters: a door adjusted to be perfect in mild spring weather, but with no margin, is set up to develop problems the very first hot spell.
For doors with a known history of summer tightness, we sometimes recommend a slightly more generous reveal setting than the absolute factory minimum specifically to build in that seasonal buffer, particularly for dark, sun-facing doors on exposed Cornish elevations.
An example from a new-build near St Austell
A relatively new anthracite grey composite door on a south-facing new-build near St Austell developed a stiff lock every July and August, resolving itself by October each year — a pattern the owner had lived with for two summers before calling. Measurement showed the door was set right at the tight edge of tolerance, meaning normal summer expansion was enough to push it over. Resetting the reveal with a slightly more generous seasonal margin resolved the recurring summer problem without affecting the door's fit the rest of the year.
Why late summer call-outs cluster around the same fault
Cornwall's warmest, most humid weeks — typically late July through early September — bring a predictable rise in composite door call-outs, because the GRP skin and foam core expand slightly with heat while the weatherseal simultaneously softens and swells with humidity, and the two effects stack on top of each other. A door that was perfectly fine in June can become noticeably stiff by August without anything mechanical having changed, which is why we always ask when a stiffness first appeared, since a sudden change tracking a heatwave points strongly to expansion rather than a hardware fault.
Holiday-let changeover pressure during peak expansion season
Peak summer expansion coincides exactly with peak holiday-let occupancy in Cornwall, which is an unfortunate overlap: the door is at its stiffest just as changeover frequency and guest unfamiliarity with the door are at their highest. Guests unaware that a slightly stiff door needs a firm, steady push rather than a sharp shove can end up forcing hardware that would otherwise have simply eased back in cooler weather, turning a temporary seasonal tightness into an actual hinge or lock fault that persists into autumn.
For managing agents, a light compression adjustment done in early summer, anticipating the expansion rather than reacting to a mid-season complaint, is generally cheaper and less disruptive than an emergency call-out during a fully booked week.
How to tell genuine expansion from a hinge fault developing at the same time
The clearest test is whether the stiffness disappears entirely once a cool, dry spell arrives — genuine seasonal expansion largely resolves itself as temperatures and humidity drop, whereas a hinge or gearbox fault will persist regardless of weather. If a door remains stiff even on a cold October morning after a hot summer, that's a sign something beyond expansion has developed, often because the door was forced while swollen and a hinge or hook has been knocked slightly out of true in the process.
We'll always ask whether a door has been forced at any point during a hot spell, since that single detail often explains why a seasonal issue has left a permanent mechanical mark rather than resolving on its own once the weather cooled.
When adjustment stops being the answer for summer stiffness
A light compression adjustment can absorb a normal amount of seasonal expansion, but if a door needs its hinges backed off to the very limit of their range just to open comfortably in summer, that's usually a sign the slab itself is oversized for its frame, or the frame opening has tightened slightly from surrounding structural movement. In either case, hinge adjustment has reached the end of what it can safely do, and the next step is either easing the door's edge fractionally by a specialist or, in a small number of cases, addressing the frame opening directly rather than continuing to compensate through the hinges alone.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Seasonal reveal check and assessment | £65–£95 | Diagnoses whether a margin adjustment is needed |
| Reveal and compression reset with seasonal margin | £100–£160 | Standard fix for recurring summer tightness |
| Hook/keep realignment (if hooks have been forced) | £120–£200 | Where forcing has already caused damage |
| Gearbox replacement (from repeated forcing) | £180–£320 | Worst-case outcome of prolonged forcing |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- For a summer-margin adjustment.
Prevention
- Allow slightly more reveal in summer (or lighter colour on south-facing installs).
