Quick answer: Heat makes the door expand. Composite doors grow by a millimetre or two on a hot day, especially south or west facing ones in direct sun, and darker colours absorb even more. That tiny bit of extra size is enough to make the door drag on the frame and stop the mechanism lifting into the locked position.
Every summer I get the same run of calls. The sun comes out, the temperature climbs, and suddenly people can’t lock their front door properly. Composite doors that behaved perfectly all winter start sticking, dragging on the frame, or refusing to lift into the locked position. It catches folk by surprise, and I understand why. Nothing has changed about the door, so why has it stopped working?
The short answer is heat. In nearly 20 years working across Southampton and the Waterside, I’ve seen this pattern repeat every single warm spell. It’s one of the most common jobs I get through the summer months, and the good news is that most of the time it’s a simple fix. Let me talk you through what’s actually going on and what you can do about it.
Why does a composite door stick when it gets hot?
Composite doors are made from a mix of materials, usually a solid core wrapped in a tough outer skin, often with a uPVC or timber frame around them. Like most materials, they expand when they warm up and contract when they cool down. On a hot day, that expansion is enough to change the fit between the door and the frame by a millimetre or two. That doesn’t sound like much, but a door is a tightly engineered thing, and even a small change can be the difference between locking smoothly and not locking at all.
The other factor is direct sunlight. A door that faces south or west catches the full afternoon sun, and darker coloured doors soak up even more heat. I see far more sticking problems on anthracite grey and dark blue doors than I do on white ones, simply because they get hotter. The skin of the door heats up, expands, and starts to bind against the frame. Once the sun goes down and things cool off, the door often works fine again, which is why people sometimes think they imagined the problem.
What actually goes wrong with the lock
Most composite doors use a multipoint locking mechanism. When you lift the handle, a series of hooks and bolts shoot out along the edge of the door and slot into keeps in the frame. For all of that to line up, the door has to sit in exactly the right position. When heat expansion pushes the door slightly out of alignment, those hooks and bolts no longer meet their keeps cleanly. You feel it as resistance when you lift the handle, or the handle won’t go all the way up at all.
The temptation here is to force it. Please don’t. If you heave on a handle that won’t lift, you can bend or snap the gearbox inside the mechanism, and then a simple summer adjustment turns into a full lock replacement. If your handle is stiff, stop, and either wait for the door to cool or give me a ring. If the mechanism has already failed you can read more about what’s involved on my page covering multipoint lock repair in Southampton, but often it hasn’t come to that yet.
Simple adjustments that usually sort it
Here’s the reassuring part. In the vast majority of cases, a sticking composite door doesn’t need any new parts. It needs adjusting. Composite doors are designed to be tweaked, and there are a few things I check when I come out.
The hinges are the first port of call. Most modern composite door hinges are adjustable in two or three directions, which means I can shift the door within the frame by a small amount to bring everything back into line. A gentle move away from the sticking edge is often all it takes. I also check and lubricate the multipoint mechanism, because a bit of the right grease helps the bolts glide into their keeps even when things are running tight. The keeps themselves can sometimes be adjusted too, giving the hooks a fraction more room to drop in.
There are a couple of things you can try yourself before you call anyone. If the door is only sticking in the hottest part of the afternoon, try locking it in the morning or evening when it’s cooler, and see if that confirms heat is the cause. A light spray of silicone lubricant on the moving parts of the lock can help too. What I’d steer you away from is any DIY sanding or planing of the door. That solves the summer problem but leaves you with a gappy, draughty door come winter when everything shrinks back.
When to call me out
If your door is sticking every day, if the handle is getting harder to lift, or if you’re worried about being locked out, it’s worth getting it looked at properly. I cover Southampton and the whole Waterside area, I’m available 24 hours, and there’s no call-out fee, so you’re not paying just for me to turn up and take a look. A summer adjustment is a quick job, and it’s far cheaper than waiting until the mechanism gives up entirely. Catch it early, get it adjusted, and your door will see you right through the warm months and beyond.
Should you force the handle when a hot composite door won’t lock?
No, and this is the one thing I really want you to take away. When the door has swelled and the hooks won’t line up with the keeps, it is tempting to haul up on the handle to force it home. Please don’t. Behind that handle sits the gearbox, the heart of the multipoint mechanism, and forcing it against a door that is bound in the frame is exactly how you snap it. A door that would have cooled down and worked fine by evening becomes a full lock replacement instead. If it won’t lift with normal effort, stop, give the door a moment to cool, and try again gently rather than fighting it.
According to my nearly 20 years across Southampton and the Waterside, the summer jobs that turn expensive are almost always the ones where someone forced the handle instead of letting the door cool, and with no call-out fee and 24 hour cover there’s no reason to risk it.
What can you do there and then when the door won’t lock in the heat?
Here is the bit most folk miss. When a composite door is hot and dragging, you can often cool it down and get it working again. A quick wipe or rinse with cold water brings the temperature down and reduces the swelling enough to lock it. The other golden rule is never to force it. If the handle won’t lift, do not barge or slam the door, as you can shear the mechanism and turn a free cooling-down job into a new lock. Lift gently, and if it still won’t go, wait for the evening cool. In nearly 20 years I’ve seen far more broken gearboxes from forcing than from the heat itself. If it’s stuck fast, give me a ring. No call-out fee, 24 hours.
If your door catches all year round, not just in the heat, it’s alignment. Here’s a composite door I sorted in Chandler’s Ford.
Frequently asked questions
Why does my composite door only stick in hot weather?
Heat makes the door expand by a millimetre or two, which is enough to change how it fits the frame. Direct sun makes it worse, especially on dark doors that soak up more heat. Once things cool down in the evening, the door often works normally again, which surprises a lot of people.
Should I force the handle if it won’t lift when it’s hot?
Please don’t. Forcing a stiff handle can bend or snap the gearbox inside the multipoint mechanism, turning a simple adjustment into a full lock replacement. If the handle won’t lift, stop and either wait for the door to cool down or give me a ring for a proper look.
Can I fix a sticking composite door myself?
You can try a few things safely. Lock it in the cooler morning or evening to confirm heat is the cause, and add a light silicone spray to the moving parts. Avoid sanding or planing the door though, as that leaves you with a draughty gap once everything shrinks back in winter.
What will it cost to have a sticking door adjusted?
In nearly 20 years across Southampton and the Waterside, I’ve found most sticking doors just need adjusting, not new parts, so it’s a quick, affordable job. There’s no call-out fee and I’m available 24 hours, so you won’t pay just for me to turn up and take a look.
