Why uPVC moves in the heat
uPVC profile has a higher rate of thermal expansion than timber or aluminium, so a south facing door that sits in direct sun for several hours can genuinely be a fraction of a millimetre larger across the frame by mid afternoon than it was first thing in the morning. Dark colours, particularly anthracite grey and black foils, absorb more heat than white and tend to show this more.
On its own, this movement is small, but a door is a tight fitting mechanism, and a lock that only has a millimetre or two of tolerance can feel the difference between a door that swings shut easily and one that needs a shoulder against it.
What normal expansion feels like
A mild version of this is a handle that needs slightly firmer lifting to engage the hooks in hot weather, or a door that closes with a bit more resistance in the afternoon than it did that morning. It typically eases off in cooler evenings and mornings and is worse on the hottest, sunniest days.
- Door feels stiffer only on hot, sunny afternoons
- Handle needs a firmer lift to lock, but does lock
- Problem eases in the evening or on cooler days
- Same door was fine through spring and cooler months
Door only playing up in the heat?
Describe when it sticks and how long it has been happening and we will let you know whether an adjustment visit is likely to sort it.
What points to a real fault rather than heat
If a door will not lock at all in hot weather, rather than just needing more effort, or if the sticking is happening at moderate temperatures too, or has been getting steadily worse over more than one summer, heat is probably exposing an existing issue rather than causing all of it. A hinge that already has a little wear, or a frame that has shifted slightly in the opening, has less spare tolerance to absorb the seasonal expansion, so it shows up first on the hottest days.
In practice this means summer is often when a borderline door tips over into being a genuine problem, and it is worth having it looked at rather than assuming it will sort itself out once the weather cools.
What helps day to day
A small hinge adjustment, the same grub screw adjustment used for a dropped door, can give a sticking door a little more clearance and make it behave better through summer. Keeping hinges and the lock's moving parts lightly lubricated with a light oil or silicone spray also reduces friction, which matters more when the fit is already tighter than usual.
Avoid forcing a stiff handle hard to make the lock throw, since that is one of the more common ways a gearbox gets damaged, turning a seasonal inconvenience into a proper repair.