Composite doors are among the toughest front doors you can fit — a solid, dense slab wrapped in a weatherproof skin, locked by a multipoint mechanism that bolts top to bottom. But that same strength creates a distinctive set of problems: a heavy slab puts real strain on the lock, and when something goes wrong it is almost never the part you can see. This guide explains what actually fails on a composite door, what you can safely sort yourself, and when to stop before a stiff handle becomes a snapped one. (uPVC doors share the same multipoint system — our guide to a uPVC door that won’t lock covers those — but composite doors behave differently, mainly because the slab is so much heavier and prone to seasonal movement.)
Behind the handle sits a central gearbox, and that is where the handle spindle and the key cylinder meet. When you lift the handle, the gearbox drives a strip running the full height of the door edge — hooks and bolts at top and bottom, plus rollers or mushroom cams that pull the door tight against its seals. These engage into keeps, the metal receivers in the frame. Turning the key then deadlocks the lot. The practical takeaway: when a composite door will not lock, the fault is usually in the mechanism or the alignment, not the visible lock — which is why forcing it rarely helps and often harms.
A composite slab is heavy, and over time it can settle on its hinges or swell slightly in hot, damp weather. Either way, the locking points no longer line up with the keeps. The tell-tale signs are a door you have to “lift and slam” to close, a handle that needs real force to raise, or a door that locks fine in winter but fights you in summer. The good news is that this is the most fixable problem of all: composite hinges are usually adjustable in two or three directions with an Allen key, so the door can be nudged back into alignment.
If the handle needs heavy force to lift, the mechanism is already fighting misalignment. Every forced lift wears the gearbox toward failure — the cheap alignment fix today becomes a full mechanism replacement if you keep straining it.
The gearbox is the heart of the system and the most serious thing to fail. The signs are unmistakable: the handle won’t lift at all, or it flops loosely with no spring back, or it lifts but nothing engages, or several locking points stop working at once. This is the centre case having worn out or broken — common on older or lower-quality mechanisms, and hastened by months of being forced against a dropped door. Do not keep operating it; a locksmith can usually replace just the gearbox if the connecting rails and hooks are sound, rather than the whole strip.
If the handle works but the key is the problem, suspect the cylinder. A key that will not turn often points to debris or corrosion in the cylinder, a worn or poorly cut key, or — again — slight misalignment loading the mechanism. A key that spins freely without throwing the bolts usually means the cam or tailpiece inside the cylinder has failed. A cylinder is an affordable part to replace, and on a composite door it is also the security-critical one, so it is worth fitting a proper anti-snap cylinder while it is open.
Before calling anyone, a few low-risk checks are worth a go:
If any of that needs force, or the handle and key still will not work, stop — you risk turning a small repair into a big one.
A named, vetted master locksmith covers your area — no call-out fee.
A failed gearbox, a key stuck or snapped in the cylinder, or a door that simply will not secure are all jobs for a professional. Multipoint mechanisms are not interchangeable — the gearbox type, backset and hook positions have to match the cut-outs in your door — so a locksmith will measure and source the correct part rather than guess. As a rough guide, a composite-door lock repair typically runs around £85–£150, and a full mechanism replacement around £150–£300 depending on the system; our price guide sets out the wider picture.
One last word: a composite front door is usually a security-rated, insurance-relevant door, so a botched DIY repair can cost you on both fronts. Keeping it correctly fitted and to standard protects your home insurance cover as well as your security. Every LocksmithLocal locksmith carries common gearboxes and cylinders, opens jammed doors without destroying them wherever possible, and fits insurance-rated parts — find your local locksmith if your composite door needs putting right.
Around a fifth of break-ins come through a back or patio door. The specific weak points of sliding, French and bifold doors — lifting, prying, glass — and the cheap, often no-drill fixes that turn the soft side of the house into the secure one.
For the jobs best left to a pro, your local locksmith network has you covered nationwide.
Your shed isn't a low-priority target: it holds valuable tools, and burglars use those same tools to break into the house. Fixing the hinges and hasp, choosing a padlock that actually resists attack, defending the garage, and protecting what's inside.
Lost your keys and wondering whether to change the locks? A calm walk through the decision — lost vs stolen, who can link them to your door, the insurance trap — and the cheap cylinder swap most people don't realise is all they need.