Quick answer
The most common composite door installer mistakes are wrong-length cylinders, keep screws too short to reach brick, missing head drip cills, unpacked fixing points, external glazing beads, mismatched hinges, uneven thresholds, poor exterior silicone, blocked drainage slots, and no CERTASS certificate issued. Any of these warrants a proper installation audit.
Overview
The mistakes we most often see when called out to fix someone else's install.
Why these mistakes keep recurring
We compiled this list from actual call-outs, not a generic checklist — these are the ten faults we most often find when asked to sort out someone else's installation. Most stem from the same root cause: a fitter working to a fixed day rate on a job that should really take longer, especially on a non-standard Cornish opening. None of these mistakes are hard to avoid; they're simply skipped when time is tight.
The frustrating thing about most of them is that they're invisible to a homeowner on the day of installation. The door looks finished, it locks, and everyone's happy — until a mechanism fails, a leak appears, or a break-in attempt exposes exactly how vulnerable the install really was.
The security-critical mistakes
A wrong-length euro cylinder is one of the most serious mistakes on this list because it creates a genuine snap-vulnerability — cylinders that protrude too far from the handle plate are far easier to snap in a forced-entry attempt, defeating the anti-snap protection the cylinder was designed to have. Similarly, external glazing beads (the strips holding the glass unit in from outside rather than inside) let an intruder simply remove the glass to gain access, regardless of how good the lock is.
Keep screws too short to reach masonry sound almost trivial but genuinely aren't — the keeps are what the multi-point lock's bolts engage into, and if they're only fixed into the plastic frame rather than through to brick behind, a determined kick can tear them straight out.
- Wrong-length cylinder — snap vulnerability
- Keep screws too short to reach masonry
- External glazing beads — glass removable from outside
The weatherproofing mistakes
No head drip cill on an exposed elevation is a mistake that only shows its cost after the first proper storm — the drip cill diverts water running down the wall away from the top of the frame, and without it, water tracks straight down behind the frame into the fixing points and, eventually, any timber sub-frame. Poor exterior silicone application — thin, inconsistent, or applied over a dirty or wet surface — fails far sooner than a properly applied joint and is one of the most common causes of the gaps and draughts we're called out for.
Blocked drainage slots are an easy one to overlook because they're small and low down on the outer frame, but if they're not left clear during installation (sometimes literally covered by sealant or debris), water that should drain away instead pools and eventually finds its way inside.
- No head drip cill on exposed elevations
- Poor or inconsistent external silicone
- Drainage slots blocked with sealant or debris
The mechanical and structural mistakes
Frames not packed at every fixing point leads directly to the frame-flexing problems covered elsewhere on this site — it's one of the single biggest predictors of an install needing remedial work within a couple of years. Wrong hinges for the door's weight is less common but does happen, particularly with heavier composite slabs or ones with large glazed panels, where standard hinges simply wear out faster than they should.
An unlevel threshold causes the door to drag, wear unevenly, and in some cases lets water pool right at the point where it's most likely to find its way inside. It's a simple thing to get right with a spirit level during install and a genuinely annoying thing to live with if it's wrong.
- Frame not packed at every fixing point
- Wrong hinges specified for door weight
- Threshold not level, causing drag and water pooling
The paperwork mistake that matters more than it seems
No CERTASS certificate issued sits at the end of the list but shouldn't be treated as an afterthought. A missing certificate usually means the work was never properly self-certified against Building Regulations Part L and Part Q, which can become a real problem if you come to sell the property and a solicitor asks for compliance documentation you simply don't have.
If you're missing a certificate for a past installation, it's worth contacting the installer directly first; if they're no longer trading or unresponsive, CERTASS itself may be able to advise on next steps, though retrospective certification isn't always straightforward.
What to do if you recognise several of these
Finding one of these mistakes on your own door doesn't necessarily mean a wholesale refit is needed — several of them (silicone, drainage, keep fixings) are quick, inexpensive fixes on their own. Finding three or four together, though, usually points to a generally rushed installation and is worth a full audit rather than chasing each symptom individually.
Raising a complaint about installer mistakes the right way
The most effective complaints are specific and evidenced. List each mistake separately — under-fixing, missing drainage slots, no compliance certificate — rather than a general 'not happy with the door', and back each one with a photo or measurement. Reference the Consumer Rights Act 2015 requirement that services be carried out with reasonable care and skill, and set a clear, reasonable deadline for a response.
If the installer is CERTASS registered, you can escalate an unresolved complaint to CERTASS directly, who can investigate member installers and, in serious cases, take action against their registration. Keep this route in reserve rather than leading with it — most legitimate businesses respond faster to a direct, well-evidenced complaint than to the threat of a regulatory one.
Insurance-backed guarantees when the installer is at fault
If the mistakes trace back to workmanship rather than the door itself, a genuine insurance-backed guarantee should still respond even where the installer disputes the claim or has ceased trading, because the guarantee sits with an underwriter, not the company. Track down the policy number rather than relying on a generic certificate, and contact the underwriter with your evidenced complaint and any independent survey findings.
Where no IBG was ever issued, your recourse is limited to a direct claim against the installer or their insurance, which is considerably harder once a small firm has stopped trading — one more reason to check for a genuine IBG before appointing anyone, not after something's gone wrong.
What correcting common installer mistakes costs
Costs scale with how deep the mistake goes. Missing or blocked drainage slots: usually under £100 to clear or re-cut. Under-fixed frames needing extra fixings and packers: £120 to £220. No compliance certificate issued: potentially free to resolve if the installer is still trading and registered, since a retrospective certificate may be possible, though this isn't guaranteed. A frame that's fundamentally the wrong size or badly racked: £450 to £850 for a full refit.
What good practice looks like, mistake by mistake
Against each common mistake there's a straightforward correct version: fixings into masonry, not render, at every point; drainage slots left open and tested with water before sign-off; a continuous, fully bonded external silicone bead rather than a thin decorative pass; and a compliance certificate handed over on completion, not promised and forgotten. None of these require specialist tools or extra time worth mentioning — they're the difference between a job done properly and one done quickly.
Second homes and the mistakes that go unseen longest
Installer mistakes on a second home or holiday let often surface only when a tenant or guest reports a problem, by which point the fault may have been present for a full season or more. Missing drainage slots, in particular, cause damage that's invisible until water has already tracked into the sub-frame — exactly the kind of fault an absent owner has no way of spotting between visits without asking someone to check specifically for it.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Full installation audit (all ten points checked) | £45–£75 | Written summary provided |
| Cylinder replacement to correct length | £75–£130 | Security-critical, quick fix |
| Keep re-fixing into masonry | £90–£160 | Per affected keep |
| Drip cill retrofit | £120–£220 | Where none was fitted originally |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- For an installation audit.
Prevention
- Choose CERTASS installers with reviews.
