Quick answer
The clearest signs of a bad composite door installation are an uneven reveal, a frame that flexes when pushed, keep screws that don't reach brick, poor external silicone, and a door that needs force to lock. Check all eight points before an installer leaves site, or arrange an independent inspection if the door's already fitted and something feels wrong.
Overview
Inspection points to check before you sign off a new install.
Why sign-off matters more than most people think
Most homeowners sign off a new door install the moment it looks finished — the sheet is on, the handle works, the fitter's packing up. That's the worst possible moment to stop checking, because a rushed final ten minutes is exactly when corners get cut on fixings and sealant. Once the van's gone, getting an installer back to correct their own mistakes gets progressively harder the longer you wait, and some smaller outfits simply become unreachable.
In Cornwall this matters even more because a lot of composite door work is done by small teams travelling in from further afield to cover a run of jobs. If the sign-off happens on the day, problems get fixed on the day. If it doesn't, you're relying on a warranty claim process that can take weeks, and in the meantime a coastal winter is testing every weak point in the install.
The eight-point check, explained
Point one and two are about the frame's relationship to the opening: is the reveal even all the way round, and does the frame stay rock solid when you lean on it? Any flex at all means fixings are wrong. Points three and four concern security and weatherproofing at the same time — keep screws should visibly bite into masonry, not just plastic or plaster, and the compression seal should squeeze evenly with the door shut, not gap at the top or bottom.
The remaining points are easy to overlook but expensive if missed: clear drainage slots along the bottom of the outer frame stop water pooling and rotting the sub-frame beneath; a neat, continuous external silicone joint is your main defence against wind-driven coastal rain; a head drip cill diverts water away from the top of the frame on exposed elevations; and the lock should turn and engage without you needing to lift, shoulder, or wrestle the door.
- 1. Reveal is even all the way round the frame
- 2. Frame doesn't flex when pushed near hinges or keep
- 3. Keep screws visibly reach into masonry
- 4. Compression seal touches evenly at every point
- 5. Drainage slots along the cill are open and clear
- 6. External silicone is continuous, neat, and fully bedded
- 7. Head drip cill fitted on exposed elevations
- 8. Lock turns and engages without extra effort
What to do if a point fails on the day
If any of these checks fail while the installer is still there, say so before you sign anything or make final payment. Reputable installers expect this and will correct fixings, re-run silicone, or clear drainage slots on the spot — it's a five-to-thirty-minute job for most of these points. Note down what failed and what was corrected, ideally with a couple of photos, in case a related issue shows up later.
If the installer pushes back, insists it's 'fine as it is', or has already left before you've had a chance to check, don't panic — none of these issues are urgent life-safety problems on day one, but they do get worse with time and coastal weather. Get them documented and book an independent assessment within the first few weeks rather than waiting for a full winter to expose every weakness.
CERTASS certification and what it should include
A CERTASS-registered installer should provide a compliance certificate confirming the work meets Building Regulations Part L for thermal performance and Part Q for security, along with the manufacturer's guarantee documentation for the door itself. If you weren't given a certificate, ask for one — it's a required part of a compliant install, not an optional extra, and you'll want it if you ever sell or insure the property.
No certificate at all is itself one of the clearest signs of a bad installation, because it usually means the work wasn't self-certified or notified to building control, leaving you technically non-compliant even if the door looks and functions fine.
Second homes and unattended sign-offs
Cornwall has a high proportion of second homes and holiday lets where the owner isn't on-site during installation and relies on a keyholder, agent, or the installer's own word that everything's correct. This is exactly the situation where problems go unnoticed longest — nobody's pushing on the frame or checking the silicone line week to week, and the first sign of trouble is often a damp patch reported by a guest months later.
If you can't be there for the install yourself, it's worth arranging an independent check within the first month, particularly before the property's first full winter with the new door in place.
How we handle post-install inspections
When we're asked to check someone else's new installation, we run through all eight points methodically and give a written summary — pass, fail, or borderline for each — rather than a vague verbal opinion. That document is genuinely useful if you need to go back to the original installer, raise a complaint with CERTASS, or simply want peace of mind that the door will hold up over a Cornish winter.
What a proper remedial survey actually checks
An independent survey after a suspect install goes well beyond eyeballing the sealant line. We measure the reveal at three points per side to confirm the frame is square, not just level; check diagonals corner to corner (equal diagonals confirm a true square); push-test the frame under load at every hinge and keep position; and inspect drainage slots with water to confirm they actually clear rather than just look open.
We also check the multi-point lock cycle under no load and under a light pull on the handle, because a lock that only engages cleanly when the frame isn't under tension is a sign the frame is being held slightly out of shape by its own fixings. All of this gets written up with photos, so you have something concrete whether you're pursuing the original installer or just deciding on remedial work.
Insurance-backed guarantees and what your certificate should show
A genuine insurance-backed guarantee (IBG) protects you if the installer goes out of business — the guarantee is underwritten separately, so a claim doesn't depend on the original firm still existing. Ask to see the actual policy document, not just a certificate with a logo on it; a real IBG has a policy number you can verify with the underwriter.
Separately, your CERTASS compliance certificate confirms the work was self-certified against Building Regulations Part L and Part Q and notified to building control. These are two different documents doing two different jobs, and a bad install is often accompanied by neither — which tells you plenty about how the job as a whole was run.
Raising a complaint that actually gets a response
Vague complaints get vague responses. Structure yours around the eight-point check: state clearly which points failed, with photos and measurements where possible, and reference the Consumer Rights Act 2015 requirement for work to be carried out with reasonable care and skill. Give a specific, reasonable deadline for a response — fourteen days is standard — and say plainly what you want done: re-inspection, re-fix, or refund of the difference for remedial work you've already had to commission elsewhere.
If the installer is CERTASS registered, you can also raise a formal complaint with CERTASS itself, who operate a dispute resolution process for member installers. Keep all correspondence in writing; a paper trail matters far more than a friendly phone call if this needs to go further.
Cost of catching problems early versus late
Catching a failed sign-off check on installation day costs nothing beyond the installer's time — most of the eight points take under thirty minutes to correct while they're still on site. Catching the same issue six months later after a coastal winter has tested it typically means an independent survey (£45–£75) plus remedial work from £120 for a re-seal up to £850 for a full refit.
Leave it two winters and you're often looking at water damage to the timber sub-frame or internal plasterwork on top of the door repair itself — a cost that has no fixed ceiling and depends entirely on how far damp has travelled.
A framework for deciding what to do next
If you're still within the first weeks of a new install, go back to the eight-point check, document any failures in writing, and give the installer a fair chance to fix it before paying any outstanding balance. If it's been longer and the installer isn't responding, commission an independent survey so you have evidence rather than opinion. If the survey shows a genuine installation fault rather than normal wear, pursue costs through a formal written complaint first and small claims only if that fails.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Independent post-install inspection | £45–£75 | Written report, all eight points checked |
| Minor corrections (fixings, silicone, drainage) | £75–£180 | If issues found and remediated same visit |
| Full re-fix of frame | £120–£220 | Where multiple fixing points fail |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Any point failing - request installer returns.
Prevention
- Choose CERTASS installers.
