Quick answer
Water leaking through a composite door comes from the threshold, the frame-to-wall sealant, blocked drainage slots, or a missing head drip cill - never the slab itself. Matching where the water actually appears to one of these four points is the fastest way to identify the cause before any repair work starts.
Overview
Water ingress at a composite door is nearly always at the threshold or where the frame meets the surrounding structure. The slab and seals are designed to shed water outward if the drainage slots are clear.
Symptoms
- Puddle inside the door after rain.
- Water tracking down from the top of the frame.
- Damp patches on the plaster around the door.
Common causes
- Blocked drainage slots at the bottom of the door.
- Failed frame sealant to brickwork.
- Threshold worn or misaligned.
- Missing head drip cill.
The slab sheds water - it doesn't let it in
A composite door slab is manufactured to shed water off its face, with the GRP or PVC skin providing a genuinely waterproof outer surface. In ten years of repair callouts across Cornwall, we can count on one hand the times water ingress traced back to a fault in the slab material itself - it is almost always something around the door, not the door.
That means chasing a leak starts with the frame perimeter, the threshold, and the drainage system, not with worrying about the panel itself. Establishing exactly where water first appears inside the property is the single most useful piece of information for diagnosis.
Matching the symptom to the source
Water pooling right at the base of the door, inside, points to the threshold - either a worn gasket, a misaligned blade, or blocked drainage slots pushing water back rather than letting it run away outside. Water tracking down the wall from above the frame points to a missing or damaged head drip cill, the small lip designed to throw rainwater clear of the frame-to-wall joint.
Damp patches on the plaster to either side of the frame, rather than at the base, usually mean the sealant between frame and brickwork has failed - a very common finding on doors installed more than five to eight years ago that haven't been re-siliconed. Water that only appears during driving, wind-blown rain (rather than steady vertical rain) points toward pressure-driven ingress through a marginal seal rather than a structural leak.
- Water at the base, inside - check threshold and drainage
- Water tracking from above the frame - check head drip cill
- Damp plaster either side of the frame - check perimeter sealant
- Only in driving rain - check seal compression
Drainage slots: the most overlooked cause
Every composite door frame has small drainage slots along the bottom of the outer frame designed to let any water that gets into the frame cavity drain back outside. When these slots are blocked with grit, moss, old paint, or a poorly-fitted cill, water that would otherwise drain harmlessly instead backs up and finds its way inside.
Clearing these slots with a thin, non-metallic tool takes minutes and resolves a surprising number of 'mystery leak' callouts on its own, with no parts required at all. It's the first thing we check on any water ingress job because it's quick, free to fix, and commonly the actual cause.
Frame sealant and why it needs renewing periodically
The silicone sealant between the door frame and the surrounding brick or render is a wear item, not a permanent seal. UV exposure, thermal movement, and - in Cornwall especially - salt-laden driving rain degrade it faster than an inland installation would experience. Cracked, shrunk, or missing sections of sealant are visible on close inspection and are a common source of leaks that only show up during storms.
Re-sealing the perimeter is a straightforward maintenance job that we'd recommend every five to eight years on any coastal property, and it's far cheaper than dealing with the plaster and decoration damage that follows a winter of undetected water ingress.
Head drip cills in exposed positions
A head drip cill is a small aluminium or uPVC projection fitted above the door frame, angled to throw rainwater running down the wall clear of the frame-to-wall joint rather than letting it run straight down onto it. Many older or budget installations were fitted without one, which is fine in a sheltered position but a genuine liability on an exposed, rain-facing elevation.
Retrofitting a head drip cill is a relatively small job and often solves a leak that's been misdiagnosed as a frame or seal fault for years, because the water was always coming from above rather than through the frame itself.
Cornwall's exposure makes this more urgent, not optional
Atlantic-driven rain hits exposed Cornish elevations with a force and frequency that most manufacturers' standard specifications weren't originally tested against - which is why professional installers in this region often recommend upgrades that would be unnecessary a few miles inland. BS 6375 weather ratings for doorsets describe exactly this kind of performance testing under wind and water pressure, and a door on a fully exposed coastal elevation benefits from being specified, or subsequently upgraded, to a higher rating.
If you own a holiday let on an exposed stretch of coast, we'd generally recommend a proactive perimeter and drainage check ahead of the winter storm season rather than waiting for a leak to be reported by a guest.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Leak diagnosis and drainage clear | £75-£120 | Often resolves the issue on its own |
| Frame perimeter re-sealing | £95-£180 | Full perimeter, both faces where accessible |
| Head drip cill supply and fit | £110-£220 | Depends on access and wall finish |
| Threshold replacement | £140-£280 | If gasket or blade is worn beyond adjustment |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Water is coming in from the frame perimeter - that's an installation issue.
Prevention
- Keep drainage slots clear.
- Re-silicone the exterior frame perimeter every 5-8 years.
