Quick answer
Gaps around a composite door frame are almost always down to failed exterior silicone or a frame that was set too far into the opening during installation. Small gaps (under about 10mm) usually need re-sealing; larger gaps mean the frame needs foam-packing or the surrounding plaster and render needs remediation.
Overview
Gaps between the frame and the surrounding brick or plaster mean the exterior silicone has failed or the frame was set with too much reveal.
Symptoms
- Visible gap outside.
- Daylight visible from inside around the frame.
- Draught from the perimeter, not the door itself.
Common causes
- Failed silicone.
- Frame packed too far in.
- Building movement.
Two very different causes, one visible symptom
A gap around the frame can come from two quite different places, and telling them apart matters because the fixes are completely different. The first is simple sealant failure — the silicone that was applied at installation has shrunk, cracked, or lost adhesion, leaving a visible line where there used to be a seal. This is by far the more common cause and is essentially routine maintenance.
The second is a structural gap that was there from day one because the frame was set too far back into the opening, leaving an oversized void between frame and masonry that was foam-filled inadequately or not at all. This is an installation fault, not a maintenance issue, and re-sealing alone won't fix it because there's nothing solid behind the sealant to bond to.
Why coastal properties see this faster
Silicone sealant has a working life, typically five to eight years, but UV exposure, salt-laden air and constant wetting-drying cycles from driving coastal rain shorten that considerably on exposed Cornish elevations. A door facing prevailing south-westerlies can see its external seal degrade in three or four years where an equivalent inland installation would comfortably reach eight.
Granite and cob openings compound the problem because both materials expand, contract and occasionally shift more than modern brick or block, which puts additional strain on the sealant joint at exactly the point it's already weakest. If your door sits in an older stone or cob reveal, budget for more frequent inspection than you would for a standard brick opening.
Diagnosing which type of gap you have
Look closely at the gap itself. If you can see remnants of old, cracked or peeling silicone and the gap is fairly consistent and under about 10mm, this is very likely a sealant failure — rake out what's left and it should reveal a sound, continuous frame edge behind it. If instead you find a wide void with no sign old sealant ever properly bridged it, or you can see straight through in places, that's a structural gap from the original install.
Daylight visible from inside around the frame, particularly at the top corners, is a strong indicator of the structural type. A draught that seems to come from the perimeter rather than the door edge itself, even with the door shut and locked, points the same way.
- Cracked or peeling old silicone with a visible gap beneath: sealant failure
- Wide void with daylight visible around the frame: installation gap
- Draught tracks the frame perimeter, not the door edge: could be either — needs inspection
- Gap widens noticeably after a cold snap or storm: structural movement, needs assessment
Fixing sealant failure properly
This isn't a job for whatever silicone's in the shed. Old sealant needs raking out completely — any remaining perished material stops a new bead adhering properly and you'll be back doing it again within a year. The surface needs cleaning and, ideally, priming before a low-modulus, neutral-cure sealant is applied in a continuous, well-tooled bead. Cheap acetic-cure bathroom sealant will fail fast outdoors and can even attack some frame finishes.
Done properly, external re-sealing is a straightforward hour-or-so job per elevation, and it's the kind of maintenance task worth doing proactively every five to eight years rather than waiting for a visible gap to appear.
Fixing structural gaps
Where the gap exists because the frame was never properly packed into the opening, the fix depends on severity. Smaller voids can sometimes be addressed with low-expansion foam injected behind the frame followed by a proper sealant finish, restoring both the thermal seal and the weatherproofing. Larger voids, particularly where surrounding plaster or render has been damaged or was never made good after installation, need that finishing work redone before a lasting seal is possible.
In rare cases where the frame itself is undersized for the opening by a significant margin, no amount of packing genuinely resolves it long-term, and refitting a correctly sized frame is the only durable answer. We'll always try the less invasive option first and only recommend a refit where it's genuinely the only way to stop the problem recurring.
What happens if gaps are left unaddressed
A gap that lets water in behind the frame doesn't stay a cosmetic problem for long. Water tracking behind a composite door frame can reach the timber sub-frame many manufacturers still use internally, and sustained damp exposure there leads to rot that's far more expensive to put right than the original gap ever was. On holiday lets, this kind of slow-building damage is particularly costly because it's rarely spotted early — by the time a guest reports a smell or a soft patch of skirting, the damage has often been building for a season or more.
Servicing intervals matched to your exposure
How often a sealed perimeter needs checking depends heavily on where the door faces. A door on an exposed coastal elevation taking direct south-westerly weather should have its seals and silicone checked every 12 months; the same door set back under a porch or facing inland might comfortably run to 24 months between checks. Salt-laden wind degrades silicone and compression seals faster than plain rain does, so 'sheltered but coastal' still needs closer attention than fully inland properties.
As a rule of thumb: exposed coastal elevation — annual check; sheltered coastal or exposed inland — every 18 months; sheltered inland — every 24 months. These are checks, not full services; a five-minute visual inspection of the perimeter seal and silicone line is usually enough to tell you whether anything needs attention.
What good sealing practice looks like on installation day
The external joint between frame and wall should be filled with a continuous bead of low-modulus silicone, tooled to a smooth, slightly concave finish that sheds water rather than holding it. It should bond to both surfaces without gaps, bubbles or thin patches — a common shortcut is running one thin pass where two are needed on rougher render or stone.
Internally, the gap between frame and structural opening should be filled with low-expansion foam before any trim goes on, not left as a hollow void behind a bead of decorator's caulk. On granite or cob openings in particular, where the wall thickness and irregularity vary a lot, this internal fill matters more than usual because it's doing real thermal and structural work, not just cosmetic finishing.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Re-seal perimeter, one elevation | £90–£160 | Rake out and re-apply low-modulus silicone |
| Foam-pack and re-seal structural gap | £150–£280 | Small to moderate voids |
| Render/plaster remediation around frame | £200–£400 | Where finishing was never completed |
| Full frame refit for undersized frame | £450–£850 | Rare, only where packing won't hold |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Large gaps (>10mm) - needs foam or plaster remediation.
Prevention
- Re-silicone every 5-8 years.
