Quick answer
Composite door seals are the compression gaskets running around the frame and threshold that the slab presses against when locked. They're a wear item, not a manufacturing fault, and in Cornwall's salt air and driven rain they typically need attention every three to five years. Replacement is a straightforward push-fit job once the correct profile is identified.
Overview
Seals are push-fit rubber or TPE profiles that sit in a groove around the door and frame. Replacement is straightforward but the corners must be mitred cleanly or leaks return within weeks.
Symptoms
- Any draught, rattle or whistle.
- Visible perished or torn seal.
Common causes
- UV, salt air, age.
What a composite door seal actually does
The seal is a soft rubber or TPE gasket set into a groove running around the frame, sometimes duplicated with a second seal on the door slab itself. When the multi-point lock engages and pulls the slab home, the seal compresses against the frame face and closes the gap that would otherwise let air and water through. It's a simple mechanical job, but it only works while the material still has spring in it.
Because the seal is doing all the sealing work at the perimeter - the slab itself is effectively solid and waterproof - a failed seal shows up as draughts, water tracking in at low points, or a whistling noise in wind long before anything else on the door looks wrong. Most homeowners never think to check the seal until one of those symptoms appears.
Seals are supplied in continuous lengths cut to the doorset's perimeter and threshold dimensions, and different manufacturers use slightly different profiles, so matching the existing groove shape matters more than matching the door brand name.
Signs a seal has failed
Visually, a failed seal often looks flattened along one section, cracked at the corners where it's mitred, or has pulled away from its groove in short lengths. Running a finger along the length will usually find a section that's gone hard and brittle compared to the rest, particularly on the south or west-facing side of the door where UV and salt exposure are worst.
Functionally, the tell-tale signs are a cold line felt with the back of the hand around the perimeter, a whistling draught in windy weather, or water appearing at the base of the door during driving rain. None of these confirm a seal fault on their own, which is why we check compression and threshold condition alongside the seal itself before quoting.
- Flattened or cracked sections, especially at mitred corners
- Hard, brittle rubber compared to the rest of the length
- Cold line felt by hand around the perimeter
- Whistling in wind or water at the base during rain
Why Cornwall accelerates seal wear
Salt-laden air degrades rubber and TPE compounds faster than an equivalent inland property sees, and doors facing the prevailing south-westerlies take the additional punishment of driven rain under pressure, which forces water against a seal far harder than still rain ever would. Properties within a mile or two of open water, or on exposed high ground, are the ones we see needing seals replaced soonest.
Rendered coastal properties in particular seem to show seal wear a season or two earlier than granite or stone-built equivalents, likely because rendered walls tend to sit in more exposed, wind-funnelled positions. It's not a hard rule, but it's a consistent pattern from our callout history.
The replacement process
We start by identifying the exact seal profile in use - most major composite door manufacturers use one of a handful of standard push-fit profiles, though a few use proprietary shapes that need ordering specifically. Once the correct seal is on hand, the old length is drawn out of its groove, the groove is cleaned of old adhesive or debris, and the new seal is pushed in by hand along its full run, working from one corner around to the other.
A threshold gasket, if it's part of the same job, is usually a separate insert that clips or screws into the threshold extrusion rather than a push-fit rubber length, so it's fitted and checked independently. The whole job, including a compression check afterwards, typically takes under an hour for a single door.
Compression checks after fitting
A new seal is only doing its job if the door is pulling it into full compression when locked - so after fitting, we check the keeps or cams on the multi-point mechanism and adjust if the slab has settled or the hinges have dropped slightly over time. Fitting a seal without checking compression is a common shortcut that leaves the customer with the same draught within a year.
We test with a strip of paper at several points around the closed door - if it pulls free easily at any point, compression isn't reaching that section and needs an adjustment before we consider the job finished.
How long a new seal should last
A quality replacement seal, correctly compressed, should comfortably last several years even in an exposed coastal position, though we'd always recommend an annual visual check for coastal properties rather than waiting for symptoms to reappear. Sheltered, inland-facing doors can often go longer between checks.
Seal life is also affected by how the door is used day to day - doors that are regularly slammed, or that have a habit of being left slightly ajar in wind, wear their seals faster through repeated impact and edge abrasion than doors closed gently and locked properly each time.
Seals versus a full weatherproofing overhaul
A seal replacement on its own fixes the seal - but if the threshold gasket is also worn, or the mechanism has lost adjustment, replacing just the seal will leave other symptoms unresolved. We always inspect the whole perimeter, threshold and mechanism together and quote for what's actually needed rather than assuming a single component is responsible.
For most households this ends up being either a straightforward seal swap or a slightly larger weatherproofing service covering seal, threshold and compression together, which is usually the more cost-effective route if more than one component shows wear.
Holiday lets and managed properties
Holiday lets see continuous door use by different guests with no single occupant around long enough to notice gradual seal degradation, so problems tend to surface only when a changeover cleaner mentions a draught or a guest leaves a complaint about a cold hallway. A scheduled annual seal check ahead of the letting season, alongside other doors and windows, catches most of this before it affects a booking.
Repair versus full doorset replacement
A worn seal is almost never a reason to replace the whole door. The slab, hinges and glazing typically outlast several generations of seal, and a seal swap costs a small fraction of a new doorset. We'd only ever suggest replacement if the frame itself has warped, the slab has genuinely failed structurally, or the door is so old that matching parts is no longer realistically available - none of which apply to the vast majority of seal complaints we see.
If you've been quoted for a full replacement door purely because of a draught or a worn seal, it's worth getting a second opinion before committing, since a straightforward seal and compression service resolves the great majority of these cases at a fraction of the cost.
Aftercare once a new seal is fitted
A newly fitted seal needs very little maintenance, but a couple of simple habits extend its life noticeably. Wiping down the seal and frame groove occasionally removes salt residue and grit before it can work its way into the rubber and accelerate wear, and lightly lubricating the multi-point mechanism's moving parts annually keeps compression consistent rather than letting it drift as the year goes on.
Avoid painting over seals during any redecoration of the frame - paint stiffens rubber and stops it compressing properly, which can undo a perfectly good seal replacement within a season. If seals do need masking during painting work, we're happy to advise on the best approach.
Getting the right profile the first time
There are a handful of push-fit seal profiles used across the major UK composite door manufacturers, but a few brands use proprietary shapes that only fit their own frame groove. Turning up with a generic seal and forcing it into the wrong groove either fails to compress properly or won't stay seated at all, so identifying the frame manufacturer and groove profile before ordering is worth the extra few minutes it takes.
Where the original manufacturer can't be identified from any visible labelling, we take a physical profile impression on site so the replacement is guaranteed to seat and compress correctly rather than guessing from a photograph.
This attention to matching is part of why a professional seal replacement tends to outlast a generic DIY-sourced seal bought without checking the groove shape first.
Booking a seal replacement
Most seal jobs can be quoted from a couple of photographs of the frame groove and door corner sent ahead of a visit, which lets us bring the correct profile on the day rather than needing a separate measuring visit first. For unusual or older doorsets, a short measuring visit ahead of ordering avoids any risk of the wrong part turning up.
Once the seal is on hand, fitting and a full compression check is normally completed within an hour, with no mess, no redecoration needed, and the door usable again immediately.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Seal replacement (single door, standard profile) | £85-£150 | Depends on profile and total perimeter length |
| Seal replacement (proprietary/non-standard profile) | £120-£180 | Some manufacturer-specific profiles cost more to source |
| Threshold gasket replacement alongside seal | £90-£160 | Often bundled with seal work on the same visit |
| Compression/cam adjustment check | Included with repair, or £75 standalone | Confirms new seal is fully compressed |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Coastal properties: heavier storm-rated seals worth having a pro fit.
Prevention
- Silicone-treat seals twice yearly.
