Quick answer
A composite door threshold rarely needs full replacement - most problems are a worn seal, a cracked drainage channel or a loose fixing letting water track through. Repairs typically cost £75-£320 depending on whether it's a seal swap or a full low-profile threshold replacement. Left alone, a failing threshold lets water reach the floor structure and door frame beneath.
Overview
Standard composite door thresholds are aluminium extrusions with replaceable rubber or brush seals. Full threshold replacement means unscrewing and lifting out; seal replacement is a push-fit.
Symptoms
- Worn threshold surface.
- Water ingress.
- Damaged threshold blade.
Common causes
- Foot traffic.
- Weather.
- Impact.
How a composite door threshold is built
Most composite doors sit on an aluminium threshold bar that bridges the gap between the floor finish inside and the outside step or path. Inside that bar is usually a thermal break, a drainage channel running to weep holes, and a compression seal that the bottom of the door presses against when it's shut and locked.
Each of those parts does a specific job. The thermal break stops cold bridging into the room. The drainage channel carries away any rain that gets past the outer seal before it can pool. The compression seal is the last line of defence, and it's also the part that wears fastest because it's compressed and released every time the door opens and closes.
Some manufacturers use a two-part seal system - a bulb seal on the door itself plus a brush or fin seal set into the threshold bar - which gives a degree of redundancy if one element starts to fail. Knowing which system is fitted to a particular door model helps when sourcing the right replacement part rather than a generic strip that may not match the profile exactly.
The three failure patterns we see most
The first and most common is a perished or flattened seal - after several years the rubber loses its memory and stops springing back, so it no longer makes contact along its full length. The second is a blocked or cracked drainage channel, often full of sand, grit and salt residue on coastal properties, which stops water draining away and forces it back under the door instead.
The third is a threshold bar that's come loose from the sub-frame, usually because the original fixing screws have corroded or because timber packers beneath it have rotted. A loose threshold flexes slightly underfoot, which breaks the seal contact even when the rubber itself is fine.
It's also worth noting that not every threshold problem originates at the threshold. A door that's dropped slightly on worn hinges can present with exactly the same symptoms - a draught or a trickle of water at one corner - even though the seal and drainage channel are both in perfectly good condition. Ruling out hinge wear before ordering a new seal avoids paying for a repair that doesn't address the actual cause.
- Perished or compressed seal no longer sealing on contact
- Drainage channel silted up or physically cracked
- Threshold bar loose or lifting from the sub-frame beneath
Diagnosing which one you've got
A visual check with the door open tells you most of what you need to know. Run a finger along the seal - if it feels hard, flat or has visible splits, it's compressed beyond usefulness. Look into the drainage channel for standing water, grit or moss; a channel that should be dry after a few hours often stays damp when it's blocked.
Press down firmly on the threshold bar itself in a few places. Any perceptible movement or a hollow sound suggests the fixings or the packing beneath have failed, which is a different repair to a seal swap and usually needs the bar lifted to inspect what's underneath.
Grit and salt residue are a particular nuisance on coastal Cornish properties because they don't just block the channel, they also abrade the seal every time the door opens and closes across them, shortening its working life. A quick sweep or vacuum of the channel every few weeks in winter is a cheap way to extend the interval between proper repairs.
Why Cornwall wears thresholds out faster
Driving rain off the Atlantic doesn't fall neatly onto a doorstep the way a still day's rain does - it's pushed sideways by wind, which puts far more water pressure against a threshold seal than the same amount of rainfall inland. Salt-laden air also degrades cheaper rubber compounds and speeds up corrosion in steel fixing screws, so a threshold that would last well in a sheltered inland spot can fail years earlier on an exposed coastal property.
Granite doorsteps and cob wall thresholds, common on older Cornish cottages, add a further complication: they're rarely perfectly level or square, so a standard off-the-shelf threshold bar sometimes needs packing or a bespoke cut to seal properly against them.
On a handful of jobs we've also found the threshold bar was the wrong depth for the floor build-up from the original installation, leaving a permanent low spot where water could gather even with a good seal. That's not something a homeowner can easily spot without lifting the door, which is one reason a professional inspection is worth having if a seal replacement doesn't fully solve the problem.
When a seal repair isn't enough
If the aluminium bar itself is corroded through, cracked at a fixing point, or was undersized for the doorway in the first place, a seal replacement alone won't solve the problem - water will still find the underlying fault. In that situation the sensible job is lifting the whole threshold, checking and if needed replacing the timber or DPC packing beneath it, and fitting a new bar sized correctly for the opening.
This is more involved than a seal swap but still a repair rather than a full door replacement in the majority of cases, provided the door slab and frame themselves are sound.
Where a full threshold has to come out, we also take the opportunity to check the condition of the door frame's bottom corners, since these are the other common entry point for water once a threshold has been failing for a while, and a frame corner repaired at the same time saves a second call-out later.
What a proper repair visit involves
A competent repair starts with removing the door from its hinges or propping it open fully, rather than trying to work around a hanging door. The old seal is cut out, the channel cleaned back to bare aluminium, and a new seal bedded in with the correct adhesive rather than just pushed into the groove, which is where DIY seal kits often fall down within a season.
Where the bar itself needs lifting, we check the sub-frame and any timber packers for rot before refitting, because putting a new threshold back onto a soft or damp base just repeats the failure in a year or two.
It's a straightforward job for a fitter carrying the right parts, but not one that lends itself well to a DIY seal kit bought online, since correct adhesive choice, channel preparation and seal tensioning all affect how long the repair lasts - a badly seated seal can fail within months even though it looked fine on the day it was fitted.
Preventing the same failure recurring
A yearly check before winter - clearing the drainage channel of grit and leaves and confirming the seal still springs back when pressed - catches most problems while they're still a five-minute fix rather than a water-damage repair. On exposed coastal properties we'd suggest checking twice a year rather than once.
A threshold repair carried out properly also protects the rest of the door: prolonged damp at the bottom edge of a composite slab, while far more resistant than timber, can still eventually affect the core material and the bonding between skin and frame if left unaddressed for years, so treating a small leak early is genuinely cheaper than waiting.
We'd also encourage homeowners to mention any recent flooring changes when booking a threshold repair, since a newly fitted floor that sits slightly higher or lower than before can itself affect how well an existing threshold seals, independent of any wear in the door hardware.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Threshold seal replacement | £75-£140 | Most common fix, done in one visit |
| Drainage channel clean & clear | £65-£95 | Often combined with a seal check |
| Threshold bar refit (loose fixings) | £110-£190 | Includes checking sub-frame condition |
| Full threshold replacement with packer repair | £220-£320 | Where timber packing has rotted beneath the bar |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Full threshold replacement.
Prevention
- Clean monthly.
