Quick answer
Water tracking under a composite door almost always comes from one of three places: a failed threshold seal, a step or path outside that slopes towards the house rather than away from it, or wind-driven rain forced under the door during a storm. On Cornwall's exposed coasts the third cause is far more common than inland, and it needs a different fix to a simple seal replacement.
Overview
Water under the door means the threshold seal or drainage has failed, or the threshold isn't level. All three are fixable.
Symptoms
- Puddle on floor after rain.
- Damp mat.
Common causes
- Worn threshold gasket.
- Blocked drainage slots.
- Threshold not level.
Why this is a bigger deal than a wet mat
Water that gets under a door once in a while during a storm is an inconvenience. Water that gets under a door repeatedly is a structural problem waiting to happen, because floor coverings, chipboard subfloors and timber joists don't dry out as fast as they get wet. Damp trapped under laminate or carpet near a doorway is a classic starting point for rot and mould that isn't visible until it's well established.
It's worth treating repeated water ingress as a fault to diagnose rather than something to mop up, especially in a hallway that leads to other rooms, where damp can spread sideways under skirting before it's noticed.
It's also worth checking whether the water appears only on the inside face of the threshold or whether it's visible pooling outside too, since standing water outside that never seems to clear points more towards a drainage or ground-level issue than a door fault.
External causes: what's happening outside the door
Before assuming the door itself is at fault, it's worth checking the ground outside. A path, step or patio that slopes even slightly towards the threshold rather than away from it will direct rainwater straight at the door regardless of how good the seal is. This is a common issue on older Cornish properties where paving has settled or been relaid over the years without attention to fall.
A blocked or missing drainage gully immediately outside the door has the same effect - water that should disperse instead pools against the threshold until it finds a way in.
A guttering or downpipe discharging directly above or very close to a doorway is another external factor worth ruling out - a blocked gutter overflowing straight onto a doorstep during heavy rain can easily be mistaken for a threshold leak when the actual cause is entirely separate from the door itself.
Door-side causes: what's happening at the threshold
On the door itself, the usual suspects are a compressed or split threshold seal that's no longer sealing under the door's full weight, a drainage channel inside the threshold bar that's silted up with grit and can't carry water away, or a door that's dropped slightly on its hinges so it no longer sits square against the seal along its whole length.
It's also worth checking that the seal makes contact evenly by sliding a piece of paper along the closed door at ground level - if it pulls out easily at any point without resistance, that section isn't sealing.
- Threshold seal flattened or split and no longer compressing
- Drainage channel blocked with sand, salt residue or leaves
- Door dropped on hinges, leaving a gap along part of the seal
- External ground sloping towards rather than away from the door
Wind-driven rain: Cornwall's particular problem
Standard weathertightness testing under BS 6375 assesses a door's resistance to combined wind and water pressure, and a well-installed composite door meeting that standard should cope with the vast majority of UK weather. But Cornwall's Atlantic-facing coastline regularly sees rainfall driven near-horizontally during winter gales, generating pressure at the threshold that a sheltered inland property never experiences.
On genuinely exposed elevations, even a door and threshold in good condition can occasionally admit a small amount of water during the worst storms. This is a different situation to a leak that happens in ordinary rain, and it usually points towards additional measures like a storm guard or a canopy rather than a fault with the door itself.
In a handful of severe storm cases we've dealt with, the underlying cause turned out to be a combination of factors working together - a slightly worn seal, a bit of ground slope, and one particularly bad gale - none of which alone would have caused a problem, but together were enough to let water through.
Working out which cause applies to you
Timing is the most useful clue. Water that appears in any rain, light or heavy, points to a seal or drainage fault at the threshold. Water that only appears during severe gales with rain hitting the door directly points more towards wind-driven rain overwhelming an otherwise sound door, or ground-level drainage outside adding to the volume.
Checking whether water appears evenly along the whole threshold or concentrated at one end also helps narrow it down - concentrated ingress usually means a localised seal gap or a dropped hinge corner, while even ingress across the width points to a blocked drainage channel or ground slope.
Where the diagnosis points to genuine wind-driven rain overwhelming an otherwise sound door on a very exposed elevation, it's worth discussing realistic expectations rather than promising a total fix; the aim becomes reducing the frequency and severity of ingress to an acceptable level rather than eliminating it in every conceivable storm.
What gets fixed and in what order
The sensible approach is to rule out and fix the cheapest, most likely causes first: seal condition and drainage channel clearance. Only if water persists after those are corrected does it make sense to look at hinge adjustment, ground regrading, or additional storm protection, since those are more involved and costly interventions.
Keeping a simple log of when water appears - date, weather conditions, wind direction if known - is genuinely useful for a fitter trying to diagnose an intermittent problem, since it narrows down whether the cause is everyday rain or only the worst of Cornwall's winter gales.
Damage that's already happened
If water has been getting under a door for a while, it's worth lifting the edge of any flooring near the threshold to check the subfloor before booking any door repair - a soft or discoloured chipboard floor or a musty smell right at the doorway usually means moisture has already travelled further than it looks from the surface.
Condensation is occasionally mistaken for a threshold leak, particularly in a cold hallway during a mild, humid spell - it's worth distinguishing between water appearing specifically during or just after rain, which points to ingress, and a general dampness that appears regardless of weather, which points towards condensation or a different moisture source entirely.
If water under the door coincides with a new smell of damp in an adjacent room or a visible tide mark on skirting, it's worth having both the threshold and the affected room checked together, since by that stage water may have been travelling for longer than the visible symptom suggests.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Diagnostic visit & water test | £65-£95 | Identifies source before quoting a fix |
| Threshold seal replacement | £75-£140 | Fixes the majority of everyday leaks |
| Hinge adjustment for dropped door | £65-£110 | Restores even seal contact |
| Storm guard / rain deflector fitting | £120-£220 | For genuinely exposed, storm-facing doors |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Threshold levelling or replacement.
Prevention
- Clear drainage monthly.
