Roof leaks
Why does my roof leak only during heavy rain?
Volume-dependent leaks: overwhelmed valleys and gutters, slow routes with large catchments, and details that only fail above a threshold.
- Evidence before expenditure
- Options, not pressure
- Updated 2026-09-16
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Leaks that appear only in heavy rain are usually about volume rather than a large opening. Valleys, gutters and outlets cope until a threshold is passed, then water backs up and finds a route inside. A small defect with a large catchment behaves the same way: it needs sustained rainfall before enough water arrives to show on a ceiling.
Drainage that runs out of capacity
Valleys, box gutters, hidden gutters behind parapets and small outlets all have a limit. Partly blocked with silt, moss or leaves, that limit drops sharply. Once water backs up above the level of a lap or an upstand, it enters — and it stops as soon as the flow reduces.
This is one of the most common patterns we are called to, and it is frequently resolved by restoring drainage rather than by repairing the covering.
- Valleys silted or holding vegetation
- Gutters running the wrong way or with sagging joints
- Outlets and hopper heads restricted
- Downpipes blocked below ground level
Small defect, large catchment
A hairline split in a flashing or a single lifted lap may admit a trickle. If the slope feeding it is large, sustained rain delivers far more water than short showers, so the trickle becomes visible internally only after a couple of hours of steady rainfall.
That threshold behaviour is why these leaks are so often reported as intermittent and dismissed as minor.
How the cause is established
The pattern itself is the best clue: note how long it takes for the symptom to appear, whether it stops soon after the rain does, and whether it happens regardless of wind direction. Wind independence points to volume rather than driven rain.
Where the pattern cannot be reproduced by observation, controlled water testing of specific areas, working upwards, is the reliable method. It is slow, and it is how these leaks get properly identified.
What evidence should you look for?
These are indications, not conclusions. Each one narrows where to look; only an inspection confirms a cause.
Leak begins an hour or more into heavy rain and stops soon after it ends.
Drainage capacity or a slow route with a large catchment is more likely than an open defect.
Areas that may need investigating
- valleys
- gutters and outlets
- underlay laps above the symptom
Happens in heavy rain regardless of wind direction.
Volume rather than wind-driven rain is likely to be the governing factor.
Areas that may need investigating
- drainage paths
- capacity of valleys and hidden gutters
Urgency: Arrange an inspection
Intermittent ingress still wets structure each time it happens. It is not usually an emergency, but it does not improve on its own either.
Treat it as more urgent if you see:
- Water runs rather than seeps during heavy rain
- A ceiling is staining more widely each time
- Water is near electrical fittings
Repair or replace?
Where drainage is the cause, maintenance and detail repair usually resolve it. Where a covering or membrane is being overwhelmed because of its design or condition, renewing that area is the more durable answer than repeated clearing.
Maintenance may be enough when
- Blocked or restricted valleys, gutters and outlets
A local repair is realistic when
- An identifiable split, lap or upstand defect
Further investigation is the right next step when
- Pattern cannot be reproduced without controlled water testing
How we can help
- Roof Leak DetectionRoof leak detection traces water back from where it appears inside to where it actually enters the roof. Water frequently travels along battens, rafters and purlins before it drops, so the ceiling stain is often several metres from the defect. Detection combines internal tracing, external inspection, drone imagery and, where needed, controlled water testing.
- Gutter CleaningGutter cleaning clears leaves, moss and silt from gutters, hoppers, outlets and downpipes so rainwater leaves the building rather than overflowing down the wall. It is the cheapest preventative maintenance available and prevents damp, pointing erosion and fascia rot.
- Roof RepairsPlanned roof repair fixes an identified defect — slipped slates, cracked tiles, failed flashings, valley or ridge problems — before it becomes water ingress. Repairs are appropriate where the covering and structure are otherwise sound and the fault is localised.
Related questions
What should you do next?
Record the timing next time it rains heavily — when it starts, when it stops — and have the drainage paths checked before any covering work is priced.
Last updated 2026-09-16.
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