Why does my valley only leak in heavy rain?

That pattern almost always means capacity rather than a hole. Either debris has reduced the channel, or the valley is too narrow for the roof area draining into it. Water rises above the edge of the lining and runs under the tiles instead of down the channel.

Can I clear the valley myself?

No. Valleys are the weakest surface on a pitched roof — thin lining over boards that are often the first timber to rot. Standing or kneeling in one is how people go through a roof. Clearing is quick and cheap for a roofer with proper access.

Should I fit a valley diverter or splash guard?

Those address water shooting past the gutter at the bottom of the valley, which is a different problem from a leaking valley. If the valley itself is failing, a diverter changes nothing. Deal with the lining and the channel first.

Is a mortar fillet along the valley edges normal?

It was common practice and you will see it on many UK roofs, but it is a maintenance liability: the mortar cracks, drops into the channel, and helps block it. Modern practice is to clip the cut tiles mechanically and leave the channel clear.

High risk

Updated 30 August 2026

Roof Valley Leaking

Water is getting in along the internal angle where two roof slopes meet, often above a bay, extension or L-shaped roof.

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What's happening?

A valley is the internal gutter formed where two roof slopes meet at an angle, and it carries the run-off from both of them. That makes it the highest-volume, highest-velocity part of a pitched roof and the least forgiving of a small defect. Valleys are lined with lead, GRP, or a preformed valley trough, and the tiles either side are cut on the rake to leave a clear channel. Two things go wrong: the lining fails, or the channel silts up so water backs up over the edge of the lining and under the tiles. Because valleys sit above the junction of two roof areas, a valley leak commonly appears in a bedroom ceiling or over a bay window rather than anywhere near the valley itself.

What usually causes it?

Debris blockage — leaves, moss shed from the tiles, and grit building up in the channel until water overtops the lining
Cracked or split lead at a fold, from thermal movement where the lining was laid in over-long pieces
Valley channel too narrow for the catchment it serves, so it overtops in heavy rain even when clean
Cut tiles projecting too far into the channel, restricting flow and trapping debris
Mortar fillet used to close the tile edges cracking and dropping debris into the valley
Failed valley boards or battens beneath the lining, letting it sag and pond
Nail holes or fixings driven through the lining during earlier work

Is it dangerous?

Yes, in terms of water damage. A valley shifts a large volume of water fast, so when it fails the ingress is heavy rather than a drip — enough to soak insulation, run along joists and bring down a plasterboard ceiling. Because it enters at a structural junction, it wets rafter ends, valley boards and sometimes a wall plate, and prolonged wetting there leads to rot. Electrically, water tracking along a ceiling void towards light fittings is a real hazard; if water is anywhere near a light or a downlight, turn that circuit off at the consumer unit.

Can I fix it myself?

Diagnose from safe vantage points. Valleys are often visible from an upstairs window of your own or a neighbouring elevation — look for standing water, a mat of moss and leaves, or a dark split line. Note whether the leak only happens in heavy rain, which points to blockage or undersizing rather than a hole. In the loft, follow the wet timber upslope to find where the water enters. Do not walk in a valley: the lining is thin, the boards beneath may be rotten, and it is the part of a roof most likely to give way. Do not push a hose or a rod down it from a ladder.

When should you call a roofer?

Call a roofer within days, and the same day if water is coming through a ceiling, if a ceiling is sagging, or if water is near any light fitting. Valley leaks do not stabilise — the volume involved means each period of rain does more damage than the last. If the leak only appears in exceptional downpours, it still needs looking at, because that pattern usually means the valley is undersized or partly blocked rather than sound.

What will the roofer actually do?

Gain safe access with a roof ladder or scaffold, spreading load so no weight goes onto the valley lining itself
Clear the channel completely and check whether the leak stops, since blockage is the most common cause and the cheapest fix
Inspect the lining for splits, fatigue cracks at folds, punctures and ponding, and check the boards beneath for rot
Re-line the valley where needed in appropriately sized lead sections, GRP or a preformed valley trough, renewing valley boards and battens where the substrate has failed rather than laying a new lining over soft timber
Re-cut and re-bed the tiles either side to leave a correctly sized clear channel, using clips rather than relying on a mortar fillet
Check the outlet where the valley discharges into the gutter, because a valley emptying into a blocked or undersized gutter backs up

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Lead Flashing Coming Away From The WallCaution Gutter Overflowing In Heavy RainCaution Water Stain On The Ceiling But No Obvious LeakCaution

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