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Roof valley leaks: why the seam between two slopes fails first

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If a roof has a valley and a leak, the valley is the first place to look. It is doing several times the work of the slope beside it.

A valley is the internal angle where two roof planes meet, and it is the busiest square footage on the whole roof. Every drop of rain that lands on either of those two slopes is funnelled into it. On a roof with a couple of gables and a dormer, a single valley can be handling the runoff from a large share of the total area, moving fast, concentrated into a channel a few inches wide.

That has three consequences, and they explain almost every valley problem.

It wears out first

Water moving quickly, carrying grit, over the same narrow strip for twenty years abrades whatever is under it. On a roof with shingles woven or cut into the valley, that means the granules go first in the channel, then the mat underneath. This is why a roof can look perfectly respectable across the slopes and be worn through in the valleys, and why a close look at the valleys from the ground with binoculars is one of the most informative thirty seconds you can spend on a roof.

It catches everything

Whatever lands on those two slopes ends up in the same channel: pine needles, leaves, seed pods, moss, shingle granules, the debris from an overhanging branch. That mat holds water against the surface long after the rain stops, which accelerates the wear. Worse, it can dam — and a dammed valley makes water go sideways, under the shingles on either side, because shingles are lapped to handle water flowing downhill and have no answer at all to water flowing across or backwards.

This is the mechanism behind the classic complaint of a valley that leaks only in heavy rain: in ordinary rain the flow gets past the obstruction, and in a downpour it backs up above it and goes under the courses.

The two ways valleys are built

Worth knowing, because it changes what you are looking at. An open valley leaves a strip of metal exposed down the centre, with the shingles cut back on each side. A closed valley runs the shingles across the channel, either woven from both sides or cut so one side laps over the other. Both work when done properly. Open valleys shed debris better and the metal is visible so you can see its condition; closed valleys are more common on a lot of American housing and hide their condition completely.

An open valley gives you real information from the ground: if you can see rust, a split, a seam that has opened, or nails driven through the exposed metal where they should never be, you have found something concrete. A closed valley gives you almost nothing visually and is diagnosed mostly by where the water shows up inside.

Where a valley leak appears inside

Usually some distance from the valley. Water that gets under the courses lands on the deck, runs down the slope of the deck, and drops wherever it meets an obstruction — so the stain can be well off to one side. This is the standard reason people insist the valley cannot be the problem, because the wet patch is nowhere near it. Trace it in the attic rather than assuming: follow the wet run up the underside of the deck and see where it starts.

What actually helps

The single most useful thing is keeping the valley clear, and the single most useful way to do that is from the ground: cut back the branches that are feeding it. An overhanging limb is the difference between a valley that stays clear and one that is permanently packed. Everything else here — assessing the metal, re-lining a valley, replacing worn courses on either side — is work on a steep, fast-draining, often high part of the roof, and it belongs to somebody with equipment and training.

What you can do usefully is describe it well. When you call, being able to say "the leak only happens in heavy rain, it shows up eight feet to the left of the valley on the north side, and there is a branch over that valley" puts the person on the phone several steps ahead.

Frequently asked questions

Why does my valley only leak in heavy rain?

Because in ordinary rain the water gets past whatever is obstructing the channel, and in a downpour it backs up above it. Backed-up water goes sideways under the shingle courses, which are lapped for downhill flow and have no defence against water moving across or upward.

The wet patch is nowhere near the valley. Can it still be the valley?

Very often, yes. Water that gets under the courses lands on the deck and runs down the deck until something interrupts it, so the drip point can be several feet to one side. Tracing the wet run in the attic settles it.

Is an open or a closed valley better?

Both work when built properly. An open valley sheds debris more easily and lets you see the metal's condition from the ground, which is genuinely useful. A closed valley hides its condition entirely, so problems are diagnosed by symptoms instead.

Can I just clear the debris out of the valley myself?

Not from the roof, and that is the point of this site. The useful version of that job is done from the ground: cut back the branches feeding the valley, which is what stops it filling up in the first place.

Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.

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