What Causes Roof Leaks Around Valleys?

What Modern Valleys Are Made Of

A traditional valley is lead on boards. A great deal of housing built in the last thirty years uses something else.

GRP valley troughs - preformed fibreglass sections, laid in lengths and lapped, sitting on the valley boards with the tiles cut over them.

Dry valley systems - proprietary plastic or composite troughs, often with integral upstands and clip details, designed to be laid quickly without lead dressing.

Both are legitimate products and both work when installed correctly. Both also fail differently from lead, and knowing how changes where to look.

They Fail at the Joins

Lead fails in the middle of a piece, from fatigue. GRP and dry valley troughs do not fatigue in the same way - they fail where two lengths meet.

A valley longer than a single trough section has laps, and each lap has to have adequate overlap, be the right way round relative to the flow, and stay in position. A short lap, or one that has shifted, lets water under the upper section and onto the boards.

On dry systems the lap detail is often a specific designed feature with a defined overlap, and it is exactly the thing that gets shortened when somebody is cutting a run to fit.

GRP also cracks where it is stressed - at a lap, at a fixing, or where the boards beneath deflect. It is rigid, and rigid materials fail where things move.

And at the Bottom

The single most common modern valley fault we see is at the foot, where the valley discharges into the gutter.

The valley has to deliver its water into the gutter, past the fascia. If the trough stops short, or if the last section is not dressed over the gutter properly, the entire flow of the valley - which is the run-off from two slopes - goes behind the gutter and down the wall.

From inside that shows as damp at the top of a wall in the room below, and it is frequently blamed on the covering or on the gutter itself.

It is worth looking at during heavy rain. A curtain of water behind the gutter at the bottom of a valley is unmistakable once you are looking for it.

Debris Matters More, Not Less

People assume a smooth modern valley trough does not need clearing. The opposite is closer to true.

Many are shallower in profile than a traditional lead valley, and some have upstands or ribs that catch material. Leaf litter and moss dam them readily, and once dammed they overflow sideways in exactly the same way.

On the newer Warrington estates there is a specific pattern: estates planted at the same time have all matured at the same time, so a decade or so after they were built the trees start dropping enough to matter and valleys begin overflowing across a whole neighbourhood in the same autumn.

That is a clearing job, not a roofing one, and it is the cheapest possible outcome.

The Warranty Question

If your house is inside its structural warranty period, a valley fault is worth establishing before you instruct anybody to repair it.

Cover on new housing typically runs for ten years, with the first two normally being the builder's responsibility to put right. Short laps, a valley that stops short of the gutter and tiles cut too tightly into the channel are all workmanship rather than wear - which is exactly what that cover is for.

We will come and tell you what has failed either way. Knowing precisely what is wrong is what makes a warranty conversation go anywhere, and a written finding naming the defect is more use than a report of a damp patch.

If it is genuinely urgent, that comes first and the paperwork follows.