What Causes a Roof Valley to Fail?

Fault One: Cut Too Tight

The tiles or slates either side of a valley are cut on the rake, and there has to be a clear channel of open lining between the two cut edges for the water to run down.

How wide that channel needs to be depends on the roof area draining into it, the pitch and the exposure. What it must not be is decorative. A valley cut so the tiles almost meet looks neat and is under-capacity from the first day.

The symptom is unmistakable once you know it: the valley copes with ordinary rain and overflows sideways in a downpour, every time, for years. Nothing is broken and nothing is worn. The roof has simply never been able to carry what lands on it.

It is the commonest built-in valley fault and it is only cured by lifting the tiles and re-cutting them.

Fault Two: Lining Pieces Made Too Long

Lead expands and contracts substantially with temperature, and a valley - open to the sky, dark, and often in sun for part of the day - goes through a wide range.

A lining formed in pieces that are too long cannot accommodate that movement, so it fatigues and splits. The split runs across the channel, along a fold, and it is usually in the middle of a long run.

Guidance from the Lead Sheet Association sets maximum piece lengths for valley linings, with each piece lapped over the one below. A valley run in a single continuous length is a split waiting to happen, and repairing that split without shortening the pieces gets you the same failure again.

Fault Three: No Tilting Fillets, or the Wrong Shape Underneath

A valley lining does not sit on a flat board and stop. It sits on a valley board with tilting fillets - triangular battens along each side - which lift the edge of the tiles slightly and stop them lying flat into the channel.

Without them, the cut tiles sit down into the valley, obstructing the flow and holding water against the lining. The channel is narrower than it looks and the lining stays permanently wet.

Related: the lining needs adequate upstand at each side and the tiles need to lap over it far enough that wind-driven rain cannot get under. A lining that is too narrow for the covering leaves that lap marginal.

None of this is visible without lifting tiles, which is why it survives so long undiagnosed.

Fault Four: Sealant Where a Detail Should Be

The commonest thing we find on a previously repaired valley: mastic run along the cut edges of the tiles, or over a split in the lining.

It sheds water for a season. Then it shrinks, cracks and, because it is bonded along one edge, holds water against the defect rather than letting it drain. The valley is worse than before anybody touched it, and the sealant has to be removed before a proper repair can be made.

Valleys work by shape and gravity - a clear channel, adequate laps and upstands, and water directed rather than blocked. Nothing you apply over a failed valley turns it back into a working one.

Why Heswall Roofs Have More of Them

The houses here are large, built on a slope and extended more than once. Every extension that meets the main roof at an angle creates a valley.

Valleys formed as part of an addition are, in our experience, considerably more likely to carry the built-in faults above than the original valleys of the house. They are made on site, at the end of a job, joining new work to old, often by somebody whose main task was the extension rather than the roof.

So on a much-extended Heswall house, the valley to look at first is not the oldest one. It is the newest.

The other local factor is size. Large complicated roofs concentrate more water into each valley, so a channel that would be adequate on a semi is marginal here.