Roof valley repair

What a roof valley does

A valley is the internal angle where two roof slopes meet, such as where a rear wing joins the main house or a dormer meets the roof. Rain from both slopes funnels into it and runs down to the gutter, so the valley handles a concentrated flow during every downpour.

Because of that, any weakness here shows up faster than it would on an open slope, and leaks tend to appear on the ceilings and walls near the junction of the two roofs.

Lead valleys

Many older roofs have a valley lined with sheet lead laid on boards, with tiles cut to sit either side. Lead moves as it heats and cools; if the sheets were too long or fixed too tightly, that movement eventually causes splits. Thin lead, or lead laid without proper laps between lengths, is more prone to this.

A short split can sometimes be repaired, but if the lead has cracked in several places or worn thin, relining with new lead in correctly sized lengths is more reliable.

Mortar valleys

On some roofs, particularly with plain tiles, the valley is formed with mortar bedded over a lining, or with tiles cut and set in mortar. Mortar shrinks, cracks and works loose with frost and movement, and pieces end up in the gutter. Once the mortar opens, water tracks underneath into the roof space.

Repointing can help for a while, but a valley where mortar keeps breaking out usually benefits from being re-formed.

Pre-formed valleys

Many newer roofs use a factory-made valley trough, usually a moulded plastic or composite section laid under the tile edges. These avoid large amounts of mortar. They can still leak if the tiles beside them are cut short, if the trough is damaged during other work, or if debris blocks it.

Debris and blockages

Leaves, moss and silt collect in valleys because water slows there. A blockage forces rain sideways under the tile edges instead of down the channel. Overhanging trees make this a recurring issue, and it is often the simplest cause to rule out first.

How a valley leak shows indoors

Water entering at a valley often travels along rafters or the lining before it drips, so the stain may appear a little way from the valley itself. Typical signs include damp on the ceiling near where two roofs meet, wet timbers in that corner of the loft, or staining on an upstairs wall below the junction. Leaks that only show in heavy or prolonged rain are common, because the valley only overflows at high flow.

Our guide to roof leaking causes covers other sources that can look similar, such as flashings and ridges.

Repair or reline

A repair is suitable when the fault is local: a single split in otherwise sound lead, a short length of failed mortar, or a few tiles cut too short. Relining means lifting the tiles along both sides, replacing the lining and boards if needed, and refitting the tiles, usually with new cut pieces.

Relining is more work, but on an old valley with several faults it is the way to stop the leaks for good. Our leadwork and roof repairs pages describe both sides of this work, and roof repair cost explains how repair jobs are priced.

Keeping valleys clear

Checking valleys from an upstairs window after autumn leaf fall is a good habit. If you can see debris building up, or mortar in the gutter below, it is worth having it looked at before the next heavy rain. We look at the roof, explain the options and give a clear written quotation before any work starts.

Frequently asked questions