What is a valley on a roof?

Where valleys are and what they do

A roof with two slopes meeting at a right angle, as on an L-shaped house or one with a rear extension, has a valley where they join. The ridge runs along the top of a roof and the hips run down its external corners, but a valley runs down an internal corner, a dip between two slopes.

Everything that falls on both slopes runs towards that line. Rain, leaf litter, moss and grit gather in it, so the valley handles a heavier flow than the plain slopes either side. The tiles on either side are cut to follow its line, and the lining underneath is the layer that actually stops water getting into the roof space.

A valley meets the gutter at its lower end, and the water leaves the roof there. A blocked gutter outlet at the foot of a valley can back water up into the roof, which can leave a ceiling stain that appears only in heavy rain.

Where two roofs meet at different heights, the same idea applies, with a flashing at the join in place of a valley. The terms are explained again on the page on plain tile roofs.

The usual types of valley

Three constructions are set out below, and each handles water in its own way.

Lead is soft, shapes to any angle and has a long record. MyJobQuote puts lead flashing at over 80 years of life, a minimum, not a promise. The weight of lead sheet is set by its code, and Royston Lead lists pitched valley gutters among the uses of its sheet. The code for a given valley is set in the specification, never guessed.

GRP is a manufactured plastic and is light to handle. A dry valley avoids mortar and the cracking that comes with it, since the tiles are held by fixings and the channel is shaped to carry the flow.

How valleys go wrong

Valleys fail in ordinary ways, and many can be spotted from the ground or the loft. The common ones are these:

• Debris and moss build up in the channel, slow the water, and push it sideways under the tile edges.
• Lead splits or cracks where it has been bent too tightly or has moved with the heat of the sun, and water gets through the crack.
• The tiles on either side are cut badly or chipped, leaving gaps where wind-driven rain can get in.
• Old mortar bedding along the edge of the valley has crumbled, so the tiles shift.
• The underlay beneath the lining has torn or rotted, and the lining carries on without a back-up layer.

Signs inside the house are brown stains on the ceiling under the join of the two roofs, damp on the wall below the valley, or drips in the loft after heavy rain. The place the water shows inside is often not the place it came in, because it can run along a timber first.

Clearing debris in good time prevents a lot of this. Checking the valley is part of any tiled roof survey, and the page on roof underlay and battens covers what lies beneath it.

Repair or replace a valley

A small split in a lead valley can be patched, and a blocked channel can be cleared. A valley with a failed lining, rotted boards underneath or a run of cracked tiles beside it is replaced, which means lifting the tiles either side, stripping the lining, checking the timber and relaying.

MyJobQuote gives valley replacement at around £540 in September 2026, a typical UK figure from a national guide and not a quote for your roof. How the work goes depends on how many tiles must be lifted, the pitch and the access, and a long valley on a steep roof asks more of the day than a short one.

If the valley is the source of a leak, a survey comes first and then one written total. A roofer may recommend renewing the valley while the tiles around it are being replaced anyway, because the lining sits under the tiles and the work is easiest done once.

The service page for pitched roofing covers the wider job, and a slate re-roof, in order shows where valleys fall in a full re-roof.