What Is a Lead Valley?

How a valley works

Wherever two pitched roofs meet at an inside corner, a valley forms. All the rain from both slopes runs towards it, so it needs a waterproof lining that can cope with a lot of water and with debris, moss and leaves. Lead has been used for this for centuries because it can be shaped to the angle, it does not rot and it moves with the temperature.

The lead sits on a valley board, a timber platform fixed between the rafters, with a layer of underlay below it to prevent the lead from sticking to the timber. The tiles or slates either side are cut to follow the angle and overlap the lead edge by a set margin, which leaves a strip of metal visible down the middle. The width of that strip is called the exposed width. Too narrow, and the channel overflows in a heavy shower. Too wide, and moss and debris settle on the lead.

The anatomy of a good lead valley

A good valley is built in lengths, with each length lapping the one below it by a set distance. The edges are held down with lead clips, never nails through the face of the sheet, because a hole through the lead is a leak waiting to happen. The sheets are limited in length, since long runs expand and contract with the heat and eventually crack. If you want to know why, read our page on why lead cracks.

Thickness is specified by a code number. A valley takes heavier lead than a flashing because it carries more water and sees more foot traffic when roofers work on it. The code to use depends on the length of the run and the pitch, and the lead trade body publishes guidance on that.

Where the roof has a hip or a dormer, the valley may turn into a more complex shape, and the lead around it is cut and folded by hand. Some roofs carry the lead valley into a concealed gutter behind a parapet. That is a more demanding job again.

Why lead valleys fail

The most common cause is debris. Leaves, moss and silt collect in the channel, hold moisture against the lead and block the flow, so water backs up under the tile edges. The second cause is movement: lead that was laid in one long run, or fixed too tightly, splits at a crease. The third is a bad lap or a hole made by an aerial or a satellite dish bracket. Nails driven through the lead from above are another culprit, as are previous repairs with bitumen tape.

On the ground you may see staining on the ceiling directly below the valley line, or a patch of green moss collecting on the tile edges beside it. From the roof, the signs are a split along the centre, daylight at a lap or tiles sitting proud where the board has rotted underneath.

What a valley costs to leave alone

A valley that is only slightly worn is easy to ignore, because the water still reaches the gutter on most days. The trouble is that a valley fails from the inside out. Water gets under a lap, wets the board beneath, and the board swells and rots slowly. By the time a stain shows on the ceiling, the felt, the board and the rafter beside it may all be wet.

That is why we look at the underside of the valley from the loft whenever we can. A dark, damp valley board or a row of rusting nails is clear evidence that the lead above has been letting water through for some time. Catching it early often means a section can be repaired instead of the whole run being relined.

Repair, relining or an alternative

A short split can be repaired by welding or by letting in a new section, but a valley with failing lead along its length is better relined. That means lifting the tiles along both sides, replacing any rotten board, laying new lead and relaying the tiles. A fibreglass or GRP valley is a cheaper alternative, and our comparison of lead valleys versus GRP valleys sets out the differences. Whichever you pick, the survey photos show where the fault sits before you choose.

For more on our leadwork, see the lead roofing and leadwork hub.