If a pitched roof has a weak point, it is usually a valley. Two slopes feed water into a single narrow channel, so the valley carries the run-off of both, concentrated into a stream. It is also where the covering is at its most awkward, with tiles cut to a diagonal and a lining beneath. When a valley fails, water goes straight into the roof space beside the valley rafter, and the stain on the ceiling appears a long way from the cause. This guide explains what valleys are made of, how they fail and what can be done.
What a valley is and why it works hard
Take a house with an L-shaped plan, or a main roof with a gabled extension against it. Where the two roofs meet, the slopes form a V when seen from above, and that V is the valley. Rain falling on both slopes runs down to the line of the V and then along it to the gutter. The amount of water is far greater than on a single slope of the same size, and the water arrives with more force, because two streams are merged.
Underneath, a valley rafter runs diagonally from the ridge down to the eaves, and a lining of lead, GRP or another material sits on boards or battens along it. The tiles on each side are cut to the angle of the valley and rest on that lining. Because they are cut, they are shaped to leave a gap that either exposes the lining or covers it, depending on the type.
The main types of valley
There are several ways to form a valley, and the choice affects both appearance and maintenance.
Open valley. The lining is visible, with a clear strip between the tiles on either side. Traditionally the lining is lead, bossed into the angle of the valley. More modern open valleys use GRP or a preformed trough. Open valleys are simple, easy to inspect and easy to clear, and they are the most common on modern roofs.
Closed or laced valley. The tiles from the two slopes meet over the lining, with no gap, hiding it. In a traditional laced or swept valley on a plain tile roof, special tiles curve round the angle in a continuous sweep. The look is neat and the lining is protected from the weather, but the valley is harder to clear and repair, because you have to lift tiles to reach it.
Valley trough. A preformed channel of plastic or metal, supplied in lengths, that sits on the valley rafter. It is cheaper than lead and quick to fit, and it is common on postwar and modern roofs.
Valley tiles. Some interlocking tile ranges include a specially made valley tile that fits into the angle, making a valley without a separate lining. They are used with the manufacturer's system.
Lead and GRP each have their strengths. Our comparison of lead valleys and GRP valleys sets out how they compare on cost, life and repair.
Why valleys fail
The causes fall into a short list, and many valley leaks combine two or three.
- Debris. Leaves, moss, twigs and dirt collect in the channel and dam it. Water backs up and runs over the edge of the lining and under the tiles.
- Cracked or split lining. Lead can split along its length when it has been laid too long or with too few expansion joints. Old GRP can crack and craze in sunlight. Plastic troughs can crack under impact or age.
- Lifted or slipped edge tiles. The tiles that cut along the valley are the most disturbed on the roof. If they slip, they expose the edge of the lining and the underlay.
- Underlay failure. The membrane under the valley lining is the last defence. If it is torn or perished, water that gets past the lining reaches the timber.
- Poor workmanship. Valleys laid with too narrow a channel, or with mortar and sealant used where a proper lining should be, tend to fail early.
Moss and debris are the most avoidable. A roof under trees will need its valleys cleared at least once a year, and our page on cleaning slate, clay and concrete roofs covers safe methods.
Spotting a valley leak
A valley leak tends to appear as a brown stain on a ceiling or wall below the valley, or as damp in the loft along the valley rafter. It often follows heavy or prolonged rain, because the channel only overflows when it is overloaded. From outside, look along the valley for visible debris, moss growth, tiles that are cracked or out of line, or a lining that looks lifted or split. In the loft, a torch along the valley rafter may show dark, damp timber or a water trail.
Do not climb onto the roof to look. A valley is a narrow, sloping, wet channel, and the tiles beside it are cut and easily broken. A pair of binoculars from the ground, or a phone photograph taken from a window, will often show enough. Our guide on signs of a failing tiled roof lists other warning signs, and the page on tracing a roof leak explains how to follow water that appears away from its source.
| Valley type | Common problem | Typical repair |
|---|---|---|
| Open lead valley | Splits, lifted edges, debris | Clear, patch or re-line in lead |
| Open GRP valley | Crazing and cracks in the surface | Re-coat locally or replace the lining |
| Valley trough | Cracked or displaced section, gaps at joints | Replace the trough sections |
| Closed or laced valley | Hidden lining failure, slipped cut tiles | Lift tiles, renew lining, relay tiles |
| Valley tiles | Cracked or displaced tile, blocked channel | Replace tile, clear the channel |
Repairs: from clearing to relining
The right repair depends on what has failed.
Clearing. If the valley is simply blocked, clearing it can stop the leak. The debris is removed by hand, the lining is checked for damage, and the channel is flushed. It is the cheapest fix and the one most often skipped.
Local repair. A small split in lead or crack in GRP can be patched. Lead can be welded or patched with a new section lapped under the old, and GRP re-laminated. This works where the surrounding lining is sound and the fault is confined.
Re-lining. When the lining is worn out along its length, the sensible fix is to lift the tiles beside the valley, strip the old lining, check and repair the boards or rafter beneath, lay new underlay and a new lining, and refix the tiles. This is a bigger job than patching, and it is the proper answer where failures keep recurring.
Rebuilding the valley. In extreme cases, rot has spread into the valley rafter and boards, and the timbers need repairing before a new lining goes on. Our guide to roof timbers explains why the valley rafter is a common casualty.
What it costs, and the access question
We have no reliable sourced UK range for valley repair, because the cost depends so heavily on the length, the lining chosen, the roof height and the access. A short local repair on a low roof is a small job. A full re-line on a two-storey roof, needing scaffold, is a much bigger one. For orientation on access alone, one UK guide for slate repairs reports an access tower or scaffold adding £300 to £800, which applies to any roof job needing it. A fixed written quote follows our survey, and we price the valley as part of the job, not as an add-on.
If you are having other work done, the valley is the time to deal with it. A valley that is nearing the end of its life will be hard to reach again once the scaffold is down.
Valleys, tiles and the rest of the roof
A valley is only as good as the work around it. Tiles cut at the valley should be fixed with corrosion-resistant fixings, as the tiling standard sets out, because cut tiles are the ones most likely to lift. Our explainer on BS 5534 and tile fixing covers the point. The underlay and the lining should lap correctly, and the first and last tiles of each course should not leave a gap. A new valley on an old roof should be matched to the pitch and tile type, and the page on minimum roof pitch for tiles notes that valleys on shallow slopes need special care.
Where a house is listed or sits in a conservation area, like-for-like lining is usually expected, so talk to the council's conservation officer before swapping lead for something else.
Valleys and the roof around them
A valley does not fail in isolation. The cut tiles beside it, the underlay below, the lining and the timber under that are one assembly, and the leak you see is the first symptom of whichever layer gave up. That is why a good repair starts with lifting a few tiles to see what is actually under them. We often find that a "leaking valley" is a lining in good order with a cut tile that has cracked, or a sound lining that has been dammed by moss at the foot of the channel where it meets the gutter. Fixing the true cause is cheaper than re-lining on a guess.
The foot of the valley deserves special attention. It is where all the water collects before it drops into the gutter, and where an undersized gutter or a blocked downpipe turns a short valley into an overflowing one. A valley that discharges into a poorly supported gutter can also soak the fascia and eaves timber. Our section on gutters belongs with the roof work, and a quick check of the outlet is part of every valley visit.
Two further points are worth knowing. Valleys in the shade of trees fill with leaf mould, which holds water against the lining for months and can eat through lead from the underside. And pigeons and squirrels sometimes nest in the sheltered channel; nesting birds are protected, so work may need to be timed around them.
How we check a valley
On a survey we examine each valley from the roof wherever access allows and from the loft along the valley rafter. We photograph the lining, the cut tiles and the timber, so you see the exact fault on your own roof. If clearing and a small patch will do, that is what the quote says. If the lining is finished, we explain why a re-line is the better answer and what it would involve. The written quote is fixed, the debris is cleared and taken away, and the repair carries our written ten-year workmanship guarantee. More on what we do is on our pitched roofing in Basingstoke page.
