The short answer
A valley leaks when the lining in the channel splits, the channel blocks and water backs up, the slates or tiles along its edges slip, or the boards beneath rot. Lead valleys also fail where the sheets are too long for the movement of the metal or where old fixings pierce them. Finding the exact point matters, since patching the wrong spot only moves the stain.
What a valley is and why it takes the load
A valley is the internal angle formed where two roof slopes meet and fall towards each other. Rain falling on both slopes runs into that angle and is carried to the gutter at the bottom. The channel in the middle is lined with lead or a similar sheet material, and the slates or tiles on each side either stop short of the centre to leave it open, or are cut to meet it in a closed arrangement.
Because so much water collects there, a valley has little tolerance for faults. A hole in a plain slope lets in the rain that falls on that spot. A hole in a valley lets in the rain from a large area of both roof slopes. The leak often shows up well below the valley, on a ceiling, because water follows the line of the rafters before it drips.
The lining sits on boards, usually timber, laid between the rafters. If water gets in for long enough, the boards soften and the lining loses its support. Those boards are hidden, which is why a valley repair so often turns into a short inspection of what is beneath it. Our guide to lead valley gutters covers open, closed and repaired valleys in more detail.
The common causes, one by one
Most valley leaks come from a short list of faults. Knowing them helps you judge a quote.
- Split or cracked lead. Lead moves with temperature. A sheet laid too long for its position can crack across its width as it is stretched.
- Blocked channel. Moss, leaves and silt build up and water backs over the edge of the lead and under the slates. This is easily mistaken for a failure of the lead itself.
- Slipped or broken slates or tiles at the edges. The cheeks of the valley, where the covering is cut to the line, are the first to loosen. Water then runs behind the lead edge.
- Holes from nails or fixings. A nail driven through the lead in the wrong place, or a wire or fixing left from earlier work, makes a pinhole that lets water through.
- Rotten boards. A soft or sagging board lets the lead pool and crack.
- Sealed or bedded valleys that have failed. Where mortar or sealant stands in for lapped lead, it usually cracks first.
None of these can be told apart from the ground. They are told apart on the roof, with the covering lifted at the edge of the channel.
How the leak is found
A valley leak is found by tracing, not guessing. The survey starts in the loft, where we look for staining on rafters and boards under the valley line and for damp insulation. It then moves outside, with the channel cleared of debris so the lead is visible from end to end. We look for lifted edges, splits, hairline cracks, pinholes and any place where the lining has been stitched or sealed in the past.
Where water has been travelling a long way, a hose test can help. One section of roof is wetted at a time, beginning at the bottom and moving up, with someone indoors watching the ceiling. It sounds simple, and it is slow, but it avoids replacing a sound valley because the stain was in the wrong place.
The photographs we take on a survey show the lining, the edges and the timber below. They make the diagnosis plain and give you something to set beside any other quote, so that two firms can be compared on the same facts.
Repair or relay
Whether a valley is repaired or relaid turns on how much of the lining is sound and what state the boards underneath are in. A single split on an otherwise sound run can be dealt with by cutting out and replacing a section, or by a welded patch where the lead suits it. Keeping sound sheet in place is sensible, and it respects the original build.
A relay is the better course where the lead is thin, cracked in several places or already carries old repairs. The covering each side is lifted back, rotten boards are replaced, and new lead is laid in lapped lengths. Lead for pitched valleys is typically code 4 or heavier, with code 5 or 7 used where more weight is wanted. The covering at each edge is then relaid in sequence so that water sheds cleanly.
Sealant, mastic and cement fillets are poor substitutes. They crack as the lead moves, and our page on sealant on lead flashings explains why the same logic holds for valleys. Where the roof is listed, the choice of material also matters, as set out in our note on a lead alternative on a listed building.
Keeping a valley working
A valley that has been put right will last a long time if the water can leave it freely. Clearing out leaves, moss and silt each spring and autumn, and after a storm, is the most useful habit. It can be done from a ladder in many cases, or by a roofer at the same time as the gutters.
Look at the trees above the roof. Seeding trees and evergreens drop a lot into a channel, and a valley under them can need attention more often. Keep aerials and other fixings off the lead, and avoid walking along the channel. Lead is soft and the board beneath it is not designed for people.
If you have a leak now, place a bucket, move belongings, and note when the drip is worst. Wind-driven rain points to an edge or a slipped slate; steady dripping after any rain points to the lining. A leak is made safe first, and the repair is planned once the roof is dry. On older stone buildings in Bath, valley and gutter leadwork is part of the original design, which is why we repair it like for like. See our notes on leadwork on listed buildings and our main lead roofing page.
