A valley is the inside angle between two roof slopes, found where a back extension meets the main house or where the arms of an L-shaped plan cross. Water from both slopes funnels into it, so a valley sees more rain per metre than the rest of the covering does. Underneath the tiles or slates sits a lining that keeps that water moving to the gutter, and on many British houses that lining is sheet lead.
When a lead valley fails, the symptom is usually a stain on a ceiling below, often in a corner of an upstairs room, and the cause is tucked out of sight under the edges of the covering. This guide explains what a replacement involves, the lead codes and lap details normally used, how a valley differs on tile and slate, and how to tell a lining that needs replacing from one that only needs clearing. The wider subject is covered on our lead roofing page.
How a lead valley is built
A lead valley sits on a timber board, usually called a valley board, laid between the rafters at the angle where the slopes meet. Over the board goes a layer of underlay, and the lead is laid on top in lengths that overlap working from the top of the roof downwards. Each upper length passes over the one below, so water always runs from a higher sheet onto a lower one and never against a joint.
The edges of the lead are turned up slightly at each side or tucked beneath the tiles or slates. The covering is cut to follow the line of the valley, leaving a clear channel of exposed lead between the two sides. How wide that channel is depends on the roof, the covering and how much water the valley carries, and a wider channel copes better with leaves, moss and heavy rain.
Why lead rather than another liner?
Lead can be dressed into the angle without kinks and still last. MyJobQuote gives a correctly installed lead flashing a life of over 80 years, and Less puts a lead flat roof at 50 to 100 years, so a valley lining that is detailed well can outlive the covering on either side. Its main enemy is movement and poor detailing, not time.
What sits on either side?
On a tiled roof the tiles are cut and bedded or clipped to the edge of the valley. On a slate roof the slates are cut to a mitre or left to run into the lead, with a short slate known as a tingle (a little strip of lead or copper used to hold a slate in place) occasionally used on repairs. The detail at the edge is where most hidden leaks begin.
Spotting a valley that needs replacing
Valleys usually give away that something is wrong from the ground, a ladder or the loft. Look for slates or tiles beside the valley that are lifted, cracked or slipped, a line of dark staining on the ceiling that runs beneath the valley, or moss and debris built up in the channel. Inside the loft, daylight at the valley line, wet battens or damp on the underlay below the lead point towards a failed lining.
Splits in the lead itself are the most definite sign. They follow the fold of the angle, where metal has been stretched by years of heating and cooling, or they open along an overlap where lengths were fixed too tightly or laid with too little lap. Pinholes can appear where the lead has been eaten by run-off from moss, and a lining that has been patched with sealant or bitumen tape several times is telling you something too.
Debris can make a sound lining overflow. Clearing the channel comes first, and some valleys we inspect need only that plus a few refixed slates.
Repair or replace?
If the damage is a single split in an otherwise good lining, a lead weld or a short new section lapped properly may be enough, and a small, well-made repair is what we would rather price. If the lead is thin, creased all along its length or cracked in several places, a full replacement of the valley is cleaner than a series of patches that move the leak elsewhere. For a closer look at the cracking itself, see our guide to lead flashing repairs.
Which lead code for a valley
Lead sheet is sold by code, a number that tells you its thickness and weight. Code 4 is 1.80 mm and 20.41 kg per square metre; Code 5 is 2.24 mm and 25.40 kg; Code 6 is 2.65 mm and 30.10 kg. Royston Lead lists Code 4 as suitable for pitched valley gutters and Code 5 and above for the same purpose, with the heavier codes put to work on bigger roofs.
In practice, a short valley on a modest roof can take a lighter code, while a long valley, a valley carrying the runoff of a large slope or a valley in an exposed position benefits from the heavier sheet. A heavier code is more resistant to the fatigue that cracks lead and gives you more material to dress neatly into the angle. The cost difference between codes is usually small against the labour of reopening a roof, which is why we often suggest going up a step.
The standard that covers rolled lead sheet is BS EN 12588. Leadworx says its codes comply with it, and the colour marking on a roll of sheet tells a fitter which code is in front of them. Our flashing repair guide covers the heavier codes in other positions, while the separate piece on lead soakers explains why the lightest code is kept for those.
Laps, widths and the details that stop leaks
A lead valley relies on laps rather than sealant. Each length overlaps the one below, and the overlap has to be long enough for the pitch: the shallower the roof, the greater the lap needed to stop water driven uphill by wind from slipping under. The lead sheet manuals set minimum laps by pitch and by the code used, and we work to those, never to a rule of thumb.
Length matters just as much. Long sheets expand and contract with temperature, and if they are fixed at the sides without room to move they buckle, creep down the slope and eventually crack. For that reason valley lengths are limited, and laps are used to break up a long valley into manageable bays. Our note on lead sheet lengths goes into why.
How is the lead fixed?
Lead is held at its upper edge with a few clips or copper nails in a line, never driven through the middle of the channel where water will run. The sides are secured by the covering above them and, on some roofs, by a clip at the edge. Allowing the metal to slide gently within these fixings is the whole principle.
What about the edges of the channel?
The tiles or slates must sit clear of the central channel so that debris can pass through, but close enough that the lead is shielded from the weather. A narrow gap collects leaves; an overly wide one exposes too much metal to the sun. Getting this balance right is a matter of judgement on site, and photographs from the survey help us explain what we found.
Tile valleys and slate valleys are not the same job
On a plain-tiled roof, the tiles at the valley are cut at an angle to follow the line, and the cut edges are usually bedded on mortar or held on a batten with tile clips. Concrete interlocking tiles are trimmed in the same way, though their larger size means fewer cuts and a wider channel. In both cases the lead is normally laid over a board and lapped from the ridge down.
Slate valleys are fussier. The slates must be cut accurately to the angle, since a ragged mitre lets wind-driven rain past it. In some older buildings the valley is built as a swept valley, in which cut slates sweep in a gentle curve toward the valley instead of meeting it at a hard angle. Swept valleys appear on traditional stone and slate roofs, and where the original was swept it should be rebuilt that way. For Cotswold stone, Cotswold District Council's technical guidance says lead valleys of the open or mitred kind are cheaper to build but "neither appear as satisfactory as the traditional valley".
Does the pitch limit the choice?
Cupa Pizarras reports a 20 degree minimum for natural slate under BS 5534, and valley detailing is part of keeping a roof near that pitch watertight. A shallow valley sheds water slowly, so laps and lead code are set more carefully than they would be on a steeper slope.
What happens where the valley meets the ridge?
The lining runs up beneath the ridge or hip cover so the top is never open to rain. If ridge tiles need refixing, doing both together avoids opening the roof twice.
What a replacement involves day to day
Replacing a lead valley starts with taking back the covering on either side of it. The tiles or slates along the valley are numbered or stacked in order so they go back in the same sequence. The old lining is lifted out, and the board and battens beneath are inspected. Soft or stained timber is cut out and replaced, because new lead on a rotten board is a short-term repair.
Next comes a fresh layer of underlay, the new lead, and the covering re-laid with the cut edges trimmed to follow the channel. Any slates cracked in the removal are swapped for sound ones. Finally the work is checked from the loft for daylight, damp and fixings, and the roof is left clean.
How long does it take?
A typical valley is a one- or two-day job for a small team, though access can change that. Higher roofs need scaffolding, and Scaffolding for a house of two storeys is quoted by MyBuilder's 2026 national guide at around £850 to £1,200. When you compare quotes, ask each one whether VAT and scaffolding are inside the price. Ours arrives in writing after the survey.
Can it be done in the wet?
Short repairs can be tackled between showers with the roof covered. A full valley replacement is best done in settled weather because the roof stays open while the lead goes in, and we plan around the forecast and cover the work each evening.
A finished valley carries a 10-year workmanship guarantee in writing, and we cart away the old lead and broken slates.
Keeping a valley working once it is done
Even a perfectly lined valley needs a little attention. Leaves, moss and seeds collect in the channel and, when they pile up, rain banks up and runs over the lead edge onto the board. Clearing the valley once a year, or after autumn leaf-fall near trees, is the best protection. Where trees overhang the roof, more frequent clearing may be sensible.
Moss and lichen are worth watching. Their run-off is mildly acidic and, left long enough, can pit the lead along its central line. Removing them gently by hand, without wire brushes or high-pressure jets, avoids scratching the metal and driving water under the covering. Our moss removal service covers this.
Fresh lead will go through a colour change as the surface oxidises. White streaks or chalky marks on new metal are normal and are caused by rain and carbon dioxide reacting with the surface, and a treatment sometimes applied at the start reduces them. They are cosmetic rather than a sign that the valley is failing.
Should I think about theft?
Valleys on low roofs and extensions are within a climber's reach. Our guide to lead theft sets out deterrents.