Look at an L-shaped house in Nailsworth or Dursley, or any roof with a gable pushed out of the main slope. The dip where the two planes meet is the valley. Every drop landing on both slopes ends up there, so a valley carries far more water than the same length of plain roof, and it has to cope for decades without anyone looking at it.
This guide stays with that one detail. It covers laced, swept, mitred, lined and dry-fix valleys, what typically goes wrong with each and what a repair involves, from re-dressing a strip of lead to rebuilding the whole line. For every other cause of a leaking tiled roof, read our wider guide to why tiled roofs leak and how they are fixed. For the full service, start at pitched roofing.
Why a valley is the busiest line on the roof
On a plain slope, water leaves each tile and drops onto the next course in a thin sheet. In a valley the sheets from both sides merge. The flow speeds up, picks up moss, grit and leaf litter, and runs along a channel that may be only a few inches wide. Whatever sits in that channel decides whether the loft stays dry.
A valley is built in three layers. At the bottom is the underlay, the membrane laid over the rafters beneath the battens (the small timbers that carry the tiles). Over it goes the valley lining: lead, a glass-reinforced plastic (GRP) trough, or a woven tile detail. On top, the tiles from each side are cut to follow the angle. If one layer is missing, badly lapped or worn through, water finds the gap.
Why they are hard to inspect
From the ground you see tile ends and perhaps a line of metal. You cannot see the underlay, the side battens or the timber below. A ceiling stain under a valley tells you where to start, but the entry point can be a metre or two upslope. That is why we survey at the valley itself and photograph what we find on your roof.
Laced and swept valleys: tile all the way down
On older clay plain-tile roofs, and on stone roofs along the Cotswold edge, the valley was often built from the covering itself, with no metal showing.
Laced valleys
In a laced valley the tiles from both slopes are interwoven course by course around the angle. Curved valley tiles, or ordinary tiles cut to a taper, are laid so each course on one side overlaps its partner on the other. The result is a rounded, all-tile channel. Water runs over clay, not sheet metal, so the detail depends on an exact lap between hand-sorted pieces and a sound lining beneath.
Swept valleys
A swept valley turns the courses round the corner in a smooth curve. Cotswold District Council's 2000 technical guidance on stone slate roofing, which records traditional practice, says these valleys use specially cut slates with names of their own (a bottomer, lie-byes, side skews). It calls mitred and open-lead alternatives cheaper to build but less satisfying to the eye. That is a heritage view, and it matters on a listed house or inside a conservation area, where a change to the valley is a question for the council's planning team.
Where they fail
The cut pieces at the centre of the angle are small and often slip or crack. The lining under them, usually old felt or thin lead, wears out long before the clay does. And because the cutting is fiddly, patch repairs tend to leave the lap too shallow. A leaking laced or swept valley is a job for a roofer who can cut and sort tiles, not only fix a strip of metal.
Mitred valleys and the mortar that splits
A mitred valley is the shortcut. The tiles on each slope are cut at an angle along the valley line and butted together, usually bedded in mortar, with a strip of lead or underlay beneath. From the ground it looks tidy: no channel, no lead.
Why the join cracks
The weak point is the mortar. A mitre is a straight cut in a roof that moves, held by a thin line of cement or lime that cannot flex. Frost, sun and frame movement open it. Once it splits, rain goes straight onto the lining, which was never meant to carry it, and moss in the crack holds damp against the timber.
What we check
We follow the mortar line end to end, lift a few cut tiles to see the lining and test the battens under them. If the lining is good and only the pointing has failed, the repair can be as small as raking out and re-bedding. If the lining is torn, tiles come off along the valley, new lining goes in and the tiles are re-laid.
Mortar alone is an unreliable seal in a valley. If a mitre keeps splitting after repointing, a lead-lined or GRP trough usually lasts longer.
Mitres turn up most often where a roof was recovered in concrete interlocking tiles, since the cutting is quick and the look is similar. Our guide to roof tile types explains how those coverings differ.
Lead and GRP valley gutters
The commonest valley on a tiled roof is a gutter lined with metal or plastic, with the tiles stopping short of the centre so a strip of lining shows between them. A narrow gap looks neat but clogs easily; a wide one sheds more water.
Lead
Lead sheet is sold in codes that relate to thickness. Code 4 (1.80 mm) is the usual choice for flashings, and Code 5 upward is kept for jobs where more water has to be carried, such as a pitched valley gutter. MyJobQuote (updated September 2026) says correctly fitted lead flashing can last over 80 years, and longer on higher codes. Lead is laid in separate lengths, since the metal moves with the temperature. Long runs buckle and split along the lap, which we find on valleys lined in one piece, often in a thin code.
GRP
GRP, often called fibreglass, is laid wet over a timber deck and cures as a single piece, so it has no laps to open. Less (updated March 2026) gives 25 to 40 years for GRP on flat roofs; a valley is a different shape, but the material is the same. The trouble is the edges. Where resin runs up under the tiles, they need a clear air gap, or damp gets trapped. Resin also fails where it was laid on damp boards or where the frame has moved.
Usual faults
Splits at the laps, leaf litter damming the channel until water backs up under the tiles, and tiles cut too close to the centre so they overhang the lining and throw water at its edge. Fixing the fault is half the job. We also clear the channel and tell you how often to look at it, since a clogged valley can undo a good repair.
Dry valley troughs: the manufactured option
A dry valley is a factory-made trough, usually rigid plastic, fixed down the valley rafters. The tiles each side are cut to the edge and sit on or clip to the trough. There is no mortar and no wet work.
The test standard is BS 8612, which Marley describes as the test for dry-fixed verge, hip and ridge products. The thinking matches why BS 5534:2014 stopped treating mortar alone as a fixing for ridge and hip tiles: a manufactured system is tested, while hand-bedded mortar depends on workmanship. A dry valley goes in to its maker's instructions, so the trough, fixings and tile cutting depend on the product.
Why they fail
First, a wrong fit. The trough is made for particular tile profiles, and cutting a different tile to sit in it leaves gaps. Second, the ends: where the trough meets the ridge at the top and the gutter at the foot. If it was laid short, or the end lap is shallow, wind-driven rain gets beneath it. Debris also collects in the ribbed edges.
When to choose one
A dry trough suits a roof with concrete interlocking or modern clay tiles where the old valley has failed and the rest is sound. It is the wrong answer where the valley is a feature of a historic roof. The council decides what may change on a listed building, and the diocese does for a church in use.
Reading the signs before the ceiling stains
Most valleys show distress before they leak. The signs worth acting on:
- Moss or weed along the line, which means the channel is damp and holding debris.
- Slipped or cracked cut tiles beside the lining, showing as a gap or sagging edge.
- Lifted or curled lead at the channel edge, or white corrosion marks on it.
- A brown tide mark on the ceiling below the valley line, often with a damp smell in the loft.
- Daylight or dark, damp timber along the valley rafters.
What the loft shows
Inside the loft, the valley rafters run diagonally and the underlay should be unbroken across them. Staining on the timber, felt that has slumped or torn, or wet insulation points to a valley that has leaked for a while. Rot in the valley rafter itself is structural; our guide to roof structure defects covers it.
Not every stain is the valley
A leak below a valley may start at a chimney, a roof window or a dormer cheek. The guide to dormer roofs explains where a dormer meets the main slope, a junction that often feeds a valley. Tracing the true entry point first stops the same repair being paid for twice.
Repair options, from re-dressing to re-lining
The repair follows the valley type and what lies beneath it. In rising scale:
- Clear and re-dress. Remove debris and moss, reset a few tiles and flatten lifted lead. This works when the lining is sound.
- Patch the lining. A split in a lead strip can take a lapped or welded patch, or a section can be replaced.
- Re-line the valley. Tiles come off both sides, old lining and underlay come out, rotten battens are renewed, new lining goes in with proper laps and the tiles are re-laid.
- Change the type. A failing mitre can become a lead or GRP trough, or a dry valley can replace a worn lined one. We agree the change with you first; on a listed or conservation-area home you check with the council.
Typical prices
National figures show the scale. MyBuilder (updated August 2026) puts roof repairs at £150 to £1,500, with minor jobs at £100 to £300 and moderate ones at £500 to £1,000. Checkatrade (October 2026) gives lead flashing repairs as £50 to £70 per metre, and stepped flashing in Code 5 as £80 to £120 per metre, both excluding VAT. A valley re-line costs more than a flashing repair because the tiles have to come off and go back. Our tiled roof repair cost guide lists what moves a quote.
These are third-party national ranges, not our prices. Our written quote follows a survey and covers access, tile lifting, lining, battens, labour and waste.
Opening a valley in bad weather, and keeping it clear
A valley is a poor place to leave open. With tiles off and lining out, the rafters are bare and one shower can soak the insulation below. When the forecast is unsettled we strip only as much valley as can be closed in the same visit, and we cover the rest with sheeting fixed to the rafters, not just weighed down. Our guide to tiled roofs in winter sets out which jobs are safe in poor weather and which wait.
Keeping a valley clear
Most valley trouble is avoidable by looking. Overhanging trees drop leaves and twigs into the channel, and a clogged valley backs up under the tiles. A check once or twice a year, and after storms, catches debris before it does harm. Do not walk on the roof; use binoculars from the ground, or ask a roofer who is already on site for another reason.
What comes with the work
Valley repairs carry our 10-year workmanship guarantee in writing. We photograph the valley before and after, and take waste away. If a valley is staining a ceiling, ring 01453 933613 or message photos by WhatsApp or through the enquiry form, and we will say what we suspect before anyone climbs a ladder.