The short answer
Both work. A lead valley is a channel of sheet lead laid between the two slopes, with the slates trimmed back to leave it showing. A mitred valley has the slates cut to meet at an angle, each course resting on a small lead sheet called a soaker. Lead valleys are the usual choice on a repair, while mitred valleys give a cleaner line but ask more of the slater.
A valley carries the water from two slopes at once, so how it is built matters more than most other details on a slate roof.
What a lead valley is
A valley is the angled line where two roof slopes meet. In a lead valley, a long strip of sheet lead is laid down that line over timber boards, and the slates on each side are trimmed back to leave a clear band of metal between them. Water from both slopes runs onto the lead and straight down to the gutter.
Which lead code suits a valley?
Lead comes in numbered codes, each one a thickness. In the Royston Lead brochure for cast lead, code 4 (1.80 mm) and code 5 (2.24 mm) both appear among the codes used for a pitched valley gutter. The thicker sheet resists splitting better but costs more per metre and is harder to dress. We pick the code once we know the length and the pitch.
The lead also needs room to expand and contract as it warms and cools, so a good valley is laid in lengths with joints that allow for that. MyJobQuote says lead can last for over 80 years, which is a lower bound, though a valley that silts up or gets trodden on will fail well before then.
How a mitred valley differs
In a mitred valley no band of lead shows. The slates from each slope are cut at an angle to meet along the valley line, and every pair sits on a small sheet of lead, a soaker, tucked underneath to catch any water that creeps through the joint. The result is a tidy narrow line that suits small, fine slates on a smart front elevation.
Is a mitred valley more work?
Yes. Every course is cut by hand, every soaker is shaped to fit and the whole thing relies on accurate lapping. A poor one soon lets water in at the joints, because the gap is narrow and silt can sit in it.
For that reason many roofers choose an open lead valley for a repair, and keep the mitre for a full re-slate where they control each course. On a listed building, changing from one valley style to another needs asking of the council's planning team before any work begins, so raise it early.
A mitred valley also has less room for the debris that gathers over the years, so it benefits from a look every autumn.
Swept and laced valleys
A third option is the swept valley, where the slates themselves curve round the corner in a smooth arc, with no cut and no lead showing. It is laid with narrow slates that run in tapering courses, and lead soakers hidden beneath. It is skilled work, and only a slater who has laid one before should take it on.
Should I copy the old valley when I repair?
Usually yes. A swept valley that has lasted for generations is part of how the roof looks, and rebuilding it as an open lead channel changes the street scene. The survey should say whether the old slates are good enough to reuse, since a swept valley needs a supply of matching, tapering slates, and those are not always easy to find. Reclaimed stock from a roof being stripped elsewhere is one source, and the sound slates from a hidden rear slope are another. If neither is available, the discussion turns to whether an open lead valley would be a fair second best.
Matching matters here, and our answer on matching replacement slates covers size and shade. Where a swept valley has failed, we keep the style if the roof allows, and explain plainly if it does not.
Choosing, and what drives the price
The right valley depends on the roof, the budget and the look you want. An open lead valley is the hardest to get wrong and the easiest to inspect. A mitred or swept valley can look better, but it needs a slater who has built one before, and it gives less room for error.
What does a valley cost?
For a full roof valley replacement MyJobQuote gives around £540 as a typical UK figure (Sep 2026). What your own job costs depends on the length, the lead code, the access and how many slates must come off to reach the old valley. If the boards underneath have rotted, they are renewed in the same visit.
Leaks near a valley are common, and our answer on why a slate roof leaks explains how to trace them. Our guide to lead codes goes further on thickness, and our wider slate roofing service describes the remaining work we take on. Once we have looked at the valley from the ladder and the loft, you receive a single total in writing for the whole of it, with no surprises halfway through.
A valley that needs only a clean and a minor repair is quoted as exactly that.
More on slate valleys
How do I know if my valley lead has failed?
Look for a brown water stain on the ceiling beneath the valley, green growth along its edge, or lead that has split or lifted. From the ground, binoculars show a lot. Please do not climb up, since the slates beside a valley are slippery and easily cracked.
Is a hole in valley lead a repair job or a renewal?
One small split in otherwise sound lead may be repairable, but old lead that has thinned or cracked in several places is better renewed. Patching a tired valley tends to bring a new leak a few feet along. The survey shows which side of that line your valley sits on.
The full picture: Slate roofing in Taunton · More short answers