Swept valley
What is a swept valley on a stone roof?
The short answer
A swept valley is a valley, the line where two roof slopes meet, finished by curving the stone courses round the corner instead of laying a visible metal trough. The slabs are cut and graded so the turn sheds water smoothly, with a lining usually hidden beneath. It looks neat, it demands skilled hands, and it needs checking at the survey because it is a likely place for a leak to start.
How a swept valley is built from Horsham slabs, what hides underneath it, and the signs that this one joint has begun to let water in.
Where two slopes meet and the stone turns
A valley is the inside corner formed where two roof slopes run into each other, like the crease of an open book. All the rain from both slopes runs down into it, so it carries more water than any other line on the roof. That makes it the joint every roofer treats with respect.
On many roofs the valley is a visible channel, with tiles stopping short on each side and lead or some other metal laid in the gap. A swept valley hides that channel. The stone courses on each slope curve round the corner in an unbroken sweep, so the valley reads as a soft, rounded line rather than a hard groove. From the lane it looks as if one slope simply flows into the other.
The effect suits stone. Horsham slabs vary in colour and thickness, and a sweep lets that variety carry through the corner without a strip of metal interrupting it. Our guide to diminishing courses explains the grading from large slabs to small, which is the thing a swept valley depends on.
How the sweep is built
Making the turn takes more than bending a row. Each slab at the corner is chosen, trimmed and bedded at an angle so that it overlaps its neighbours on both slopes and leaves no gap through which water can run. The slabs are generally smaller and narrower along the sweep than across the open slope, because a big slab cannot follow a tight curve without leaving open joints.
Underneath, a swept valley is not held up by stone alone. A lining normally sits below the slabs to catch any water that creeps past the joints, much as a gutter catches what runs off a roof. What that lining is made of, and how well it has aged, varies from house to house. It may be lead, it may be a later patch, and it may be nothing you can see without lifting slabs.
Lead is the usual choice for valley linings in the trade, and lead sheet comes in numbered codes by weight. Our guide to lead valley replacement covers the lining side in detail.
Why the valley is a likely place for a leak
A valley takes the water from two slopes at once, so any weakness there is tested more often than elsewhere. Several faults commonly show up at the same spot.
- A slab at the sweep has cracked or slipped, which opens a path to the lining.
- The lining beneath has split, lifted or worn through with age.
- Leaves, moss or other debris has built up in the curve and held water against the stone.
- An earlier repair has used the wrong material and shifted the way water runs.
A damp stain on a ceiling below the line of a valley is the usual first sign. Moss that grows thicker along the sweep, or water running down an inside wall in heavy rain, can say the same thing. Our answer on valley repair traces a leak from ceiling stain to source. Moss in the curve matters too, and moss on a stone roof says how to deal with it.
The fault is not always in the stone. Often the sweep is sound and the lining under it has gone, which is why the repair starts with looking, not guessing.
Repairing a swept valley without spoiling it
A small fault can be mended in place. A matching reclaimed slab can replace a cracked one at the corner, and debris can be cleared out of the curve. If the lining below has failed, the slabs along the sweep are lifted, numbered and set aside, the lining is renewed, and the same slabs are laid back in the same order.
The skill lies in keeping the sweep true. A valley relaid badly can turn out too tight, with water backing up, or too loose, with gaps showing light. A roofer who has not worked on stone before may be tempted to swap the whole sweep for a lead-lined channel because it is quicker. That changes the appearance of the roof, and on a listed building it may need consent.
Our repair or relay answer helps decide how far the work should go. We recommend the least work that properly mends the fault, and the survey photographs show where that line falls.
Cost, consent and the next step
A swept valley has no standard price. The cost turns on how long the valley runs, how many slabs must come off, whether the lining needs renewing, how far the roof is from the ground, and whether scaffolding is needed. Checkatrade's slate guide gives about £160 to £210 per square metre for slate in general, not Horsham stone, but a single valley is a small area of skilled work, so that figure is only a reference point. The written quote arrives after the survey, with a single total.
On a listed building, or within a conservation area, settle the consent position first. Works that match what is there are handled differently from changes to appearance, and the council's conservation officer can say which applies.
To start, send us photographs of the stain, the valley and the loft below it. We read it against the whole Horsham stone roofing picture and tell you what we see before we mention money.
Related questions
Swept valley queries
Is a swept valley harder to repair than an open lead valley?
Usually, because the slabs hide the lining and must be lifted to reach it. An open lead valley can be seen and worked on directly. Which is simpler on your roof depends on what the survey finds.
Can moss in a swept valley cause a leak?
It can hold water against the stone and choke the curve, so it is worth clearing gently, without prising slabs loose. A thick cushion along the sweep is also a prompt to look at the lining beneath.