Walk past a Cotswold house with a side wing and look at the line where the wing roof runs into the main roof. On a well-made old roof there is no metal gutter and no hard diagonal seam. The stone courses bend gently from one slope into the other, as if the roof had been poured round the corner. That detail is a swept valley. A valley is the inside corner formed where two roof slopes run together, so rain from both comes down it at once. It is the busiest water channel on the whole roof, so how it is covered matters more than almost any other part.
Here we cover why the swept valley exists, which special slates it is made from, why modern shortcuts in lead or mitred stone are less suitable, and what we check when one starts to leak. The technical details come from Cotswold District Council's 2000 stone slate guidance, which records traditional practice. For the wider picture of stone roofing, start at our Cotswold stone roofing page.
A roof that has to turn a corner
The plainest Cotswold roof is a single straight pitch running between two gable ends, and the council's guidance says that is the form most buildings took. A straight pitch has no valley at all. The trouble starts when a house grows a wing, a rear kitchen or a cross-gable, because then a second ridge has to meet the first at right angles. The guidance says that this need to turn a roof through 90 degrees on plan is what led to the development of the swept valley, and it calls the construction often the best example of the slater's art.
Think about what the stone has to do. On the two slopes the slates are laid in diminishing courses, long at the eaves and short at the top. At the meeting line those courses arrive from two different directions, each at its own angle, and they must hand over to each other without a gap that rain can find. A metal gutter solves that by sitting under the stone on either side and carrying the water away on its own surface. The swept valley solves it with the stone itself, by cutting slates to a changing shape so that each course follows a gentle curve round the corner.
Why the curve matters
A curve spreads the flow. Water running down both slopes gathers at the corner, and in a swept valley it travels over overlapping stone rather than dropping into a channel. Because there is no seam running the full length of the valley, there is also no single line of joints for water to follow all the way down. The detail depends on the slates being cut well and laid with generous cover, and that takes time at the bench as well as on the roof.
Most valleys we meet around Stroud sit where a later extension was added to an older house, or where an outbuilding was joined to the main range. Each one is a small piece of bespoke geometry, which is why no two look quite alike.
The cut slates: bottomer, lie-byes and skews
The council's guidance names the pieces of a swept valley, and the names are worth knowing because they describe what each slate does. In its tighter pattern, slates are laid two across the valley, alternating with three across. Where there are three, the middle slate is wedge-shaped and does not need a peg. It is called a bottomer, and it is held in place by the two slates either side of it, known as lie-byes or side skews. The next course up has to break joint, which means its vertical joints must not line up with those below, so two slates turn the valley and these are called skews.
| Slate | Where it sits | What it does |
|---|---|---|
| Bottomer | Centre of a three-across course | Wedge-shaped, no peg; held by the slates beside it |
| Lie-byes or side skews | Either side of the bottomer | Pinch the bottomer in place and carry the curve |
| Skews | The next course, turning the valley | Break the joint over the course below |
Why the wedge needs no peg
Ordinary stone slates are hung over the batten by a small cleft oak pin pushed through a hole close to the top edge. The bottomer sits at the very bottom of the angle, where a peg has no room to go and a hole would only give water a way in. It stays put because the lie-byes lock it from the sides and the courses above weigh on it. That is a neat bit of engineering, but it also means the wedge is the piece to watch. If a lie-bye slips or cracks, the bottomer loses its grip, and the first warning may be a ceiling stain below rather than anything visible from the ground.
Cutting to fit
Each wedge is cut from sound stone and dressed to the exact angle of its position. A skew that is a little short leaves too little cover; one that is a little long pushes the whole course out of line. We cut and trial-fit on the roof, which is slower than ordering a standard fitting from a catalogue, but a swept valley is not a catalogue item.
How the courses sweep through the angle
Every course in a swept valley is a small puzzle. The slates on the main slope run in horizontal rows. The slates on the wing run in their own horizontal rows, at right angles. In the valley, those two sets of rows must merge, and the merging slates are the ones that get cut.
The work usually goes from the bottom of the valley upward. The first courses set the curve, because the eaves slates are the largest and the curve is widest there. As the valley climbs, the slates get shorter and the sweep tightens, so the geometry changes on every course. A roofer has to look ahead as well as at the course under their hands, since a mistake low in the valley is repeated all the way up.
Cover and lap
Stone slates are double-lapped, so at any point on the valley two thicknesses of stone should lie between the weather and the batten. At the angle itself that rule is harder to keep than on a plain slope, because slates are cut down and the cover they offer shrinks. This is the reason a swept valley tends to use slates with a bit more length than the same course would have elsewhere, and why the roofer keeps the sound, long slates for the valley and uses the shorter ones on the open slope.
Underlay and battens in the valley
Under the stone, the valley needs more than the slope does. Battens are cut and fixed to follow the curve, and a strip of underlay sometimes goes beneath as a back-up layer. On a listed building we ask the conservation officer before adding anything unusual, since even a hidden layer alters the fabric. Our wider note on the verges and gable edges of stone roofs covers the other junction that meets the weather at the end of a slope, which is a different detail with its own faults.
Why a lead or mitred valley is less suitable
Many modern roofers leave out the swept valley. The council's guidance says they often insert a close-mitred or an open lead valley instead. It accepts that both cost less to build, yet says neither matches the traditional valley for looks, and advises steering clear of them. We agree with the direction of that advice, for reasons that go beyond appearance.
Open lead valleys
In an open lead valley a trough of sheet lead is laid down the angle and the stone slopes finish short of it on either side, leaving a strip of metal on show. Lead is a good material and we use it often elsewhere: on flashings, soakers (the small stepped pieces that tuck under slates against a wall) and abutments. Royston Lead's table lists codes 4 to 8 for pitched valley gutters, and the thickness of lead is set by its code, so a code 5 sheet is 2.24 mm thick and a code 6 is 2.65 mm. Lead properly fitted can last well over 80 years according to MyJobQuote. The difficulty in a stone valley is the way lead behaves beside a heavy, rough stone edge. The sheet expands and contracts with temperature, the stone does not, and the exposed strip changes the look of the roof from the lane for good.
Mitred valleys
A close-mitred valley cuts the slates from both slopes to meet on the diagonal, with a hidden lead strip beneath. The meeting line becomes a straight seam, a single path down which water can run, so it relies on that hidden strip to keep the roof dry. A swept valley has no such seam. Mitred valleys also ask the stone to be cut sharply on its edges, which suits a sawn slate better than a hand-split one and tends to leave thin, vulnerable corners.
If your valley has already been changed to lead or mitred work, that is not an emergency. It is a detail we can survey, photograph and explain, and we will say plainly whether a return to a swept valley is worth the cost on your roof.
Faults we look for in an existing swept valley
Two slopes drain into the valley, so it is usually the first place a stone roof shows its age. That is an observation from the roof, not a statistic, and we head for the valleys early on any stone roof survey.
Slipped or cracked slates
Frost is the main agent of decay in stone roofs, according to the council's guidance. Stone that soaks up water and then freezes can delaminate, which means it splits into thin layers along its bedding planes. In a valley the slates are cut, so the cut edge exposes more of the stone to that process. A delaminated lie-bye or skew leaves a gap, and the first heavy rain finds it.
Moss and debris
The curve of a swept valley collects moss, leaf litter and the grit shed from the stone itself. A damp mat of growth holds moisture against the slate, which feeds the frost damage. Gentle clearing keeps it in check, and our page on looking after a stone roof gives the wider routine.
Lost cover from past repairs
We often find valleys where an earlier repair swapped a damaged piece for the nearest slate to hand. A slate that is the wrong length or cut at the wrong angle throws out the cover on the course above, and the fault may take years to show. A shiny line of mortar or a dab of sealant in a valley is usually a sign that someone has tried to fill a gap that really wants a new cut slate.
What a repair involves
For a small failure, we lift the slates around it, replace the broken piece with a newly cut one in the same stone, check the batten underneath, and re-lay. For a valley that has gone widely, the courses are stripped back and rebuilt. Either way, the survey photographs show you the actual damage, and the quote covers scaffold, stripping, stone, labour and making good.
Cost, consent and a listed house
A swept valley is built as part of the slope around it, so its cost is worked out on survey rather than as a standalone item. The nearest national guide is Checkatrade's slate roof cost page, updated September 2026, which gives roughly £160 to £210 per m² for stone slate, with the quarry, new against reclaimed stone and your area all moving the figure. Nothing in it is specific to Cotswold valleys. Scaffold is a real item on a valley job because the work sits at the junction of two roofs: MyBuilder (2026) lists scaffold for a whole re-roof at about £1,125.
A valley repair is cut-and-fit work, so more of the cost is labour and cut stone than on an open slope. Your price follows the survey and arrives as a written quote.
Planning and building control
Repairing a swept valley in the same stone and form, normally needs no planning permission, because the roof looks the same once it is done. Changing a swept valley to a lead or mitred one does alter how the roof looks, so check with Stroud District Council's planning team first. Replacing the valley slates alone rarely goes near the Building Regulations thresholds, but a larger strip-and-relay can, and if the work is notifiable we see to that paperwork ourselves.
If the house is listed
On a listed building, the whole roof is protected, inside and out. A change from a swept valley to a lead valley would be an alteration affecting the building's character, and our guide to listed and conservation-area stone roofs explains the consent route. If you are weighing a cheaper substitute stone for the cut slates, read our note on reconstituted stone slates first, because cut work in artificial slate is a poor match for the shape a swept valley needs.
Once the valley is finished we photograph it, leave the site tidy, and put the 10-year workmanship guarantee into your hands in writing.