How valleys are formed on slate roofs

Elsewhere on the roof, each slate sheds water onto the next one down. In a valley, the run-off of two slopes arrives together, often with leaves, moss and grit riding along with it. That is why the valley has to be waterproofed by something other than the slates themselves, which is normally a lead gutter sitting on the rafters and boarding, under the slate edges.

Where the roof has an internal angle, such as an L-shaped house or a rear extension joining the main ridge, a valley is unavoidable. The line of the valley follows the junction of the two roof planes and runs from the ridge or a hip down to the eaves gutter. Because it is a channel, any blockage or split in it sends water sideways under the slates instead of out to the gutter.

Before the details, one term. The mitred form below relies on soakers, which are little folded tabs of lead slid under the slates to close a joint.

An open lead valley is the simplest to read from the ground. A lead gutter is laid in the angle, and the slates on each side are cut along a straight line parallel to the centre of the valley, leaving a width of lead visible between them. Water runs down the lead, never over the slate edges.

The lead itself is specified by code, which is a weight and thickness grade. Royston Lead's brochure for cast lead lists codes 4 to 8 for a pitched valley gutter, with code 4 the lightest of those at 1.80 mm. A longer or heavier-use valley normally takes a thicker code than a short one.

Lead is durable. MyJobQuote (2026) puts the life of correctly fitted lead flashing at over 80 years, and over 100 for higher codes. That figure describes flashing, though, and a valley bears more water than a flashing does. On our surveys the lead is rarely the first thing to give; more often it is the slates beside it, the nails holding them or the debris sitting in the channel.

The cut edge

Every slate along an open valley has been cut to a line. Those cut edges are the weakest slates on the roof, because a cut slate has less headlap and fewer fixings than a full one. A neat run of cut slates, each fixed twice, is a sign of careful work.

In a close-mitred valley, the slates from both slopes are cut to a diagonal and brought tightly together over the valley line, with a lead soaker under each course. From above you see only slate, with a thin diagonal join. It looks tidy, but the join is only as good as the soakers beneath it, and those are out of sight.

A swept valley takes a different route. Instead of cutting a straight line, the slater lays specially cut slates that curve round the angle, so the slope appears to flow from one plane into the other with no lead showing. Cotswold District Council's 2000 guidance on stone slate roofing, which describes the same idea in stone, names the special pieces as the bottomer, the lie-byes and the skews. That guidance is about stone, not slate, yet the principle is the same. It adds that the cheaper close-mitred and open lead alternatives look less satisfactory than the traditional swept valley, and are best avoided on those roofs.

Where a conservation officer has a say, the valley type can affect how the whole roof reads. Where the house is listed or sits in a conservation area, Stroud District Council's planning team should hear about any change to a visible valley first.

Here is what a valley survey tends to turn up, roughly in the order it shows.

• Blockage. Leaves, moss and slate fragments pile up in the channel. Water backs up, rises over the lead's edge and tracks under the slates. This one is simple to cure and easy to miss.
• Split or cracked lead. Lead expands and contracts with temperature. If it was laid without room to move, or in too thin a code, it can crack along the fold. A crack shows up as a damp stain on the ceiling below or on the rafters in the loft.
• Slipped or cracked cut slates. The cut edge loses its fixing first. Once a slate beside the valley drops, the lead is exposed to the weather and the next slate loses its support.
• Failed soakers. In a mitred valley, a rusted or missing soaker is invisible from outside until the water gets in.
• Nail fatigue. Old fixings corrode and let go; our guide to slate roof repairs covers this and the other common faults across the whole roof.

Repair depends on what the survey finds. A blocked channel needs clearing and a check that the lead is intact. A small crack in good lead can sometimes be repaired in place, but where the lead is thin, patched or buckled, the better course is to lift the adjacent slates and lay new lead to a suitable code. Neither job is quick, because the neighbouring slates must be lifted and relaid.

Replacement slates have to match the rest of the roof in size, thickness and colour. If you are fitting new slates against old weathered ones, see our answer on matching roof slates, and if the original slates came from Spain, our note on Spanish slate quality explains what to check.

Cost depends on length, access and whether scaffold is needed. Checkatrade's roof repair guide, updated in 2026, puts natural slate repair at £150 to £220 per m² before VAT, with access excluded. A valley is usually a narrow strip, so the area is small, but the work around it is fiddly. The written quote after our survey sets out each stage of the work.

We give a written 10-year guarantee on the workmanship. The rest of the trade is on our slate roofing page.