The short answer
A slate valley is usually one of three types. An open valley has a lead channel with the slates cut back from its centre line. A close-mitred valley hides lead beneath slates cut to meet at the angle. A swept valley curves the courses round the corner with specially cut slates. Lead is the lining in the first two, and the type decides how it is repaired.
What a valley does on a slate roof
Think of the inward fold between a rear wing and the main house, where one pitch runs into another: that fold is the valley. Rain from both slopes drains into it, so it carries far more water than the slates beside it. Whatever lines the valley has to cope with that flow in a downpour, and keep working while grit and moss collect along the bottom, the way they do on every roof that has trees nearby.
On a slate roof the lining beneath the slates is nearly always lead, because it can be dressed to the angle and does not crack in sun the way many plastics do. Lead sheet is sold in codes that run from 3 to 8. Leadworx and Royston Lead list Code 4 up to Code 8 as suitable for a pitched valley gutter, and Code 5 is the usual general choice. The thicker the code, the heavier the sheet and the longer it tends to stay flat in changing temperatures.
Open, close-mitred and swept: the three types
In an open valley the slates stop well short of the centre line, leaving a visible strip of lead between the two slopes. Water runs down that strip and the slate edges sit on either side of it. It is the simplest valley to build and the easiest to inspect, because you can see the lining from the garden.
A close-mitred valley hides the lead. Each slate is trimmed to the angle so the two slopes meet along the valley line, and a lead strip, often called a soaker, sits under every pair of cut edges. It looks tidier from the road, but the lead is out of sight, and each mitre depends on a slate cut accurately by hand.
A swept valley has no sharp line at all. The courses curve round the corner, using slates cut to a taper so the roof turns in a gentle sweep. Cotswold District Council's stone slate guidance describes the traditional swept form with specially cut slates. It says the cheaper alternatives, open lead or close mitres, do not look as satisfactory and are best avoided on those stone roofs.
Where slate valleys go wrong
Blockage is the commonest starting point. Leaves, moss and twigs pile up in an open valley, water backs up behind them and finds its way under the slate edges. Because the slates there are cut, not whole, they have less hold than the rest of the roof.
Lead is the second suspect. Sheet that was laid with laps too short, or fixed so tightly it could not move, splits along a fold as it warms and cools. Old lead can also wear thin where water has run down the same line for generations. A split in a close-mitred valley may sit under a slate edge where you cannot see it, so a ceiling stain can appear well away from where the water got in, and tracing it takes patience.
Third come the cut slates. Nails rust at the sharp end of a small slate, and the piece slips. Our page on nail sickness explains how that happens across a roof, and it tends to show first at the valleys, where every slate has been cut and the fixings are fewest.
Repairing a valley or relaying it
Where the lead is intact and the trouble is debris or a few slipped slates, the repair is small: clear the channel, refix or replace the cut pieces and check the laps. Where the lead is cracked in more than one place, patching a single split usually pushes the leak along to the next weak spot, and the better answer is to lift the slates each side and lay a new length.
Relining means stripping the cut slates back from both edges, which is why valley work can cost more than the area suggests. The slates come off, go back and some break, so spare slates that match the roof are worth having to hand. Our guide to matching roof slates covers how a replacement is chosen so the repair does not show.
Our survey photographs run the full length of the valley and along both cut edges, and the written quote follows from what they show. If a repair is enough, that is what goes in the quote, and if the lead has reached the end of its life we explain why a patch would not hold.
Weight, tools and getting access
Valley work is slow and fiddly because the roofer works in a narrow trough on a steep surface, often above a gutter or a lower roof. On a house of two storeys or more, a scaffold gives a safe platform and a place to lay slates down. Check whether a quote includes scaffolding and VAT before you compare it with another.
New lead adds little to the load, but a full relay changes how much slate and lead is on the roof at once, so temporary protection matters if rain is forecast. Sheet is kept clean and dry, and any joint is dressed in place rather than forced.
If you suspect a valley is leaking, you can look for a damp patch on the ceiling below and for stains on the timbers in the loft. Our guide to slate weight and our page on slate repair or reslating help you judge how far a valley fault should change the plan for the rest of the roof. Back to the main slate roofing page for the full service, and we back the work with a 10-year workmanship guarantee in writing.