The short answer
A slate valley is usually built one of two ways. An open valley has a lead-lined channel with the slates cut to a straight line on each side. A mitred valley has the slates cut to meet at the angle, with lead soakers hidden beneath. Both depend on well-fixed lead, and the valley is among the first places a slate roof leaks when that lead fails.
What a valley has to carry
A valley is the inside corner of a roof, the line where two slopes lean towards each other, as at an L-shaped house or where an extension joins the main roof. Rain from both slopes runs into it, so a valley carries far more water than the ordinary slate courses either side. In heavy rain it fills like a small stream.
That load is why the valley is built with a waterproof lining beneath the slates and not just the usual overlap. On a slated roof that lining is almost always lead sheet. Lead bends into the angle by hand, resists rust, and follows awkward changes of pitch that a stiffer material would not.
For the roofer, the questions are simple. How wide is the channel, how thick is the lead, how far does it run up under the slates on each side, and what happens at the bottom where it empties into a gutter. A valley that is too narrow overflows, one that is too thin splits, and one that is lapped too little lets water creep sideways under the slates. Our slate roofing page covers the wider job.
Lead-lined open valleys
The open valley is the traditional form on many slate roofs. A lead channel is laid along the valley, and the slates on each side are cut in a straight line parallel to its centre, leaving a strip of metal visible between them. Water runs down the lead and the slates only have to guide it.
A lead code sets the thickness. For pitched valley gutters, Royston's cast-lead brochure lists codes 4 to 8, from 1.80 mm at code 4 up to 3.55 mm at code 8, and reserves code 3 for soakers only. A thicker code tolerates more movement before it cracks. The Approved Document C for England cites BS 6915 for lead sheet roofs, so a competent valley follows that standard.
Fixings have to let the metal expand and contract without tearing, so it is clipped along its edges and never nailed through the middle of the channel. A nail through the valley floor is a leak waiting to happen. When we survey a valley, we lift the nearest slates, check the width and condition of the lead and photograph what is under them, so you can see the same view.
Mitred valleys
On some roofs the owner or the planning team wants no metal on show. The answer is a mitred valley, sometimes called a closed or secret valley. The slates from the two slopes are cut to meet at the angle of the valley, so that the line looks like a continuous fold in the slating.
Underneath, lead soakers do the waterproofing. A soaker is a little L-shaped lead sheet that slips beneath each course of slates along the angle, with one arm resting on each slope. The slate cutting must be precise so each soaker lies flat and the cut edges do not leave gaps. It is slower and more skilled than an open valley, and it hides its working parts.
The compromise is that the working parts are hidden. Moss or leaf debris can clog the narrow line between the cut slates, and when it leaks it is harder to see why, because the lead is out of sight. Our answer on head-lap on a slate roof shows why the overlaps on either side still matter here.
When a valley leaks, and what repair costs
Valleys fail in predictable ways. The lead splits along a fold or at the point where it was nailed, a joint between two sheets opens, moss and leaf litter dam the channel so water backs up under the slates, or the slates at the edge slip and expose the lead's edge to the weather.
A stain on the ceiling along a line, in an L-shaped part of the house, can be the first sign. Debris can sometimes be cleared. A split needs the lead lifted and relaid or renewed, which means taking up the slates beside it and putting them back. A tired valley across a whole roof calls for more than a patch.
For a full valley replacement MyJobQuote gives around £540 (MyJobQuote, September 2026), a typical UK figure and not our price. The real cost turns on length, access and what the slates beside it need, and the survey fixes the price in writing. If the valley meets a lean-to or dormer, the neighbouring slate roof leaking answer shows where else to look.
