What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Close, not identicalvalley
also open valley, swept valley, laced valley, valley gutter, valley trough
A SWEPT valley curves whole tiles round the angle; a LACED valley interleaves them over a wider tile. Both are craft details in plain tile with no equivalent in a profiled or sheet covering.
Australia
Close, not identicalvalley
also valley gutter, valley tray, valley iron
Almost always a metal valley tray beneath the covering, since the dominant coverings are profiled tile and sheet and neither can be swept.
United Statesyours
Same word, different thingvalley
also open valley, closed-cut valley, woven valley, California valley
A WOVEN valley interlaces shingles from both slopes across the angle with no metal at all, which is only possible because an asphalt shingle bends. No British tiled roof can be woven.
Canada
Close, not identicalvalley
also open valley, closed valley, valley flashing
As the United States, with open metal valleys preferred in snow country because a woven or closed valley holds ice.
The governing standard, by market
United Kingdom
BS 5534 / BS EN 12588
Slating and tiling including valley construction, and rolled lead sheet for valley linings with maximum piece lengths.
United States
IRC R905.2.8.2 / ASTM D1970
Valley lining requirements by roof covering type, and self-adhering membrane beneath valleys.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Why do valleys leak more than the rest of the roof?
- Because they carry the water off two slopes through one narrow channel, so the volume per metre of detail is far higher than anywhere else on the roof — and because every unit along a valley is a CUT unit. A cut tile or slate is smaller, has lost the part of itself that was nailed, and often ends up held by adhesive or by a single fixing at an awkward angle. Add that the valley is where debris collects, that it is the last place to dry, and that in cold climates it is where ice forms first, and the concentration of risk is obvious. The practical consequence is that valley detail is specified rather than left to the roofer, and that a leak reported somewhere in the middle of a roof very often traces back to a valley above it.
- What is a swept valley, and why can it not be done in every tile?
- It is a valley formed entirely in the covering, with no lining at all: the courses are carried round the angle in a curve using tapered tile-and-a-halves and purpose-made valley tiles, so the roof appears continuous through the intersection. It is possible only with a small flat unit — plain tiles or slates — because the curve is made by rotating many small pieces slightly, and a large profiled tile cannot rotate without breaking the interlock. It is expensive, it is slow, and on the right building it is the only detail that looks correct, which is why it survives on historic and high-quality work. A LACED valley is the related detail using a wider tile bridging the angle. Neither is available on an interlocking concrete tile roof, where the answer is a lined open valley.
- How wide does an open valley have to be?
- Wide enough for the water and the debris, and the second one governs more often than the first. Guidance gives a minimum exposed width for the lining — typically well over a hundred millimetres of clear channel between the cut edges of the covering — and it widens as the roof area draining into it grows and as the pitch falls, because a shallower valley moves water more slowly and backs up further. But the real-world sizing question is blockage: a valley narrow enough to be bridged by a lodged leaf pile or a tennis ball will surcharge under the tiles either side of it, and the resulting leak is nowhere near the valley. That is the argument for the wider channel and for clearing valleys as maintenance rather than waiting for a symptom.