Valleys

SLATE AND STONE VALLEYS

Valleys carry more water than any other part of a roof. Before the common availability of lead, slaters used skill and ingenuity to make a water-tight junction. They fall into three broad classes:

-those that form a raking cut in the main slates on each side and overlap a set of slates laid up a narrow strip (double cut);
-those that are cut on one side (the steeper pitch if there is a difference) and are swept on the other known as single cut or half swept, and
-those that carry the slate courses across the intersection without a break and in the same plane.

The former include Welsh, chevron and collar & tie, and the latter swept and laced versions. All of these types function perfectly well provided they are on a roof with a local traditional pitch. Because the valley pitch is less than the main pitch, roofs should never be designed at less than the local tradition.

Close mitred and open lead-lined valleys are common today but there is no reason to substitute them for the traditional types.

Collyweston laced

Chevron valley in Carboniferous sandstone

Close mitered and open lead valleys in Welsh slate

Welsh valley in sandstone

Cotswold limestone swept valley

Collar and tie. Devon and Cornwall

Collar and tie. Pembrokeshire

Welsh valley in slate

Close mitred Welsh valley. The mitred slates have been removed in the upper section to reveal the valley slates.

Valley types are generally restricted to their own regions: chevron in the Pennine counties; Welsh in Wales, collar and tie in the West Country and South Wales. The swept valley has a wide distribution following the Jurassic limestone belt from Dorset to Yorkshire. Slate versions are seen in Wales aa well as the half swept or single cut.

In the East Midlands where Collyweston limestone slates are produced the laced valley is the normal detail. But, because the slaters produced their own slates and traveled with them to wherever Collyweston slates were specified the laced valley went too. Even to America; to New York state.