Valley Installation

A valley is the internal angle where two roof slopes meet. Because it collects water from both slopes, it carries more flow than any other part of the roof and is a common source of leaks. On asphalt shingle roofs there are three main valley methods: open, closed-cut and woven. Tile, slate and metal roofs use their own valley details, usually with a metal valley gutter.

Valley lining

All methods start with a lining. Canadian guidance centres a 1 m (39 in) wide strip of felt, self-adhering underlayment or modified bitumen sheet in the valley, then trims the field underlayment so it overlaps that strip by at least 150 mm (6 in). The lining must be in place before shingling begins.

Open valley

The valley is lined with corrosion-resistant metal (or mineral-surfaced roll roofing) and left exposed.

• Metal at least 600 mm (24 in) wide, nailed at its edges every 300-450 mm (12-18 in) without puncturing the centre.
• Joints lapped 300 mm (12 in) in the direction of flow and set in cement.
• Chalk lines 100 mm (4 in) from the centre at the top, widening toward the bottom by about 10 mm per metre (1/8 in per foot).
• Shingles trimmed to the lines, upper corners clipped and the edges set in a band of plastic cement, with no exposed nails.

Closed-cut valley

Shingles from the lower or smaller slope are run through the valley at least 300 mm (12 in) onto the adjoining slope. Shingles on the other slope are laid over them and cut along a line at least 50 mm (2 in) back from the centreline, with the cut edges set in cement. No nail should be within 150 mm (6 in) of the centreline.

Woven valley

Both slopes are shingled at the same time, with courses alternately woven across the valley and each end shingle extending at least 300 mm (12 in) onto the other slope. Whole shingles are used so no joint falls in the valley. Woven valleys work best with flexible 3-tab shingles; many manufacturers do not allow them with thick laminated shingles.

Other coverings

On tiled roofs in Europe and Australia, valleys are usually lined with lead, aluminium or preformed valley troughs, with tiles cut and mechanically fixed alongside. Debris build-up in valleys is a common maintenance issue on all roof types.