Why valleys have a lower slope

Roofing · Valley Flashings · printed page 17 (PDF page 18)

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Slope of Roof Valleys Valleys Roof Slope Valley Flashing On Roofs

Text in the illustration

valley pitch
2.8 in 12
roof pitch
4 in 12
valley pitch=
4 in 17
or 2.8 in 12
rise=4
roof pitch=4 in 12
run=12
12
horizontal run=√(12²+12²) ≈ 17

Generated explanation

The upper drawing shows two intersecting roof planes, each with a pitch of 4 in 12, meeting at a valley marked with a narrow white strip. The valley has a lower pitch of 2.8 in 12. The lower geometric diagram compares the roof-plane slope with the diagonal valley slope using the same rise of 4 units. Two perpendicular horizontal runs of 12 units produce a diagonal horizontal run of approximately 17 units, giving the valley a pitch of 4 in 17, or approximately 2.8 in 12.

Components

intersecting roof planes roof valley roof ridges eaves gable edges