Mastering Roof Inspections: Asphalt Composition Shingles, Part 16

Valleys are created where two roof slopes meet. Valleys are especially vulnerable to leakage for three reasons:

1. concentrated runoff:  The combined runoff from both roof slopes is concentrated in the valley, so valleys carry more runoff than other parts of the roof;
2. interrupted roofing:  The roof-covering materials are interrupted by the change in roof direction. Any interruption in the roof-covering material increases the opportunity for leakage; and
3. less slope:  Although they carry more runoff, valleys slope less than the surrounding roof, so, in addition to carrying more runoff, they drain it more slowly. The chances for leakage increase as the slope of the roof decreases.

Where the rest of the roof slopes at an angle, such as 4:12, where the roof rises 4 inches vertically in every 12 inches of horizontal run…

The reason for this is that, although valley rafters rise the same total distance as common rafters, they do it over a longer distance because valley rafters are oriented diagonally to the ridge, and common rafters are oriented perpendicular to the ridge.

Valley Lining

Different methods have been used over the years to line valleys. It can be difficult to tell exactly how the valley is lined on existing homes, so your best bet is to look at the lower end of the valley at the roof edge.