Mastering Roof Inspections: Asphalt Composition Shingles, Part 16

by Kenton Shepard and Nick Gromicko, CMI®



The purpose of the series “Mastering Roof Inspections” is to teach home inspectors, as well as insurance and roofing professionals, how to recognize proper and improper conditions while inspecting steep-slope, residential roofs. This series covers roof framing, roofing materials, the attic, and the conditions that affect the roofing materials and components, including wind and hail. 



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…

 ...a valley would rise 4 inches vertically in every 17 inches of vertical 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.

In newer construction, the valley is often lined with self-adhesive underlayment, which comes with a peel-and-stick backing. Another method used is to line the valley with a layer of 90-pound underlayment, similar to roll roofing.


Learn how to master a roof inspection from beginning to end by reading the entire InterNACHI series: Mastering Roof Inspections.

 Take InterNACHI’s free, online 
Roofing Inspection Course
Mastering Roof Inspections
Roofing Underlayment Types
Inspecting Underlayment on Roofs
Fall-Arrest Systems
Roofing (consumer-targeted)
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