A dead valley is one that has little or no slope, which can occur between two parallel roof slopes. This causes water to drain slowly and may cause puddles of standing water. A dead valley is prone to leakage and it should be fixed with a saddle at the shared valley that slopes towards the end, or ends, of the valley—as in the diagram above. 

    Because a saddle over a dead valley is often a much lower slope than the roof, an asphalt shingle roof must have a different roof material designed for low-slope applications at the saddle. Two examples would be modified bitumen or valley metal.The absolute minimum slope allowed for application of asphalt shingles is 2/12 (2” of rise for every 12” of run).

     Shingles on a saddle with a shallow slope is a recurring problem we see in residences with a complicated roof line. Shown below is a saddle covered with the same shingles as  the roof. The surface is visibly more deteriorated than the rest of the roof, and this will be the first area to leak as the roof ages because shingles are not designed to be watertight. They are installed with overlaps to shed water that rolls down the roof surface.

    Also, see our article What are common problems of asphalt shingle roofs?

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Field Guide for Home Inspectors, a quick reference for finding the age of 154 brands of HVAC systems, water heaters, and electrical panels, plus 210 code standards for site-built and manufactured homes, and the life expectancy rating of 195 home components. Available at amazon.com for $19.95.
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  To learn more about roofs, see these other articles:

• How can I tell if a roof has more than one layer of shingles?

• Why is my roof sheathing sagging between the trusses?

 • Why is my roof leaking?

• What causes leaks at a fake roof dormer? 

• What causes a sagging roof ridge line?

 • Why does it cost so much more to replace a steep roof than a low slope roof? 

• How can I inspect my roof for hurricane damage?   

• What's the average lifespan of a roof?

• Why is it a mistake to replace a roof and not replace its flashings? 

• What can I do to prevent roof leaks?

• Are roof trusses better than roof rafters (stick framing)? 

• If my roof is not leaking, why does it need to be replaced?

• How can I be sure my roofing contractor got a permit?

• How can I make my roof last longer?  

• How can I find out the age of a roof? 

• Should I buy a house that needs a new roof?

• Why do roof edges start leaking?

• What's the difference between an "architectural" and a regular shingle roof?  

• Do stains on the ceiling mean the roof is leaking?

     Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.