What is a dead valley and are they a problem for the roof?
You may notice, although it can sometimes be hard to see, some areas where the roof drains onto lower pitched or reverse pitched areas that don’t naturally drain. This is known as a dead valley.
Dead valleys occur when two slopes come together in a way that does not allow proper/natural drainage or very slow drainage. In a dead valley there is essentially nowhere for the water or leaves to go. This can be an expensive repair because the debris that sits in that area with water can rot the wood and shingles and can cause premature damage.
A modified bitumen application should be applied to keep the water from reaching framing and causing damage and leaks. If you were to use felt paper and shingles in a dead valley, the material when exposed to the sun will wear out and you could probably expect to have a leak in 3-5 years. Water naturally runs downhill. It usually runs down the slope of your roof, into the gutters, and away from the home. In a dead valley there is nowhere for the water to go and the water can collect and drain very slowly; add leaves, twigs, and other debris that all hold water and you have the recipe for an expensive roof repair.
Our business practice is to use a modified bitumen application, usually a self-adhesive but sometimes a torch down application. This is a granulated roll installed in three feet sheets, it is mostly used on commercial flat roofs and is well adapted for residential application, especially dead valleys or other troublesome spots.
Repairing a dead valley isn’t really a DIY project for most homeowners. It is best to have a professional roofing company examine your roof and recommend the proper materials to safeguard your home. If you are planning a new build, consult with your architect and ask that their design avoid as many dead valleys as possible.