Christmas Decorations for the Interior Home

A Guide To Roof Valleys

by Russell Lowe

If your roof has multiple planes beyond a simply peaked or hipped roof design, then it likely has at least one roof valley where two planes meet at the low point. Roofing these in a manner that prevents leaks can be a challenge.

Types of Valleys

There are two main types of roof valleys — open and closed. Open valleys are not shingled over. Instead, metal flashing is installed over layers of moisture and weather guarding materials. Water flowing off the two opposing roof planes enters the flashed metal valley trough and ideally flows off the roof.

Closed valleys typically feature the same layers of moisture guard and metal flashing, but shingles or a rolled roofing material that resembles shingles is installed over the top to create a seamless appearance.

Common Issues

Leaks are common in roof valleys. They can occur when a valley isn't sealed and flashed properly, which results in gaps that allow water beneath the seam. Backflow leaks caused by a cross wash of water are another issue. When water flowing off one plane hits the valley at a high speed, it may cross over the valley and wash a short way up the opposing roof plane and beneath the shingles. This leads to a leak once water flows beneath the shingles.

Ice dams also tend to form on roof valleys. These are often the result of dirty gutters, but in some cases, it is the construction of the valley that leads to water collecting on the roof and freezing into a dam. A roofer will have to survey the area to determine if the valley is at fault for the dam formation. 


Leak prevention begins with the right flashing. Thicker flashing materials are less prone to corrosion or damage from hail, weathering, or debris. The shape of the flashing also matters, particularly in open valleys. Avoid V-shaped flashing and opt for W-shaped flashing. The W-shape is more effective at stopping cross wash from occurring due to the additional ridges.

For a closed valley, the important aspect is how the shingles are placed at the meeting point. Woven shingles look nice by creating a smooth valley trough, but they may inhibit water flow off a steep plane that meets a lower slope plane. Instead, opt for a cut valley to protect against water flow issues. A cut valley extends the shingle plane of the low slope-side across the valley and slightly onto the steeper plane. 

Contact a residential roofer for more assistance if you are concerned about the valleys on your roof. A residential roofer can provide further information.