Recommended Roof Pitch for Snow

Measuring Pitch

• Roof pitch measures how high a roof rises for every 12 inches. If every 12 inches of a roof rises 4 inches, the pitch of that roof is 4:12. Steep roofs are more expensive to build than flat roofs, but they shed snow more easily. They cost more to build because they require a larger interior support than flat roofs and workers cannot walk on them to install shingles, tiles or other covering. To encourage snow to slide off, your roof should have a minimum pitch of 3:12, dropping more than 3 feet for every 12 feet of roof. A roof pitch less than 3:12 is considered flat.

Type of Snow

• Dry or powdery snow weighs about 7 pounds per square foot. Most snow weighs an average of 15 to 20 pounds per square foot (psf), but wet or compacted snow weighs even more. Most roofs in areas with snow are designed for loads of 20 psf. Whether snow is usually dry or wet depends primarily on geography.

Geography

• If you have heavy snowfalls or the snow in your area is usually wet, you should consider a 4:12 pitch. Steep roofs also drain melting snow better. If you live in an area where heavy rains commonly fall after heavy snow, the load on your roof will jump dramatically. Drifting snow pushed by high winds can cause loads of 50 psf, with reported loads up to 300 psf, and is the greatest cause of roof failures.

Composition and Design

• Metal roofs are the most apt to collapse under heavy snow loads. More snow accumulates on unheated barns, sheds and other outbuildings than on heated homes with warm roofs. A wooden roof might be able to withstand a heavy load of wet snow for days or weeks and then collapse. Melting snow on sloped, warm roofs often refreezes, causing ice to back up under shingles or icicles to form on eaves. Drifting and sliding snow causes heavy loads in the “valleys” of sawtooth and barrel vaulted roofs. Snow from sloped or gabled roofs collects on lower bays of multilevel roofs. If your roof does not adequately slope into drains, melting snow can accumulate in low areas, a phenomenon called “ponding.” Both flat and curved roofs are prone to ponding.