Good Roof Pitch for Snow

Roof Design Considerations

• Buildings where heavy snow loads routinely occur must have roofing designed to withstand the added weight. Proper pitch allows snow to slide harmlessly from the roof before heavy precipitation accumulation causes roof failure or dangerous snowslide conditions. Snow is less likely to slide off flatter roofs than those with a greater pitch. Older buildings were often built with little insulation, so snow melted more quickly and extended periods of excessive snowload were less commom.

Pitch Calculation

• Roof pitch is calculated by determining the amount of rise over a run of 12 units on the surface of the roof. If, over a run of 12 feet, the roof rises 4 feet, the pitch of the roof is 4 in 12. The pitch measurement in this example may often be written 4/12 or 4:12. An online calculator (Resource 1) is available to assist in the calculation of roof pitch.

Pitch and Roof Coverings

• A roof with a pitch of 2:12 or less is considered to be a flat roof. Appropriate materials used in the construction of flat roofs include tin sheeting and prefabricated plastic sheeting. Heavier roofing materials require a greater pitch to allow for the overlay of materials needed to keep the roof sound and waterproof. Slate and clay tiles require a greater roof pitch than traditional cedar shakes or asphalt shingles. Roofing materials are marked with pitch-appropriate application designations.

Good Pitch for Snow

• Snow tends to accumulate and cannot easily slide off a roof that has a pitch of 3:12 or less. A higher pitch is also required for a shingled roof, as snow slides less easily off this material than it would from a metal roof. If drifting snow is an issue in your area, consider a greater roof pitch to reduce the uneven snowload caused by windblown precipitation. Local contractors and building supply stores can advise on which roof pitch will work best for your geographical location.