Roof Snow Load Capacity

Designing Snow Load Capacity

• Structural design to ensure that a roof can handle the weight of snow is calculated according to the climate of the region and the use of the building. The design load also takes into account other loads on the building, such as wind and dead loads -- the weight of the building itself -- which can be two to four times greater than the weight of the snow. But heavy snow can cause structural failure of the roof if the weight exceeds the snow load capacity.

Snow Loading

• The weight of snow applies vertical loads on the roof, causing it to sag or bow downward. The weight also transfers horizontal forces to the walls, making them deflect, or move outward, either at the top or the bottom of the wall. When the weight of snow meets the load capacity, any sagging or deflection is temporary and will be gone when the snow disappears. But sagging and deflection is permanent when the snow load capacity is exceeded, and the roof may even collapse.

Weight of Snow

• The weight of the snow is more critical than the depth. This weight varies depending on the water content, which ranges from 3 percent for dry snow to 33 percent for heavy, wet snow. The water content of ice is nearly 100 percent. One inch of water weighs 5.2 pounds per square foot. Snow accumulation is especially critical on roof overhangs, valleys, and multilevel roofs, where the lower roof catches drifting or sliding snow from the upper roof.

Assessing Snow Load

• Signs that raise concerns about overload are creaking or moaning in the building, and bowing in any of the structural members, such as rafters or columns. The condition of the building should be considered when assessing the risk of snow overload. Poor construction and maintenance can cause a roof to perform below the snow load design capacity. Removing snow and ice from the roof will reduce the pressure, but safety and potential damage to the roof have to be balanced against the need.