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Flat Roof Snow Loads Flat Roof Snow Loads

Engineering Analysis

Snow loads are influenced by elevation, general weather and moisture patterns, slope direction, exposure, roof (or trail bridge) configuration, and wind direction and severity.

Overestimation of snow loads can unnecessarily increase the cost of construction. Underestimation of snow loads can result in premature failure, high maintenance costs, resource damage, and, in some cases, safety issues. The following are the equation for determining snow loaing on flat roofs.

Flat roof snow load are determined by the following method:

The snow load, pf, on a roof with a slope equal to or less than 5 degrees◦ 1 in./ft = 4.76 deg.) shall be calculated in lb/ft2 (kN/m2) using the following formula:

pf = 0.7CeCt I pg

but not less than the following minimum values for low slope roofs


pg is 20 lb/ft2 (0.96 kN/m2) or less,

pf = (I ) pg (Importance factor times pg)

where pg exceeds 20 lb/ft2 (0.96 kN/m2)

pf = 20(I ) (20 lb/ft2 times Importance factor)

Exposure Factor,Ce. The value for Ce shall be determined from Table E-2.

Thermal Factor, Ct . The value for Ct shall be determined from Table E-3.

Importance Factor, I. The value for I shall be determined from Table E-4.

Minimum Values of pf for Low-Slope Roofs. Minimum values of pf shall apply to monoslope roofs with slopes less than 15◦, hip and gable roofs with slopes less than the larger of 2.38◦ (1/2 on 12) and (70/W) + 0.5 with W in ft (in SI: 21.3/W + 0.5, with W in m), and curved roofs where the vertical angle from the eaves to the crown is less than 10◦.

See Symbols and Notations used in Snow Loads Design Data for symbol data

Reference: American Society of Civil Engineering/Structural Engineering Institute Standards

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