ASCE 7-22 Chapter 7 | Flat and Sloped Roof Snow Loads
Snow accumulates on roofs differently than on the ground due to wind blowing snow off flat surfaces, heat escaping through the ceiling, and roof slopes causing snow to slide. The design roof snow load translates ground snow into actual roof pressure.
Code Equation
Governing Formula
Standard: ASCE 7-22Dual US Customary & SI Metric
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Notation & Variables
Flat roof design snow load
psf or kPa
Ground snow load from ASCE 7-22 national hazard maps
psf or kPa
Exposure factor (0.9 to 1.2 based on terrain and sheltering)
unitless
Thermal factor (1.0 for heated buildings, 1.2 for unheated)
unitless
Snow importance factor from Table 1.5-2
unitless
Worked Example
1
Determine ground snow load
Look up pg for site location: pg = 40 psf.
2
Determine factors
For a standard heated office (Ct = 1.0), fully exposed roof in Terrain B (Ce = 0.9), Risk Category II (Is = 1.0).
3
Calculate flat roof snow load
Multiply constants together.
When does this govern?
Governs roof truss and joist sizing throughout northern and mountainous regions.
Common Mistake
Ignoring snow drift loads at parapets and rooftop air handling units. Drifts often exceed the flat roof snow load by a factor of 3 to 5 times!