Ground Snow Load vs Roof Snow Load: The Difference
Why roof snow load is usually less than ground snow load, and the exposure, thermal and importance factors that connect them.
People often use 'snow load' to mean both the snow on the ground and the snow on the roof, but ASCE 7 treats them as two different numbers connected by a formula.
Ground snow load
Pg is what accumulates on open ground. It is a fixed property of your location and the input to the whole calculation.
Roof snow load
The flat-roof load Pf = 0.7 × Ce × Ct × Is × Pg is what the roof actually carries. The 0.7 factor reflects that wind and a little melting remove some snow from a roof compared to open ground. Exposure (Ce), warmth (Ct) and importance (Is) then adjust it up or down.
So which is bigger?
On a normal heated, partially sheltered house the roof load is about 60–70% of the ground load. But a fully exposed, unheated or essential-facility roof can have factors that push the roof load close to the ground value, and occasionally above it. Always run the conversion rather than assuming.
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