From ground snow to roof snow load
The flat-roof snow load is derived from the ground snow load using exposure, thermal, and importance factors. The ground snow load comes from local building codes or ASCE 7 maps and represents the 50-year return-period ground snow depth converted to pressure in pounds per square foot.
Sloped roofs shed snow more readily, so a slope reduction factor is applied for pitches above a code-defined threshold. Unheated or partially exposed structures use different thermal factors than fully heated buildings.
pf = 0.7 x Ce x Ct x Is x pg
pf is the flat-roof snow load, Ce is the exposure factor, Ct is the thermal factor, Is is the importance factor, and pg is the ground snow load.