How to use the roof snow load calculator
Snow load is one of the governing vertical loads on any roof structure. This calculator estimates the ground snow load from measured depth and snow density, applies the ASCE 7 basic factor of 0.7 together with the exposure, thermal and importance factors to get the flat roof snow load, and then multiplies by the slope factor for the sloped roof value.
Snow density drives the result. Fresh snow runs about 6.4 pcf, while settled snow is near 12.7 pcf and wet or repeatedly thawed and refrozen snow can reach 19.1 pcf. The same 20 inches of depth can therefore produce a load that differs by a factor of three. Whenever a mapped value is available from the ASCE 7 Hazard Tool or the local building department, enter it in the override field, because mapped pg governs design.
The slope factor Cs accounts for snow sliding off the roof. Slippery, unobstructed surfaces on heated buildings shed snow at shallow pitches, so Cs drops quickly, while rough surfaces or unheated roofs hold Cs at 1.0 until the pitch becomes quite steep. Cs only reaches zero at 70 degrees. For roofs under 15 degrees the minimum roof snow load of ASCE 7 §7.3.4 is also checked and the larger value governs.
Treat the output as a preliminary check. Drift on lower roofs, unbalanced snow across a gable, sliding snow and rain-on-snow surcharge are separate provisions, so have a licensed structural engineer confirm the numbers before the design is finalized.
Frequently asked questions
Use the ASCE 7 Hazard Tool or your local building department, since pg is mapped by location and can vary sharply over short distances. Enter that value in the override field; the depth-and-density estimate is only a fallback.
The ASCE 7 slope factor Cs only reaches zero at 70 degrees. Below that a portion of the load remains, and snow that slides off can pile up as drift on lower roofs, which is a separate check.
No. This is a preliminary check. Drift, unbalanced, sliding and rain-on-snow surcharge loads are separate ASCE 7 provisions, so have a licensed structural engineer confirm the design before construction.