Snow Load Calculator

This calculator uses the ASCE 7 balanced-load method. First it finds the flat roof snow load with pf = 0.7 times Ce times Ct times Is times pg, where pg is the ground snow load, Ce is the exposure factor, Ct is the thermal factor, and Is is the importance factor. Then it applies the roof slope factor Cs to get the sloped roof load ps = Cs times pf. Multiplying ps by the roof plan area gives the total weight in pounds.

Ground snow load (pg) is the design snow weight on the ground for your location, measured in pounds per square foot (psf). You get it from the ASCE 7 ground snow load maps or, more reliably, from your local building department, since many areas set their own case-study values for mountains and lake-effect zones. Typical values range from about 0 psf in the deep South to 40 to 100 psf or more in northern and high-elevation regions.

The exposure factor Ce (0.9 to 1.2 here) accounts for how wind exposure removes or retains snow, so a sheltered roof carries more. The thermal factor Ct (1.0 for heated, up to 1.2 for freezers or open structures) reflects how building heat melts snow. The importance factor Is (0.8 to 1.2) scales the load by the building's risk category, so hospitals and emergency buildings use higher values than minor structures.

Steeper roofs shed snow, so ASCE 7 reduces the load with the slope factor Cs. Below a threshold slope Cs stays at 1.0, then it drops linearly to 0 at 70 degrees. The threshold is lower for slippery surfaces like metal or membrane (which shed sooner) and for colder roofs. For a warm, non-slippery roof the reduction does not begin until about 30 degrees.

No. It computes only the balanced snow load on the roof. It does not include drift loads against walls and parapets, sliding snow from higher roofs, unbalanced loads on hips and gables, or rain-on-snow surcharge, any of which can govern a real design. Treat the result as a planning estimate, not a substitute for engineering.

Treat it as an educational estimate, not an engineering stamp. Building codes adopt specific ASCE 7 editions and local amendments, and a complete roof design must consider drift, sliding, and other load cases. Have a licensed structural engineer review the snow loads before you use them for permits, framing, or any structural decision.