Snow Load Calculator

ASCE 7-22 Chapter 7 provides the snow load procedure. Start with ground snow load (pg) from maps. Apply exposure factor (Ce), thermal factor (Ct), and importance factor (Is). For sloped roofs, apply slope factor (Cs). Flat roof snow load: pf = 0.7CeCtIs pg. Sloped roof: ps = Cs pf.

Fully Exposed (Ce = 0.8): Open terrain, no obstructions. Partially Exposed (Ce = 0.9): Some trees/buildings nearby. Sheltered (Ce = 1.0): Dense trees/buildings within 10× building height. Exposure reduces snow accumulation due to wind scouring. Lower Ce = less snow on roof.

Flat roofs (≤5°): Cs = 1.0 (full snow load). Low slopes (5-30°): Cs decreases linearly from 1.0 to 0.7. Steep slopes (>70°): Cs = 0 (snow slides off). For 30° slope: Cs ≈ 0.7. For 45° slope: Cs ≈ 0.5. Steeper roofs shed snow, reducing design load.

ASCE 7-22 requires minimum roof snow load: pm = Is × 20 psf for pg ≤ 20 psf, or pm = Is × pg for pg > 20 psf. This ensures roofs can handle light snow even in low-snow regions. Always check minimum vs. calculated load—use the higher value.

Thermal Factor (Ct): Cold/Unventilated (Ct = 1.0): Normal conditions. Cold/Ventilated (Ct = 1.1): Well-ventilated attics increase heat loss. Warm (Ct = 1.2): Heated buildings with R-value < 30. Higher Ct = more heat loss = more snow melting = lower snow accumulation.

Importance Factor (Is) by Risk Category: Category I: Is = 0.80. Category II: Is = 1.00. Category III: Is = 1.10. Category IV: Is = 1.20. Higher importance = higher design snow loads for critical structures like hospitals, emergency facilities.