Snow Load Calculation: Worked Examples

The ASCE 7-22 flat-roof snow load equation (Pf = 0.7 × Ce × Ct × Is × Pg) is straightforward once you know how to choose each factor. Here are three worked examples at different building types.

Example 1: Typical suburban house in Rochester, NY

Ground snow load Pg: 55 psf (Rochester, Upstate NY). Ce = 1.0 (partially exposed, suburban terrain B). Ct = 1.0 (continuously heated). Is = 1.0 (Risk Category II, ordinary occupancy). Pf = 0.7 × 1.0 × 1.0 × 1.0 × 55 = 38.5 psf. For a 6:12 slope (26.6°), the slope factor Cs for a warm roof (non-slippery) = 1.0 up to 30°, so balanced sloped load Ps = 38.5 psf.

Example 2: Unheated detached garage, suburban

Same Rochester Pg = 55 psf. Ce = 1.0. Ct = 1.2 (unheated structure). Is = 0.8 (Risk Category I, minor storage, low-hazard). Pf = 0.7 × 1.0 × 1.2 × 0.8 × 55 = 36.96 psf. The Ct increase for being cold offsets the Is reduction, and the result is close to the house load. Check the minimum: Is × Pg = 0.8 × 55 = 44 psf; the minimum governs. Design load = 44 psf.

Example 3: Fully exposed warehouse in open terrain

Location: Boise, ID, Pg = 35 psf. Ce = 0.8 (fully exposed roof, open terrain Category C with no obstruction). Ct = 1.1 (partially conditioned warehouse, maintained above freezing but not heated to normal interior temperatures). Is = 1.0. Pf = 0.7 × 0.8 × 1.1 × 1.0 × 35 = 21.56 psf. Check minimum: Is × Pg = 35 psf; minimum does NOT govern here since Pg > 20. Design flat-roof load = 21.56 psf, reduced from the 35 psf ground value by exposure.

Key takeaways

The thermal factor matters most for unheated and cold buildings: Ct = 1.2 adds 20% load. Exposure matters most for fully open industrial buildings in Category C terrain: Ce = 0.8 reduces load 20%. The minimum load check (Is × Pg) often governs for unheated low-importance buildings in heavy-snow regions. Always run both the formula result and the minimum and take the larger.