Snow Loads (ASCE 7-22)

How it calculates

The Snow Loads (ASCE 7-22) calculator computes balanced and unbalanced roof snow loads, drift loads, and rain-on-snow surcharges per ASCE 7-22 Chapter 7.

Flat roof snow load

The base calculation converts ground snow load (p_g) to flat roof snow load (p_f) using two adjustment factors:

p_f = 0.7 × C_e × C_t × p_g

• C_e - roof exposure factor, from roof exposure and ground surface roughness (for roughness B: 0.9 fully exposed, 1.0 partially exposed, 1.2 sheltered)
• C_t - thermal factor, interpolated from roof R-value and p_g for heated buildings (1.00 to 1.20). Other conditions use 0.85, 1.2 or 1.3.

There is no snow importance factor. Risk category instead sets the cap on the minimum roof snow load.

For slopes up to 15°, a minimum roof snow load p_m = p_g applies. It is capped at 25, 30, 35 or 40 psf for risk categories I to IV.

Sloped roof (balanced) snow load

The balanced design snow load on sloped roofs applies the roof slope factor (C_s):

p_s = C_s × p_f

C_s is determined separately for warm roofs, cool roofs, and cold roofs based on roof pitch and thermal condition per ASCE 7-22 Figure 7.4-1. The governing design load is max(p_s, p_m).

Rain-on-snow surcharge

A rain-on-snow surcharge of 8 psf is added to the balanced snow load per ASCE 7-22 Cl. 7.10. It applies where p_g is at or below the p_m cap (25 to 40 psf) and the slope is no more than W/50 degrees.

Unbalanced snow loads

Unbalanced loading is checked for gable roofs with pitches between 0.5:12 and 7:12. The windward side carries a reduced load while the leeward side carries an increased load. Where the eave-to-ridge distance W is 20 ft or less, the leeward side carries a uniform p_g and the windward side carries none. For wider roofs, the leeward surcharge is h_d × gamma / sqrt(S). Here gamma is snow density and S is roof slope run per unit rise. Unbalanced loads are not required outside the 0.5:12 to 7:12 pitch range.

Snow drift loads

Leeward and windward drifts are calculated on lower roofs and adjacent structures, and windward drifts at roof projections and parapets. The drift height h_d comes from ground snow load, fetch length, snow density and the winter wind parameter W_2. Drift pressure is h_d × gamma, limited by the clear height, over a width of 4 × h_d. The width is adjusted where the drift exceeds the clear height, and capped at 8 times the clear height. Drifts are not required where the clear height is 20% or less of the balanced snow depth. A note flags the exemption for projections less than 15 ft long.

Sliding snow from a sloped upper roof onto a lower roof is also calculated. This feature is in beta.

Outputs

The calculator reports p_f, p_s, p_design, the rain-on-snow surcharge where it applies, and unbalanced leeward and windward loads. It also reports drift heights and pressures for each lower roof or projection entered. Calculated values cite the ASCE 7-22 clause, equation or figure they come from.