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ASCE 7-22 Snow Drift at Roof Steps & Parapets

ASCE 7-22 Eq. 7.6-1 with the new winter wind parameter W₂: h_d = 1.5√(p_g^0.74ℓ_u^0.70W₂^1.7/γ). Leeward and windward drifts with the 7-22 width rules. Note: many calculators labelled “ASCE 7-22” still use the ASCE 7-16 equation h_d = 0.43ℓ_u^1/3(p_g + 10)^1/4 − 1.5, which ignores W₂ and can under-predict drifts at windy sites.

Site & roof

Ground snow load p_g psf

Winter wind parameter W₂

Balanced load on lower roof p_s psf

Step / parapet height ft

Upper-roof fetch ℓ_u (leeward) ft

Lower-roof fetch (windward) ft

p_g and W₂ from the ASCE 7 Hazard Tool. W₂ is the fraction of October–April with wind ≥ 10 mph (mapped 0.25–0.65).

Results

γ · h_b · h_c—

Leeward: height · width · p_d—

Windward: height · width · p_d—

Governing—

Eq. 7.6-1h_d = 1.5√(p_g^0.74ℓ_u^0.70W₂^1.7/γ); γ = 0.13p_g + 14 ≤ 30 pcf; h_b = p_s/γ, h_c = step height − h_b; no drift needed when h_c/h_b < 0.2

§7.7.1Leeward: h_d ≤ h_c → w = 4h_d; otherwise height h_c, w = 4h_d²/h_c ≤ 8h_c. Windward: height 0.75h_d (lower-roof fetch), w = 8 × windward height = 6h_d (new in 7-22). The 7-22 text does not cap windward drifts at h_c — flagged when exceeded.

VerifyASCE AMPLIFY text of §7.6.1; STRUCTURE magazine (ASCE 7-22 snow authors): W₂ spans about a factor of 5 in drift load; at W₂ = 0.45 drifts are on average ~10% below 7-16. Hand check (regression test): p_g 40 psf, ℓ_u 100 ft, W₂ 0.45 → h_d 3.41 ft, 10.5% below the 7-16 value of 3.81 ft.

ScopeRoof steps and parapets. Gable unbalanced loads, sliding snow and rain-on-snow are separate checks.

Preliminary calculation tool. Final design requires the full text of the referenced standard (see the sources on this page) and review by a licensed PE.