Snow Drift Load Calculator
Calculate ASCE 7 balanced snow load, leeward and windward snow drift surcharge height (ft), peak drift load (psf), and drift horizontal width.
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ASCE 7-16 & 7-22 Snow Drift Load Methodology
Wind blowing across a large, unobstructed upper roof fetch strips loose snow and deposits it in aerodynamic eddies against lower adjacent roofs, stepped elevations, and tall parapet walls. This localized accumulation creates triangular surcharge wedges called snow drifts that can generate structural loads two to four times higher than the uniform flat roof snow load.
ASCE 7 Mathematical Formulations
- Flat Roof Balanced Snow Load:
p_f = 0.7 × C_e × C_t × I_s × p_g - Snow Density (γ in pcf):
γ = 0.13 × p_g + 14 ≤ 30 pcf - Balanced Snow Depth:
h_b = p_f / γ - Leeward Drift Height (h_d):
h_d = 0.43 × (l_u)^(1/3) × (p_g + 10)^(1/4) - 1.5
Ifh_d + h_b > h_step, the drift is truncated to the clear height. - Drift Peak Load & Width:
p_d = h_d × γ|Width (w) = 4 × h_d
Frequently Asked Questions
Why do parapets cause snow drift surcharges?
Parapet walls act as windbreaks, creating a low-velocity aerodynamic wake that causes windblown snow to fall out of the airstream and deposit against the wall.
What is the snow density formula used in ASCE 7?
ASCE 7 defines compacted drift snow density as gamma = 0.13 * p_g + 14 (in pounds per cubic foot), capped at a maximum of 30 pcf.
How wide does a snow drift typically extend onto the roof?
Under ASCE 7, the horizontal width of a triangular snow drift wedge is typically taken as 4 times the drift surcharge height (w = 4 * h_d).