Also in: Disaster Response and Recovery
Snow Drift Surcharge at a Roof Step or Parapet (ASCE 7 Ch. 7)
The triangular surcharge that collapses lower roofs, which the flat-roof snow-load never adds.
Example
You enter
- Upwind fetch lu (ft) 100
- Ground snow load pg (psf) 30
- Winter wind parameter W2 (Figure 7.6-1) 0.5
You get
- Snow density gamma 17.9 pcf
- Drift height hd 3.47 ft
- Drift width w 13.9 ft
- Peak drift surcharge pd 62.1 psf (at the step, tapering to 0)
Checked against published worked examples from: SK&A Engineers (M. Crandall).
Details, formula, and sources
ScopeASCE 7-22 leeward drift height hd = 1.5 sqrt(pg^0.74 lu^0.70 W2^1.7 / gamma), with W2 the winter wind parameter, density gamma = 0.13 pg + 14 (capped 30 pcf), peak pd = hd gamma over a 4 hd width. A 100 ft upwind roof at pg 30 psf and W2 0.5 piles 62 psf at the wall on top of the balanced load; a heavier 50 psf snow zone drives it to 80 psf. Leeward form, on top of snow-load's balanced number. A design aid, not the engineer of record's stamped design.
Formula or tablegamma = min(0.13 pg + 14, 30) pcf; hd = 1.5 sqrt(pg^0.74 lu^0.70 W2^1.7 / gamma) ft; w = 4 hd (hd <= hc) else min(4 hd^2/hc, 8 hc) with the drift cut to hc; pd = min(hd, hc) gamma psf.
Source and editionThe ASCE 7-22 Chapter 7 leeward snow-drift provisions -- the drift height (Eq. 7.6-1, applied by 7.7.1), the snow density 7.7.1, the surcharge pd = hd gamma, and the drift width -- by name.
Free accessASCE 7 is available through the ASCE Library at ascelibrary.org; the Chapter 7 drift relations are public.
What governsEstimate. AHJ and licensed professional govern.
API field nameslu_ft, pg_psf, w2, gamma_pcf, hd_ft, w_ft, pd_psf
- Drift height hd = 1.5 sqrt(pg^0.74 lu^0.70 W2^1.7 / gamma), no lower bound neededASCE 7-22 Eq. 7.6-1
- Winter wind parameter W2 defaults to 0.65, the top of the lower-48 range, which overstates the drift where winter wind is calmer; enter the Figure 7.6-1 valueASCE 7-22 Figure 7.6-1
- Density gamma = 0.13 pg + 14, capped at 30 pcfASCE 7-22 7.7.1
- Surcharge and width pd = hd gamma at the step over a 4 hd width (full-height triangle, hd <= hc)ASCE 7-22 7.7.1