Snow Drift Calculator
psf
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Total Design Load at Drift Peak127.6 psf
Flat Roof Snow Load (pf)28.0 psf
Snow Density (γ)19.2 pcf
Balanced Snow Depth (hb)1.46 ft
Design Drift Height (hd)5.19 ft
Leeward Drift Height5.19 ft
Windward Drift Height2.86 ft
Drift Width (w)20.8 ft
Drift Surcharge (pd)99.6 psf
This snow drift calculator evaluates the drift surcharge load at a roof step, parapet, or projection following ASCE 7-22 Chapter 7. It computes the flat-roof snow load, the snow density and balanced depth, the leeward and windward drift heights from the upwind fetch, and the design drift limited by the available clear height at the step. The output gives the drift height, the drift width, the peak surcharge, and the total design load.
Formula
pf = 0.7·Ce·Ct·Is·pg ; hd = 0.43·∛lu·(pg+10)^0.25 − 1.5 ; surcharge = γ·hd
- pf
- Flat-roof snow load (psf)
- hd
- Drift height (ft); windward drift uses 75% of this value
- lu
- Upwind fetch length feeding the drift (ft)
- γ
- Snow density = min(0.13·pg + 14, 30) pcf
- pg
- Ground snow load (psf); the design drift is capped at the clear height above balanced snow
How it works
- Enter the ground snow load pg (psf) and the exposure factor Ce, thermal factor Ct, and importance factor Is from the ASCE 7 tables.
- Enter the step or parapet height (ft) and the upper- and lower-roof lengths (ft), which act as the upwind fetch for the leeward and windward drift formulas.
- The tool computes the flat-roof load pf, the snow density γ, the balanced depth hb, then the larger of the leeward and windward drifts, caps it at the clear height above the balanced snow, and reports the drift width (4·hd) and the total load pf plus surcharge.
Worked example
A roof step 8 ft high with pg = 40 psf, Ce = Ct = Is = 1.0, a 200 ft upper roof, and a 100 ft lower roof.
- Flat-roof load pf = 0.7 × 1 × 1 × 1 × 40 = 28 psf; snow density γ = 0.13 × 40 + 14 = 19.2 pcf.
- Balanced depth hb = 28 / 19.2 = 1.46 ft; clear height hc = 8 − 1.46 = 6.54 ft.
- Leeward drift = 0.43 × ∛200 × (50)^0.25 − 1.5 = 5.19 ft; windward = 0.75 × (0.43 × ∛100 × 50^0.25 − 1.5) = 2.86 ft.
- Design drift = min(max(5.19, 2.86), 6.54) = 5.19 ft; surcharge = 19.2 × 5.19 = 99.59 psf.
Drift height ≈ 5.19 ft, drift width ≈ 20.75 ft, surcharge ≈ 99.59 psf, and total design load ≈ 127.59 psf at the step.
Frequently asked questions
- What is the difference between leeward and windward drift?
- A leeward drift forms on the lower roof from snow blown off the higher upwind roof. A windward drift forms when snow blows against a wall or step from the lower roof side; ASCE 7 takes the windward drift as 75% of the leeward formula, and the larger of the two governs.
- Why is the drift height limited by the clear height?
- A drift cannot rise higher than the space available above the balanced snow at the step. The calculator caps the computed drift at the clear height hc, the step height minus the balanced snow depth, so the drift never exceeds the geometry of the obstruction.
- How is the snow density determined?
- ASCE 7 relates snow density to the ground snow load: γ = 0.13·pg + 14, capped at 30 pcf. A heavier ground snow load implies denser snow, and the density converts the snow loads into the equivalent drift depths.
- Does this calculator add rain-on-snow surcharge?
- A 5 psf rain-on-snow surcharge applies only to low-slope roofs where the ground snow load is 20 psf or less. For higher ground snow loads, as in the worked example, the rain-on-snow surcharge is not added.