Snow Loads (ASCE 7-22)
US structural engineers calculating roof snow loads to ASCE 7-22, the current edition under IBC 2024. Covers ground-to-roof conversion, balanced and unbalanced loads, drift loads on lower roofs, and rain-on-snow surcharge flagging in a single calculation. For IBC 2021 projects, use the ASCE 7-16 version instead.
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What it calculates
Get balanced and unbalanced snow loads for sloped and flat roofs to ASCE 7-22 Chapter 7. Inputs cover ground snow load, surface roughness, importance, exposure, pitch, and thermal condition. Drift loads on lower roofs and projections are returned automatically.
Code standards
- ASCE 7-22, Chapter 7
Who uses this calculator
US structural engineers calculating roof snow loads to ASCE 7-22, the current edition under IBC 2024. Covers ground-to-roof conversion, balanced and unbalanced loads, drift loads on lower roofs, and rain-on-snow surcharge flagging in a single calculation. For IBC 2021 projects, use the ASCE 7-16 version instead.
Apply ground-to-roof snow load factors aligned to ASCE 7-22 and IBC 2024, with code references on every output.
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 three adjustment factors:
p_f = 0.7 × C_e × C_t × I_s × p_g
- C_e - roof exposure factor (0.9 fully exposed, 1.0 partially exposed, 1.2 sheltered)
- C_t - thermal factor (1.0 for heated buildings, up to 1.3 for cold structures)
- I_s - importance factor (0.8 to 1.2 based on risk category per ASCE 7-22 Table 7.3-1)
A minimum roof snow load p_m = I_s × p_g is also enforced for low-sloped roofs.
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
For roofs with p_g not exceeding 20 psf and slopes below the minimum threshold, a rain-on-snow surcharge of 5 psf is added to the balanced snow load per ASCE 7-22 Cl. 7.10.
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. For rafter-supported roofs the leeward surcharge magnitude is h_d × gamma / sqrt(S) where 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 on lower roofs and at roof projections are calculated from the ASCE 7-22 Figure 7.6-1 parabolic drift height curves. The drift height h_d is a function of the upwind fetch length and ground snow load. Drift surcharge pressure is h_d × gamma, tapering over a width of 4 × h_d from the step. Drift calculations are not required where the projection height is less than 15 ft.
Outputs
The calculator reports p_f, p_s, p_design, rain-on-snow surcharge flag, unbalanced leeward and windward pressures, and drift heights and pressures for each lower roof or projection entered. All values reference the governing ASCE 7-22 clause.
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I like that Calcs.com shows the code reference section for each calculation and function. That means every time I use it, there's a potential for me to learn something.
Lawrence Bowen
Founding Principal, CPBD, VQ Design
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Aaron D. Obermiller, P.E.
Engineer, REO Engineering
Frequently asked questions
What design standard does this calculator follow?
What are the key inputs?
What outputs does the calculator return?
Does the calculator handle drift loads on lower roofs?
When should I use ASCE 7-22 instead of ASCE 7-16 for snow loads?
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