United StatesASCE 7-22Chapter 7
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.
This calculator computes both the balanced and unbalanced snow loads for sloped and flat roofs. It takes into account various factors such as ground snow load, surface roughness, building importance, roof exposure, roof pitch, and the thermal condition of the building. Additionally, the calculator determines whether a rain-on-snow surcharge load is necessary. It also provides calculations for snow drift loads on lower roofs or roof projections.
Method & scope
What it calculates
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.Calculation method
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)
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.How to use it
1
Open the calculator
Open it from Run calc in the About this calculator panel above.
2
Enter your inputs
Work through the input sections from top to bottom. Click any input label to see its reference explanation, clause, conditions and assumptions. See Checks, References, Conditions and Assumptions.
3
Review the results and export
Check the utilization of each governing check in the summary, then export a PDF report. See Views and Export.
Common questions
What design standard does this calculator follow?
What design standard does this calculator follow?
The calculator applies ASCE 7-22 Chapter 7, Snow Loads. It calculates flat roof snow load (pf) from the ground snow load (pg) using the roof exposure factor (Ce), roof thermal factor (Ct), and importance factor (Is). It then derives sloped roof snow loads, unbalanced loads, and drift surcharges from the base pf value.
What are the key inputs?
What are the key inputs?
Key inputs are ground snow load (pg) from ASCE 7-22 Figure 7.2-1 or the authority having jurisdiction, roof geometry (span, eave height, slope angle), surface roughness category, roof exposure condition (fully exposed, partially exposed, sheltered), thermal condition, and risk category for the importance factor.
What outputs does the calculator return?
What outputs does the calculator return?
Outputs include flat roof snow load (pf), balanced sloped roof snow load (ps), unbalanced snow load distribution for gable and hip roofs, drift loads on lower roofs and at obstructions (hd), sliding snow load where applicable, and a rain-on-snow surcharge flag for low-slope roofs where pg does not exceed 20 psf.
Does the calculator handle drift loads on lower roofs?
Does the calculator handle drift loads on lower roofs?
Yes. The calculator computes leeward and windward drift heights using the ASCE 7-22 Figure 7.6-1 parabolic drift curves. You input the upper roof width and the lower roof width separately, and the calculator returns the governing drift surcharge height and peak pressure for each drift condition.
When should I use ASCE 7-22 instead of ASCE 7-16 for snow loads?
When should I use ASCE 7-22 instead of ASCE 7-16 for snow loads?
Use ASCE 7-22 when your project is governed by IBC 2024 or when the authority having jurisdiction mandates the 2022 edition. ASCE 7-22 made minor adjustments to snow load procedures relative to ASCE 7-16, primarily clarifications to the risk category importance factor table. Check your local building code adoption schedule before selecting the edition.
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