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# Calculate Snow Loads (ASCE 7-22)
> Get balanced and unbalanced snow loads for sloped and flat roofs to ASCE 7-22 Chapter 7, including drift on lower roofs. Current edition for IBC 2024.
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
| Property | Detail |
| ------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| Design standards | ASCE 7-22, Chapter 7 |
| Regions | United States |
| Who it's for | US structural engineers calculating snow loads to ASCE 7-22 for IBC 2024 compliance — balanced and unbalanced conditions on sloped and flat roofs, including rain-on-snow surcharge and drift loads on lower roofs or projections. |
| What it helps with | Fully aligned to ASCE 7-22: importance factor (Is) removed (now incorporated in ground snow load Pg), updated low-slope roof and surcharge criteria, revised thermal / slope / winter wind drift factors, and Iu minimum limit removed. Aligns with wind and seismic calculators for end-to-end IBC 2024 compliance in one platform. |
| What it checks | Flat roof snow load, Sloped roof (balanced) snow load, Rain-on-snow surcharge, Unbalanced snow loads, Snow drift loads |
#### 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)
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.
## How to use it
Open it from **Run calc** in the About this calculator panel above.
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](/docs/running-calculations/checking-results/checks-references-conditions-assumptions).
Check the utilization of each governing check in the summary, then export a PDF report. See [Views and Export](/docs/running-calculations/exporting-reports/views-and-export).
## Common questions
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.
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.
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.
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.
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.
## Next steps
Worked example: balanced, unbalanced and drift snow loads for sloped and flat roofs to ASCE 7-16 Chapter 7.
Complete list of Calcs.com design calculators available for United States projects (AISC, ACI, NDS, ASCE 7, IBC, and related standards).
Calculate MWFRS and components and cladding wind pressures to ASCE 7-22 for low-rise buildings, with code references on every output.
Calculate seismic design forces and equivalent lateral loads to ASCE 7-22 for US buildings. Finds SDC, spectral accelerations, base shear, and story forces.
Worked example: seismic design category, spectral accelerations, base shear and story forces to ASCE 7-16 with the equivalent lateral force procedure.
