Snow Load Calculator

What this calculator does

This tool computes the design snow load on a sloped roof to ASCE 7-22 §7.3 and the IBC 2024. It returns the flat-roof snow load (pf), the sloped-roof snow load (ps), and each of the four governing coefficients — exposure factor (Ce), thermal factor (Ct), importance factor (Is), and slope factor (Cs) — so you can show your work in a permit submission or compare against a structural engineer’s output.

Enter the ground snow load (pg) from the ASCE 7-22 figure 7.2-1, your state-specific atlas, or the ASCE 7 Hazard Tool. Pick the roof pitch in degrees, the exposure category, the thermal classification, and the risk category of the structure. The calculator returns ps in pounds per square foot directly usable in framing, truss-design, and roof-deck calculations.

How the snow-load math works

ASCE 7-22 reduces the ground snow load (pg) to a flat-roof snow load (pf) and then to a sloped-roof load (ps) through two equations:

1. pf = 0.7 × Ce × Ct × Is × pg — the 0.7 reduction reflects empirical evidence that a balanced flat roof carries less than the ground load because of wind erosion and thermal melt.
2. ps = Cs × pf — Cs reduces the load further on steeper slippery roofs where snow sheds before reaching maximum accumulation.

The four coefficients track:

• Ce — Exposure factor, from ASCE 7-22 table 7.3-1. Ranges from 0.9 (fully exposed terrain category C) to 1.2 (densely treed or sheltered). Most suburban residential sites use 1.0.
• Ct — Thermal factor, from table 7.3-2. 1.0 for warm heated structures, 1.1 for ventilated cold roofs, 1.2 for unheated structures, 1.3 for unheated freezer warehouses.
• Is — Importance factor, from table 1.5-2. 0.8 for risk category I (minor structures), 1.0 for ordinary buildings, 1.1 for substantial occupancy, 1.2 for essential facilities like hospitals, fire stations, and emergency operations centers.
• Cs — Slope factor, from §7.4.2. Equation depends on Ct and whether the surface is slippery and unobstructed.

Reference test cases

Each row reproduces what the calculator returns when you enter the inputs in the leftmost columns.

Ground snow load (pg) — where to find it

ASCE 7-22 figure 7.2-1 is the starting point, but for many states it is superseded by a state-developed map adopted by reference in the building code. Always check what the AHJ has adopted:

• Colorado — SEAC ground snow load study (Structural Engineers Association of Colorado), updated 2021. Mountain communities run from 35 psf at Denver to 130 psf at high-elevation resort towns.
• New York — NYS Department of State ground snow load map. Western NY (Buffalo, Syracuse, Rochester) runs 50–60 psf; the Adirondacks run 80–100 psf.
• Maine — MBOIA atlas. Coastal Maine 60 psf; western mountains 90+ psf.
• New Hampshire — Joint NHDOT/SAE map. The White Mountains hit 100 psf.
• Michigan, Wisconsin, Minnesota — state DOT atlases for upper-peninsula and Lake Superior shore communities.
• Utah, Idaho, Wyoming, Montana — state-specific maps for Wasatch Front, Teton Range, Bitterroot, and Big Sky communities.

For sites not on a state map, ASCE 7-22 figure 7.2-1 gives a baseline pg by region. Above the elevation noted on the map, use ASCE 7-22 §7.2.1’s elevation correction to add ground snow at 0.4 psf per 100 ft above the map elevation.

Exposure factor (Ce) — site terrain

ASCE 7-22 table 7.3-1 maps three exposure categories to terrain category B/C/D. The most common combinations:

• Fully exposed, terrain B/C → Ce = 0.9. Open coastal sites, prairie, isolated structures with no obstructions within 10 building heights upwind.
• Partially exposed, terrain B/C → Ce = 1.0. Most suburban and urban residential sites — neighbors and trees provide partial wind shielding.
• Sheltered, any terrain → Ce = 1.2. Dense conifer cover within 10 building heights, or roofs lower than nearby tall buildings or terrain.

The 20-percent penalty for sheltered sites reflects that wind cannot remove drifting snow when the building is in the wind shadow of taller features. This matters for forested mountain cabins and for low secondary roofs against tall main buildings.

Thermal factor (Ct) — heated vs unheated

Table 7.3-2 distinguishes:

• Ct = 1.0 — All warm structures except those in the next two categories. Standard heated single-family homes, commercial offices, retail.
• Ct = 1.1 — Cold ventilated roofs. Roofs above well-insulated, well-ventilated attics where the underside of the roof deck stays at outside air temperature. Most modern code-compliant attics fall here, but the engineer typically uses 1.0 unless the design is unusually high-R-value with strong soffit-to-ridge ventilation.
• Ct = 1.2 — Unheated and open-air structures. Carports, pole barns, agricultural sheds, open canopies.
• Ct = 1.3 — Continuously held below freezing. Cold-storage warehouses, ice-rink roofs.

For most permit-stage residential calculations, use Ct = 1.0. The cold-roof bonus is conservative but not strictly required for an ordinary attic.

Importance factor (Is) — building risk category

ASCE 7-22 table 1.5-2 sets the snow importance factor:

• Risk Category I — Is = 0.8. Minor agricultural and storage structures with low occupancy.
• Risk Category II — Is = 1.0. Single-family homes, multifamily, ordinary commercial.
• Risk Category III — Is = 1.1. Buildings with substantial hazard potential — schools, large assembly, big-box retail, hotels over a certain occupancy.
• Risk Category IV — Is = 1.2. Essential facilities — hospitals, emergency operations, fire and police stations, designated emergency shelters.

The 20-percent uplift for Risk Category IV is what lets a hospital roof carry an unusually heavy snow event without compromising emergency response.

Slope factor (Cs) — geometry and surface

§7.4.2 gives three Cs curves based on Ct:

• Warm roof (Ct ≤ 1.0). Cs = 1.0 up to 30°, then linear from 1.0 at 30° to 0 at 70°.
• Cold roof (Ct = 1.1). Cs = 1.0 up to 37.5°, then linear from 1.0 at 37.5° to 0 at 70°.
• Unheated (Ct = 1.2). Cs = 1.0 up to 45°, then linear from 1.0 at 45° to 0 at 70°.

These curves apply to unobstructed slippery surfaces — standing-seam metal, smooth membrane, slate. For asphalt shingles, dimensional architectural shingles, snow guards, dormers, and roof penetrations, hold Cs at 1.0 even on steep slopes because snow no longer sheds. The calculator follows the slippery curve by default; if your roof has snow guards or asphalt shingles, take the Cs output as the lower bound and use 1.0 for design.

When the design needs more than this calculator

The calculator returns the balanced sloped-roof load. ASCE 7-22 also requires consideration of:

1. Drift loads (§7.7). Triangular drift surcharges at steps in roof, behind parapets, against tall walls. Drift design is mandatory whenever there is a step of more than 15 ft horizontal length or a higher adjoining structure.
2. Sliding loads (§7.9). When snow slides off a steep upper roof onto a lower roof, the lower roof must carry both its balanced load and 40 percent of the upper-roof flat load distributed over a 15-ft slide-zone.
3. Partial loading (§7.5). Cantilevers and continuous beams must be checked with snow on alternate spans.
4. Rain-on-snow surcharge (§7.10). Required for low-slope roofs (less than ½:12) in regions where pg ≤ 20 psf — the rain-on-snow surcharge is 5 psf added to ps.
5. Unbalanced loads (§7.6). Required for hip and gable roofs with W > 20 ft and slopes between 2:12 and 7:12. The unbalanced loading is a non-uniform distribution that loads the leeward side preferentially.

For any of these conditions, run the structural design through a licensed engineer. Permits issued for stepped-roof additions, lower-roof transitions, and parapet-equipped commercial buildings are routinely red-tagged when drift surcharges are missing from the truss reaction package.

Comparing to the 2018 IBC adoption schedule

Most US jurisdictions are now on the 2024 IBC, which references ASCE 7-22 directly. A handful of states are still on the 2018 IBC referencing ASCE 7-16. The differences for residential snow design are minor — pg values changed at fewer than 3 percent of grid points, and the equations are identical. The calculator’s output is valid under either reference.

Related calculators

• Roof Pitch Calculator — convert X/12 to degrees and back.
• Roof Truss Calculator — feed ps into truss-design loading.
• Roof Rafter Calculator — span tables for the snow-load output.