Roof Snow Load Calculator — ASCE 7-22 / IRC R301.2.3

Calculate the design roof snow load (Pf and Ps) from ground snow load, exposure, thermal condition, and importance per ASCE 7-22 Chapter 7 and IRC R301.2.3. Adds dead load to get total design load in psf.

By TradeCalc, Roofer Calculators — Code-Referenced — ASCE 7-22 §7.3.1 (flat-roof snow load Pf), ASCE 7-22 §7.4 (sloped-roof snow load Ps and Cs), ASCE 7-22 Table 7.3-2 (Ce exposure factor), ASCE 7-22 Table 7.3-3 (Ct thermal factor), ASCE 7-22 Table 1.5-2 (Is importance factor by Risk Category), IRC R301.2.3 (snow load design requirements)

Related Calculators

How to Calculate Roof Snow Load (ASCE 7-22)

Ground Snow vs Roof Snow

Roofs see less snow than the ground because wind scours snow off the roof and building heat melts the bottom layer. ASCE 7-22 Chapter 7 converts the ground snow load (Pg, published by jurisdiction) into a flat-roof design load (Pf) using the exposure, thermal, and importance factors. IRC R301.2.3 then requires every roof to be designed for either the ASCE 7 calculation or the simplified jurisdictional value, whichever controls.

The ASCE 7-22 Formula

Pf = 0.7 · Ce · Ct · Is · Pg (flat-roof snow, psf)

Ps = Pf · Cs (sloped-roof snow, psf)

Total = Dead load + Ps (combined design load)

Pg = ground snow load from the local jurisdiction (psf)
Ce = exposure factor (0.8 fully exposed, 1.0 partial, 1.2 sheltered)
Ct = thermal factor (1.0 heated, 1.1 slightly heated, 1.2 unheated)
Is = importance factor (0.8 Cat I, 1.0 Cat II, 1.1 Cat III, 1.2 Cat IV)
Cs = slope factor — reduces load on steep slippery roofs
0.7 = base conversion from ground to flat roof

The slope factor Cs reduces Pf on steep, slippery (metal/membrane) roofs where snow slides off. For a warm non-slippery roof (asphalt shingles) Cs = 1.0 up to 70°. For a warm slippery roof, Cs = 1.0 up to 30°, then drops linearly to 0 at 70°. Cold (unheated) roofs keep snow longer, so the Cs curve starts at a higher angle.

Worked Example

Scenario: A heated single-family home with a 6:12 asphalt-shingle roof, ground snow Pg = 40 psf, partially exposed site. Find the design roof load.

Pg = 40 psf (from local snow map / building department)
Ce = 1.0 (partially exposed — typical suburban lot)
Ct = 1.0 (heated, occupied building)
Is = 1.0 (Risk Category II residential)
Pf = 0.7 × 1.0 × 1.0 × 1.0 × 40 = 28.0 psf
Roof pitch = 6:12 → 26.6° (below 30° and non-slippery) → Cs = 1.0
Ps = 28.0 × 1.0 = 28.0 psf
Dead load (asphalt shingles + felt + sheathing + rafters + drywall) ≈ 10 psf
Total design load = 10 + 28 = 38 psf

A 6:12 asphalt roof at this site carries 38 psf — use that number to size rafters from IRC Table R802.5.1, which has load rows for 20/30 psf. For 38 psf you either interpolate, step up one lumber size, or use the AWC Span Calculator with the actual Ps.

Practical Tips

Pg comes from the AHJ, not the map. Always pull the jurisdiction-specific ground snow load — the ASCE 7 map is coarse. Lake-effect belts (Buffalo, Erie, Tug Hill Plateau) run 60–100 psf while 20 miles away the map shows 30 psf.
Sheltered roofs carry 50% more snow. A row of tall conifers on the north side bumps Ce from 1.0 to 1.2, adding 20% to Pf. Clear trees close to the roof or design for the higher factor.
Unheated buildings hold snow. A barn or three-season porch (Ct = 1.2) holds 20% more snow than the heated house next door. Use the right Ct for accessory structures.
Drift loads can dwarf the uniform load. ASCE 7-22 §7.7 requires drift load checks at roof steps, parapets, and rooftop AC units. A 4-ft drift at a roof step can add 50+ psf locally even when the uniform Pf is only 25 psf.
Below 30 psf the IRC lets you simplify. IRC R301.2.3 allows using the jurisdictional Pg directly (no ASCE calc) where Pg ≤ 30 psf and the roof is simple. Above 30 psf or for complex roofs, the full ASCE 7 calculation governs.

Code References

ASCE 7-22 §7.3.1 (flat-roof snow load Pf), ASCE 7-22 §7.4 (sloped-roof snow load Ps and Cs), ASCE 7-22 Table 7.3-2 (Ce exposure factor), ASCE 7-22 Table 7.3-3 (Ct thermal factor), ASCE 7-22 Table 1.5-2 (Is importance factor by Risk Category), IRC R301.2.3 (snow load design requirements)

Frequently Asked Questions

How do I convert ground snow load to roof snow load?

Use ASCE 7-22 §7.3.1: Pf = 0.7 · Ce · Ct · Is · Pg. For a typical heated, partially exposed residential roof (Ce=1.0, Ct=1.0, Is=1.0), Pf is 70% of Pg — a 40 psf ground snow load gives 28 psf on the roof. The 0.7 factor accounts for wind scour and heat melt. Sloped roofs then apply a further Cs reduction only if they are steep and slippery.

What are the ASCE 7 Ce, Ct, and Is factors?

Ce (exposure): 0.8 fully exposed, 1.0 partially exposed, 1.2 sheltered. Ct (thermal): 1.0 heated, 1.1 slightly heated, 1.2 unheated/open. Is (importance): 0.8 Risk Cat I (agricultural), 1.0 Cat II (residential), 1.1 Cat III (assembly), 1.2 Cat IV (essential facilities). Multiplying all three against 0.7·Pg gives Pf. A sheltered, unheated barn in Cat IV can carry 2× the snow of an exposed, heated house at the same Pg.

Does roof pitch reduce the snow load?

Only for steep, slippery roofs. The Cs slope factor is 1.0 (no reduction) for non-slippery asphalt shingles up to 70°. For slippery roofs (metal, smooth membrane) on a heated building, Cs = 1.0 up to 30° (about 7:12), then drops linearly to 0 at 70°. So a 12:12 (45°) metal roof carries Cs = 0.875 × Pf, while the same pitch in asphalt carries the full Pf.

Can I use the IRC simplified snow load instead of ASCE 7?

IRC R301.2.3 permits using the jurisdictional ground snow load directly (without the ASCE 7 calc) when Pg ≤ 30 psf and the roof is a simple gable/hip without valleys, dormers, or roof steps. Above 30 psf, or for any complex roof geometry, the full ASCE 7-22 calculation governs. Most heavy-snow jurisdictions require ASCE 7 for any residential roof.