Snow load is the single weather number that drives roof framing design in cold climates. Get it wrong by 10 psf and you're either paying for lumber you don't need or designing a collapse risk. The number on your engineer's drawings comes from ASCE 7, the structural engineering standard adopted by the International Building Code. Here's how the math works and where to find your inputs.
Estimate only. This guide and the calculator cover the ASCE 7 simplified method for residential roofs (Risk Category II). For commercial, multi-family, drift-prone roofs, or any structure your local jurisdiction flags as critical, get a licensed structural engineer to run the full analysis.
Ground snow vs roof snow
Two numbers matter:
- Ground snow load (pg): The weight per square foot of snow that statistically accumulates on flat open ground in your region over a 50-year return period. Set by the building department.
- Roof snow load (pf or ps): The design load on your actual roof, calculated from pg with four multipliers that account for exposure, heating, importance, and slope.
You don't design framing for the ground number — you design for the roof number, which is usually less than ground because of wind scour and slope shedding.
The simplified formula
ASCE 7-22 chapter 7 gives a simplified equation for residential pitched roofs:
pf = 0.7 × Ce × Ct × I × pg ps = Cs × pf
Where pf is the flat-roof snow load and ps is the sloped roof snow load. The 0.7 factor reduces ground snow to what actually accumulates on a roof under normal exposure. Each multiplier:
- Ce — Exposure factor: 0.9 fully exposed (open terrain), 1.0 partially exposed (typical suburban), 1.2 sheltered (dense trees)
- Ct — Thermal factor: 1.0 heated structure (house with insulated conditioned space), 1.2 unheated structure (barn, detached garage)
- I — Importance factor: 1.0 residential (Risk Cat II), 1.1 schools and assembly, 1.2 hospitals (Risk Cat IV)
- Cs — Slope factor: 1.0 if pitch ≤ 30°. Above 30° on warm slippery roofs, Cs = (70 − angle) ÷ 40, dropping to 0 at 70°
Worked example
Boston single-family house, 6/12 pitch, asphalt shingle roof, heated, suburban (some trees nearby):
- pg = 35 psf (Boston ground snow per Massachusetts code)
- Ce = 1.0 (partially exposed)
- Ct = 1.0 (heated)
- I = 1.0 (residential)
- pf = 0.7 × 1.0 × 1.0 × 1.0 × 35 = 24.5 psf
- Pitch 6/12 = 26.6° (below 30° threshold)
- Cs = 1.0
- ps = 1.0 × 24.5 = 24.5 psf design load
The truss supplier or rafter designer uses 24.5 psf as the snow load input plus dead loads (sheathing, shingles, insulation — typically 8-15 psf for residential). The snow load calculator runs this math instantly with the four multiplier dropdowns.
Where to find your ground snow load (pg)
Three reliable sources, in order of preference:
- Local building department. Most US jurisdictions publish a single pg value for the entire town or county. This is the legally binding number and what your permit reviewer will check against.
- State amendments to the IBC/IRC. Some states (Massachusetts, New York, Maine) publish state-wide pg maps with finer resolution than the ASCE 7 base map.
- ASCE 7 Hazard Tool at asce7hazardtool.online — the free online lookup that returns ground snow load for any US lat/lon. Useful for design-stage estimating before confirming with the local department.
Common US ground snow values for reference:
- Atlanta, GA: 5 psf
- Denver, CO: 25 psf
- Chicago, IL: 25 psf
- Boston, MA: 35 psf
- Buffalo, NY: 50 psf
- Burlington, VT: 60 psf
- Anchorage, AK: 50-70 psf
- Mt. Washington valley, NH: 100+ psf
When the slope factor kicks in
Cs reduces snow load for steeper warm slippery roofs because snow slides off before it accumulates to the design depth. The cutoff is 30° (about 7/12 pitch) for asphalt shingles and metal roofs.
A 12/12 pitch (45°) gives Cs = (70 − 45) ÷ 40 = 0.625 — meaning the design snow load drops by 37.5% vs a flat roof at the same ground snow. This is why steeper Cape Cod and Tudor roofs survive heavy New England winters with lighter rafters than a 6/12 ranch with the same snow region.
Cold roofs (unheated barns, ski cabins kept at outside temperature when unoccupied) don't get the slope reduction until much steeper angles — 45° instead of 30° — because the snow doesn't soften and slide.
What this number drives in framing
Once you have ps (sloped design load), it gets added to dead load for sizing:
- Rafter and truss top chord: top chord lumber size scales with span and total load (ps + dead). A 20 ft span at 30 psf snow uses 2×8; at 60 psf it's 2×10.
- Sheathing thickness: 7/16 in OSB at 24 in OC handles 30-40 psf; bumps to 5/8 in for higher snow.
- Header sizes over openings: snow load adds to roof load over walls — the header size calculator uses this.
- Foundation design: total roof load eventually transfers to footings, sized in the engineering stamp.
What pros do differently
Round up at the boundary.If your code official uses 30 psf and the calculator returns 28.5 psf, design for 30. The savings from designing to the calculated number aren't worth the risk of the inspector pulling out a different value mid-build.
Account for drifting on adjacent roofs. ASCE 7 simplified does not handle drift. If your roof has a shed dormer, a stepped second story, or sits next to a taller building, the drift load can be 2-3× the base ps in concentrated areas. Get an engineer for any building with these geometries.
Verify the local pg before ordering trusses. Truss suppliers ask for snow load on the order form. Submitting the wrong number means the trusses ship with the wrong top chord lumber. Cross-check pg with the building department, then submit.
Quick FAQ
Why is roof load less than ground load? Wind scours snow off roofs faster than off open ground. Heated roofs melt snow from below. The 0.7 factor and the multipliers account for both.
Do I need to design for snow plus rain?In shallow-pitch regions where rain-on-snow is common, IBC requires a 5 psf rain surcharge on roofs below 1/2 in per ft slope. Above that pitch, water runs off and the surcharge doesn't apply.
What if my old roof is rated lower than the current code? Existing buildings are grandfathered to the code in effect when built. New roofs and additions must meet current ground snow. Re-roofing alone (replacing shingles) does not trigger an upgrade.
Estimate only — not engineered design. The snow load calculator runs the ASCE 7 simplified method for residential roofs. For commercial, drift-prone, or multi-family structures, get a licensed structural engineer to run the full chapter 7 analysis and stamp the design.