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ASCE 7 multiplies the ground snow load by 0.7 × Ce × Ct × Is: for a heated, partially exposed house that is 0.7 × pg, so 40 psf on the ground becomes 28 psf on a flat or shingle roof up to 30°. Steeper or slippery roofs get a slope factor below 1.0; roofs under 15° have a minimum of up to 20 psf. Use your building department's ground snow load.
Roof snow load is usually about 70% of the ground snow load: ASCE 7, the standard US building codes use, sets the flat-roof snow load at pf = 0.7 × Ce × Ct × Is × pg, so a heated, partially exposed house where the ground snow load is 40 psf is designed for 28 psf on a shingle roof up to 30° (a 7/12 pitch). Steeper and slippery metal roofs shed snow and get a slope factor below 1.0; roofs under 15° have a minimum load of up to 20 psf. Ground snow load is a statistical extreme for your location, not today's snow: get it from your local building department or the ASCE Hazard Tool, because mountains, hills and lake-effect belts vary block by block. The calculator also shows the depth of settled snow that reaches the load and the total pounds on your footprint. It leaves out drifts, sliding snow and rain-on-snow, so it is a planning number — check local code and leave design to an engineer.
Method: ASCE 7 flat-roof snow load pf = 0.7 × Ce × Ct × Is × pg and sloped-roof snow load ps = Cs × pf, with the slope factor falling linearly to zero at 70°. Region ranges are broad planning ranges for low elevations, not a map — use the ground snow load from your building department or the ASCE Hazard Tool (ascehazardtool.org).
| Region | For example | Ground snow load | Roof snow load |
|---|---|---|---|
| Deep South, Gulf Coast & low deserts | FL, LA, south TX, south AZ, coastal CA | 0–5 psf | 0.0–3.5 psf |
| Mid-South & Southern Plains | NC, TN, AR, OK, north TX, GA uplands | 5–15 psf | 3.5–10.5 psf |
| Mid-Atlantic & Ohio Valley | VA, MD, DE, KY, MO, south OH/IN/IL | 15–30 psf | 10.5–21.0 psf |
| Northeast corridor & lower Great Lakes | NJ, PA, NY metro, CT, RI, north OH/IN/IL, south MI, IA | 20–40 psf | 14.0–28.0 psf |
| Upper Midwest & northern New England | MN, WI, north MI, ND, SD, VT, NH, ME | 30–70 psf | 21.0–49.0 psf |
| Mountains & high elevations | Rockies, Sierra Nevada, Cascades, Appalachian highlands | 50–300+ psf | 35.0–210.0+ psf |
| Pitch | Shingles | Metal (slippery) |
|---|---|---|
| 0/12 | 28.0 psf | 28.0 psf |
| 2/12 | 28.0 psf | 26.1 psf |
| 4/12 | 28.0 psf | 22.2 psf |
| 6/12 | 28.0 psf | 18.7 psf |
| 8/12 | 25.4 psf | 15.6 psf |
| 10/12 | 21.1 psf | 13.0 psf |
| 12/12 | 17.5 psf | 10.8 psf |
Ground snow load (pg) is the weight of snow on the ground, in pounds per square foot, that a building in your location has to be designed for — a statistical extreme (roughly a once-in-50-years value on older maps), not this winter's snow. It comes from your local building department or the ASCE Hazard Tool, and it is the starting number for every roof snow load calculation.
Roof snow load is usually lower. ASCE 7 turns ground snow into flat-roof snow load with pf = 0.7 × Ce × Ct × Is × pg: the 0.7 reflects that wind and heat leave less snow on a roof than on the ground, Ce adjusts for wind exposure, Ct for how warm the roof is, and Is for how important the building is (1.0 for a house). A steeper or slippery roof sheds more, so the sloped-roof load ps = Cs × pf can be lower still.
Ask your local building department — many towns publish the design snow load on their website — or enter the address in the free ASCE Hazard Tool (ascehazardtool.org). Mountain areas are 'case study' zones where the value depends on elevation, so a regional map or this page's presets are only a starting point.
Yes. For a warm (heated) roof with shingles, the slope factor stays at 1.0 up to 30° (about a 7/12 pitch) and then falls in a straight line to zero at 70°. A slippery, unobstructed surface such as metal starts shedding from 5°. Snow guards, valleys and lower roofs below a taller one can hold more than the average, not less.
Drifts against walls and higher roofs, snow sliding from an upper roof, rain-on-snow surcharge, unbalanced loads on gable roofs, and the newer ASCE 7-22 maps, which publish strength-level ground snow loads by risk category and read higher than older maps. Any of these can govern a real roof, so treat the result as a planning number and leave design to an engineer.
A house built under the IRC uses the ground snow load in the local code's Table R301.2. Where that value is over 70 psf, the IRC's prescriptive rafter and truss tables no longer apply and the roof must be engineered. Truss drawings and permit plans usually state the design load; your building department can look it up by address.
Want today's snow in pounds instead? The roof snow load calculator weighs it by depth and snow type.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.