Article — Snow Load Calculator
Snow load calculator: roof snow load per ASCE 7-22
Roof snow load per ASCE 7-22 is calculated as p_s = 0.7 × Ce × Ct × Is × pg × Cs. Typical US residential design loads range from 20 to 60 psf, with mountain regions exceeding 100 psf. The 0.7 base factor accounts for wind blowing snow off roofs, while Cs reduces load for steeper pitches.
Designing a roof to carry snow is not optional in most of North America. Failure under snow load is one of the leading causes of structural collapse, killing dozens and damaging thousands of buildings each year. This calculator follows ASCE 7-22 Chapter 7, the same standard adopted by the International Building Code and enforced by local building departments across the United States.
What is roof snow load?
Roof snow load is the vertical force per unit area exerted by accumulated snow on a roof. It is measured in pounds per square foot (psf) in the US or kilopascals (kPa) in metric countries. One psf equals 0.04788 kPa or about 4.88 kg/m². A 40 psf roof load means each square foot of roof carries 40 pounds of snow weight, equivalent to roughly 195 kg/m².
The weight of snow depends on its density, not just depth. Fresh powder weighs about 5 lbs per cubic foot. Settled snow can reach 15-25 lbs/ft³. Wet, heavy snow or ice can exceed 30 lbs/ft³. Two roofs with the same snow depth can have very different loads if one holds dry powder and the other a saturated slush layer.
Ground vs roof snow load
Ground snow load (p_g) is the weight of snow on the ground over a 50-year return period. ASCE 7-22 publishes p_g values by location, with values mapped from decades of National Weather Service data. Snowloadbyzip.com and the ASCE Hazard Tool give instant lookups by ZIP code or coordinates.
Roof snow load is almost always lower than ground snow load. Wind removes snow from exposed roofs. Heat conducting through the roof melts the lower layer. The 0.7 multiplier in the ASCE formula captures the typical wind effect — buildings in sheltered locations get a higher Ce factor that partially cancels this reduction.
The 50-year return period in ASCE 7-22 means there is a 2% chance each year that ground snow load will exceed the design value. Over a 50-year building life, the probability of at least one exceedance is about 64%. Hospitals and emergency facilities use Cat IV factors precisely because they cannot fail during the storms that exceed design loads.
The ASCE 7-22 snow load formula
For a flat or low-slope roof, snow load is p_f = 0.7 × Ce × Ct × Is × pg. The 0.7 factor reflects partial wind removal. Each of the C and I factors adjusts for site conditions. For a sloped roof, snow load drops by an additional slope factor Cs: p_s = Cs × p_f.
A worked example: a Minneapolis home in a typical suburb with p_g = 50 psf, asphalt shingle roof at 20° pitch, heated interior, residential occupancy. Ce = 1.0, Ct = 1.0, Is = 1.0, Cs = 1.0 (slope ≤ 30° on rough surface). The flat roof load is 0.7 × 1.0 × 1.0 × 1.0 × 50 = 35 psf. Since Cs = 1.0 at 20°, the sloped roof load also equals 35 psf.
ASCE 7-22 enforces a minimum snow load of 20 × Is psf for roofs with slope ≤ 15°, regardless of how low p_g is. A residential building in Atlanta with p_g = 5 psf still needs a 20 psf design load for a flat roof. This rule prevents unsafe construction in mild-climate zones where occasional storms exceed historical averages.
Exposure, thermal, importance factors
The exposure factor Ce ranges from 0.8 to 1.2. Sheltered roofs (surrounded by taller buildings or dense conifers) get 0.8 — they accumulate more snow because wind can't reach them. Fully exposed roofs in open country get 1.1 to 1.2 because wind blows snow away. Most suburban homes default to 1.0.
The thermal factor Ct reflects heat loss through the roof. A heated residence uses Ct = 1.0. Unheated structures like detached garages use 1.1. Cold storage above freezing uses 1.1. Continuously below freezing structures use 1.2. Walk-in freezers use 1.3. The principle: more heat means more snow melts off the bottom of the snowpack.
The importance factor Is depends on occupancy category. Cat I (temporary buildings, minor agricultural) = 0.8. Cat II (residential, most commercial) = 1.0. Cat III (schools, public assembly above 300 occupants) = 1.1. Cat IV (hospitals, fire stations, emergency operation centers) = 1.2. The factor scales the entire load — a hospital must handle 20% more snow than an equivalent home.
Roof slope effect on snow load
The slope factor Cs reflects snow sliding off pitched roofs. Cs depends on roof angle, surface material, and thermal condition. For a typical asphalt shingle roof (warm, rough surface), Cs = 1.0 up to 30° pitch, then drops linearly to 0 at 70°. For a metal standing-seam roof (slippery), Cs = 1.0 only up to 5°, then drops faster — snow slides off readily.
This is why metal roofs in snow country need ice guards or snow rails: even though they carry less snow load on average, sudden mass discharges can damage gutters, vehicles, and people below. The ASCE formula gives lower design loads for slippery roofs, but operational safety demands separate consideration.
If your roof has multiple slopes, calculate each section separately. A combined gable-and-shed roof might have one face at 30° (full snow load) and another at 50° (reduced load). Use the most severe load condition for the framing that supports both faces.
Snow load by US region
Ground snow load varies dramatically across the United States. The Gulf coast and Florida have p_g = 0 to 5 psf — essentially symbolic loads enforced only by the 20 psf minimum. The mid-Atlantic averages 15-25 psf. The Great Lakes and northern Midwest run 20-40 psf. Boston, Buffalo, and Portland Maine reach 30-60 psf.
The Rocky Mountains spike highest. Aspen, Vail, and Jackson Hole regularly design to 80-200 psf. Some ski resort buildings carry over 300 psf. These extreme loads require engineered solutions — heavy timber, steel framing, snow-shedding metal roofs, and routine snow removal during peak winters.
1 psf ≈ 4.88 kg/m²1 psf ≈ 0.04788 kPafresh snow ≈ 5 lb/ft³wet snow / ice ≈ 30 lb/ft³typical residential design 20 - 60 psfCommon snow load design mistakes
The most expensive errors come from ignoring drift loads. The balanced snow load calculated here is only the starting point. Where snow piles against parapets, taller adjacent buildings, or roof steps, drift loads can reach 2-3 times the balanced load. ASCE 7-22 Section 7.7 covers drift calculations and they require separate spreadsheets or design software.
A second common error is misusing the exposure factor. Roofs covered by tree canopies do not always get Ce = 0.8 — only if the trees are taller than the building and within 10 height-units of the roof. Roofs near taller buildings get sheltered treatment only on the windward side. Field judgment matters.
- 0.7 base factor in ASCE 7-22 — accounts for wind removal of snow from typical roofs
- 20 psf × Is minimum design load for low-slope roofs regardless of p_g
- 50-year return period for p_g — 2% annual exceedance probability
- 30 degrees threshold for slope factor reduction on rough surfaces
- 5 degrees threshold for slope factor reduction on slippery surfaces
- 0.8 - 1.2 range for Ce, Ct, Is factors
- 2-3 times balanced load is typical near drift surfaces
- 15 lb/ft³ typical density of settled snow used in depth-to-load conversion