Snow is heavy: a foot of fresh snow can weigh 5 to 20 pounds per square foot, and wet snow or ice weighs much more. Roofs are designed to carry a specified snow load, and the value comes from the ground snow load at the site, adjusted for how exposed the roof is, how warm the building is, how important the building is and how steep the roof is. This calculator follows the simplified procedure of ASCE 7, the standard that building codes use in the United States.
How to use the roof snow load calculator
- Enter the ground snow load (pg) for your site. Get it from your local building department or the ASCE 7 maps and tables; do not guess it.
- Select the exposure: fully exposed, partially exposed or sheltered by trees and nearby buildings.
- Select the thermal condition and the risk category of the structure.
- Enter the roof pitch and read the flat-roof and sloped-roof loads.
Snow load formula
Minimum: pm = Is × pg if pg ≤ 20 psf; pm = 20 × Is if pg > 20 psf
Sloped roof: ps = Cs × pf
Use the greater of ps and pm
Ce is the exposure factor, Ct the thermal factor, Is the importance factor and Cs the slope factor. For warm roofs (Ct ≤ 1.0) with unobstructed non-slippery surfaces, Cs is 1.0 up to 30° and falls linearly to 0 at 70°; for colder roofs the reduction starts at 37.5°.
Worked example: a house with 30 psf ground snow, 6/12 roof
- Factors: Ce = 1.0 (partially exposed), Ct = 1.0 (heated), Is = 1.0 (risk category II).
- Flat-roof load: 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21.0 psf.
- Minimum load: pg is above 20, so pm = 20 × 1.0 = 20.0 psf.
- Roof angle 26.6° is below 30°, so Cs = 1 and ps = 21.0 psf. The design value is the larger of ps and pm: 21.0 psf.
Flat-roof load for typical ground snow loads
Partially exposed, heated, risk category II.
| Ground snow load pg (psf) | Flat-roof pf (psf) | Minimum pm (psf) | Value to use (psf) |
|---|---|---|---|
| 10 | 7.0 | 10.0 | 10.0 |
| 20 | 14.0 | 20.0 | 20.0 |
| 30 | 21.0 | 20.0 | 21.0 |
| 40 | 28.0 | 20.0 | 28.0 |
| 50 | 35.0 | 20.0 | 35.0 |
| 60 | 42.0 | 20.0 | 42.0 |
What this does not cover
- Drifting against parapets, taller walls, dormers and rooftop units can double the load locally.
- Unbalanced loads on gable roofs, where wind blows snow from one side to the other.
- Rain-on-snow, sliding snow from a higher roof and snow retention devices.
- Combination with other loads such as dead load and wind in the load cases of ASCE 7.
Why it matters
Understanding snow load helps you talk with an engineer or truss supplier, to check whether an old roof can take a new heavy roofing material, or to decide when to remove snow safely. Never rely on this page for structural sizing. A truss designer or engineer will calculate the loads for your county, and the building department must approve them.
Roof geometry tools: Roof Pitch Calculator, Rafter Length Calculator and Roof Sheathing Calculator. To protect the eaves from ice dams, see the Ice and Water Shield Calculator, and for materials, the Roofing Calculator.
Frequently asked questions
What is ground snow load?
The weight of snow on the ground at your location, in pounds per square foot (psf), used as a starting point for roof design. Local building departments and ASCE 7 give the values; they range from near zero in warm areas to more than 100 psf in mountain towns.
What is the formula for roof snow load?
For a flat roof, ASCE 7 gives pf = 0.7 × Ce × Ct × Is × pg. For a sloped roof, multiply by the slope factor Cs: ps = Cs × pf. The calculator applies the minimum-load rules as well.
Why does a steeper roof carry less snow?
Snow slides off steeper roofs, particularly slippery ones. The slope factor Cs reduces the design load for slopes above 30° for heated (warm) roofs and above 37.5° for cold roofs, down to zero at 70°.
Is this enough to design a roof?
No. This is a simplified educational estimate. Real designs include drift, unbalanced and rain-on-snow loads, sliding snow and local rules. Structural members must be sized by a qualified engineer or from a truss designer’s drawings.
What does the thermal factor Ct mean?
Heat escaping through the roof melts snow, so heated buildings (Ct = 1.0) carry less snow than unheated ones (1.2). Roofs kept just above freezing have 1.1.