Understanding the snow load factors
Each factor adjusts the mapped ground snow load for a specific site and roof condition — none of them should be guessed, since they materially change the design load.
| Factor | Values used here | Effect |
|---|---|---|
| Exposure (Ce) | Sheltered 1.2 / Normal 1.0 / Windswept 0.9 | Sheltered sites retain more snow (higher load); windswept sites lose snow to wind (lower load) |
| Thermal (Ct) | Heated 1.0 / Unheated 1.2 | Heated buildings lose some snow to melt from below (lower load); unheated structures retain the full load |
| Slope (Cs) | 1.0 up to 30°; reduces to 0 by 70° | Steeper, slippery roofs shed snow more readily, reducing the sloped-roof design load below the flat-roof value |
- The ground snow load (pg) must be taken from the applicable local building code's snow load map or a site-specific study — it is not something this calculator can estimate, and using an incorrect value invalidates every downstream result.
- The 'Design guidance' result always reads 'designPerLocalCode' — a fixed reminder attached to every calculation, indicating that final roof snow load design must follow the applicable local building code, which may specify different exposure categories, thermal conditions, importance factors or slope-factor curves (including a distinction between slippery and non-slippery roof surfaces) than this simplified ASCE 7 form.
- This calculator does not account for snow drifting against parapets, adjacent higher roofs or other obstructions, which can create localized loads well above the uniform load calculated here.
What is roof snow load and the ASCE 7 form?
Roof snow load is the design load, expressed in kilopascals (kN/m²), that a roof structure must be able to carry from accumulated snow. Building codes in snow regions derive it from a mapped 'ground snow load' (pg) for the site, then adjust it for factors specific to the roof: how exposed the site is to wind (which can blow snow off the roof), whether the building is heated (which can partially melt snow off the roof), and the roof's own slope (steep roofs shed snow more readily than flat ones).
This calculator uses the widely referenced ASCE 7 flat-roof snow load form: pf = 0.7 × Ce × Ct × Is × pg, where Ce is the exposure factor, Ct is the thermal factor, and Is is the importance factor (fixed at 1.0 here for typical residential occupancy). A slope factor Cs is then applied to convert the flat-roof load into a sloped-roof load, since steeper roofs are assumed to shed more snow.
How to use this roof snow load calculator
- Enter the ground snow load (pg) for your site in kPa — this figure is set by the applicable local building code's snow load map, not measured directly on site.
- Select the terrain exposure: sheltered (increases load, since snow can't blow off as easily), normal/unobstructed, or windswept/fully exposed (reduces load, since wind removes more snow).
- Select the thermal condition: heated structures reduce load slightly (some snowmelt from below), while unheated or cold structures use a higher factor.
- Enter the roof pitch as X-in-12 to apply the slope reduction factor.
- Read the flat-roof design snow load (pf), the sloped-roof design snow load (ps) after the slope reduction, and the roof angle in degrees.
The formula behind the ASCE 7 snow load estimate
The flat-roof snow load is pf = 0.7 × Ce × Ct × Is × pg, where Ce (exposure factor) ranges from 0.9 (windswept) to 1.2 (sheltered), Ct (thermal factor) is 1.0 for heated structures and 1.2 for unheated structures, and Is (importance factor) is held at 1.0 for typical residential occupancy in this calculator. The sloped-roof load then applies a slope factor Cs: Cs = 1.0 for roof angles up to 30°, decreasing linearly toward 0 as the angle increases from 30° to 70° (reflecting that snow slides off steeper, slippery roofs more readily), giving ps = pf × Cs.
Worked example (calculator defaults): pg = 1.5 kPa, normal exposure (Ce = 1.0), heated structure (Ct = 1.0), 4:12 pitch. Flat-roof load pf = 0.7 × 1.0 × 1.0 × 1.0 × 1.5 = 1.05 kPa. Roof angle = arctan(4 ÷ 12) ≈ 18.43°, which is under the 30° threshold, so Cs = 1.0 and the sloped-roof load ps also equals 1.05 kPa.
Common mistakes
- Guessing or estimating the ground snow load (pg) instead of looking it up from the applicable local building code's snow load map for the actual site.
- Selecting the wrong exposure category — sheltered sites (surrounded by taller structures or dense trees) retain significantly more snow than fully exposed, windswept sites.
- Applying the slope reduction factor to a roof surface that isn't 'slippery' per code definitions (e.g., asphalt shingles vs. metal roofing can be treated differently), which can overstate the snow-shedding benefit.
- Ignoring drift loads near parapets, roof step-downs or adjacent taller structures, which building codes address separately from the uniform snow load calculated here.
Frequently asked questions
What is the ASCE 7 formula for roof snow load?
The flat-roof snow load formula is pf = 0.7 × Ce × Ct × Is × pg, where pg is the ground snow load, Ce is an exposure factor, Ct is a thermal factor, and Is is an importance factor. A slope factor is then applied separately to get the sloped-roof design load.
Where do I find the ground snow load for my site?
Ground snow load (pg) is set by the applicable local building code, typically from a snow load map or a site-specific study for higher-elevation or unusual terrain — it should never be estimated or guessed, since every other result in this calculator scales directly from it.
Why does a steeper roof have a lower snow load?
Steeper roofs shed accumulated snow more readily by sliding, especially with slippery roofing surfaces, so codes apply a slope reduction factor (Cs) that lowers the design snow load as roof angle increases beyond about 30°, down toward zero by very steep angles around 70°.
Why does an unheated building have a higher snow load factor?
Heated buildings lose some snow to gradual melting from warmth escaping through the roof, which can reduce accumulated snow load. Unheated or cold structures (garages, sheds without heating) don't get this melting effect, so codes apply a higher thermal factor (Ct = 1.2) to account for the full snow load being retained.
Is this calculator a substitute for a code-compliant snow load calculation?
No. It applies the general ASCE 7 flat-roof form as an educational estimate using simplified factor values. Actual roof design snow load — including drift loads, unbalanced loads and the correct local ground snow load — must be determined per the applicable building code, typically with input from a licensed structural engineer.
References
- ASCE/SEI 7 — Minimum Design Loads and Associated Criteria for Buildings and Other Structures: source of the flat-roof snow load form (pf = 0.7 · Ce · Ct · Is · pg) and slope-factor concept used here.
- International Code Council (ICC) — International Building Code (IBC) / International Residential Code (IRC), snow load provisions referencing ASCE 7.
- Local/state building code snow load maps — the authoritative source for ground snow load (pg) at a given site.