Ground snow load is looked up, not measured
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 — a figure set by the applicable local building code's snow load map or a site-specific study, never estimated or guessed on site. Every other result in the formula scales directly from this one number, so using the wrong pg invalidates the whole calculation.
The ASCE 7 flat-roof formula
The widely referenced ASCE 7 flat-roof snow load form is pf = 0.7 × Ce × Ct × Is × pg. Ce is an exposure factor ranging from 0.9 (windswept, fully exposed sites that lose snow to wind) to 1.2 (sheltered sites that retain more snow). Ct is a thermal factor: 1.0 for heated structures, which lose some snow to melt from below, or 1.2 for unheated structures like garages and sheds, which retain the full load. Is is an importance factor, held at 1.0 for typical residential occupancy.
A slope factor Cs then converts the flat-roof load into a sloped-roof design load, since steeper roofs shed snow more readily: Cs = 1.0 for roof angles up to 30°, decreasing linearly toward 0 as the angle increases from 30° to 70°, giving the sloped-roof load ps = pf × Cs.
- Flat-roof snow load pf = 0.7 × Ce × Ct × Is × pg
- Sloped-roof snow load ps = pf × Cs
Worked example
With a ground snow load pg = 1.5 kPa, normal exposure (Ce = 1.0), a heated structure (Ct = 1.0) and a 4:12 roof pitch: the flat-roof load is pf = 0.7 × 1.0 × 1.0 × 1.0 × 1.5 = 1.05 kPa. The roof angle is arctan(4 ÷ 12) ≈ 18.43°, which is under the 30° threshold where the slope factor begins reducing the load, so Cs = 1.0 and the sloped-roof design load ps also equals 1.05 kPa.
| Factor | Value used |
|---|---|
| Ground snow load (pg) | 1.5 kPa |
| Exposure factor (Ce) | 1.0 (normal) |
| Thermal factor (Ct) | 1.0 (heated) |
| Importance factor (Is) | 1.0 (residential) |
| Flat-roof load (pf) | 1.05 kPa |
| Roof angle (4:12 pitch) | ≈ 18.43° (Cs = 1.0) |
| Sloped-roof load (ps) | 1.05 kPa |
This is an educational estimate, not a stamped design
This formula is a widely referenced ASCE 7 structural estimating tool, useful for understanding how the factors interact — it is not a substitute for a stamped structural design. The ground snow load (pg) must always be taken from the applicable local building code's snow load map, and final roof framing must be verified by a licensed structural engineer against the governing code for the actual site.
Frequently asked questions
What is the ASCE 7 snow load formula?
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.
Why does a heated building have a lower snow load?
Heated buildings lose some snow to gradual melting from warmth escaping through the roof, which can reduce accumulated snow load. Unheated structures like garages and sheds 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.
Does roof pitch always reduce the snow load?
Only above a threshold. The slope factor Cs stays at 1.0 for roof angles up to 30° and only starts decreasing toward 0 as the angle rises from 30° to 70°, reflecting that snow slides off steeper, slippery roofs more readily.
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.
- Local/state building code snow load maps — the authoritative source for ground snow load (pg) at a given site.
- International Code Council (ICC) — International Building Code (IBC), structural snow load provisions.