How Roof Snow Load Is Calculated
Structural engineers don't design a roof for the raw ground snow load — they convert it into a flat-roof snow load (Pf) using ASCE 7's simplified formula, then apply a slope factor (Cs) because snow slides off steep roofs and doesn't accumulate the same way it does on flat ones.
The formula: Pf = 0.7 × Ce × Ct × I × Pg. Then: Ps = Cs × Pf.
Ground Snow Load (Pg) — where to find yours
Ground snow load is set by your local building code, not something you estimate. It typically ranges from near 0 psf in the Deep South to over 100 psf in parts of the Mountain West and interior New England, and it can vary significantly within the same county due to elevation. Always confirm your Pg value with your local building department or the ASCE 7 Hazard Tool — this calculator only converts the ground value into a roof design value, it does not supply the ground value for your specific address.
Slope (Cs) Reduces the Load on Steep Roofs
| Roof Pitch | Angle | Typical Cs (warm roof, unobstructed) |
|---|---|---|
| 0/12 – 5/12 | 0° – 22.6° | 1.00 (no reduction) |
| 6/12 | 26.6° | ~0.85 |
| 8/12 | 33.7° | ~0.65 |
| 10/12 | 39.8° | ~0.45 |
| 12/12 | 45.0° | ~0.25 |
| Steeper than 70° | 70°+ | 0 (snow does not accumulate) |
Slope factor curves also depend on whether the roof is "slippery" (metal, unobstructed) versus "non-slippery" (asphalt shingles with snow guards or roof obstructions), and on the thermal factor — unheated structures hold snow longer and get less slope credit. This calculator uses the standard warm-roof curve; a licensed engineer will apply the exact curve for your roof's specific surface and obstruction conditions.
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What is a good snow load for a roof?
There's no universal "good" number — it depends entirely on your location's code-required ground snow load. Most of the continental US falls between 10 and 50 psf ground snow load, with mountain and far-northern areas requiring 60-100+ psf. Your local building department sets the required design value for your address.
How do I calculate the snow load on my roof?
Start with your area's ground snow load (Pg) from your local building code. Multiply by 0.7, then by your exposure factor, thermal factor, and importance factor to get the flat-roof load (Pf). Multiply that by the slope factor for your roof's pitch to get the final design roof snow load (Ps). Use the calculator above to run the full formula.
Why does a steeper roof have a lower snow load?
Snow slides off steep roofs rather than accumulating, so codes apply a slope factor that reduces the design load as pitch increases. A 12/12 roof carries roughly a quarter of the load of a flat roof under the same ground snow conditions, because most of the snow sheds before it can build up.
What is the difference between ground snow load and roof snow load?
Ground snow load (Pg) is the raw measured/coded snow depth-equivalent load at ground level for your region. Roof snow load (Pf or Ps) is what the roof structure is actually designed to carry, which is almost always lower than Pg because of wind exposure, roof heat loss, and slope shedding.
Do I need an engineer for snow load calculations?
For new construction or any structural framing change (new roof pitch, removed load-bearing wall, added dormer, solar panel racking), yes — a licensed structural engineer should verify the final snow load calculation and stamp the design where your jurisdiction requires it. This calculator is a planning and educational tool, not a substitute.