Ground Snow Load to Roof Snow Load Calculator
Roof snow load is the ground snow load converted for your specific roof: Pf = 0.7 × Ce × Ct × Is × Pg, then × Cs for slope. Enter your ground snow load below and adjust the factors to see exactly how the roof value is derived.
The worksheet
ASCE 7-22At 18° the §7.3.4 minimum does not apply, so the balanced load of 21 psf governs.
≈ 31,500 lb total on 1,500 sq ft of roof plan
Large gable roof (W > 20 ft): windward side at 0.3·Ps, leeward side at Ps plus a drift surcharge from the ridge.
surcharge 18.98 psf over 8.7 ft · hd 1.86 ft · γ 17.9 pcf
Flat-roof load Pf = 0.7 × Ce(1) × Ct(1) × Is(1) × Pg(30) = 21 psf. Sloped-roof balanced load Ps = Cs(1) × Pf = 21 psf.
| Ground snow load, Pg | 30 psf | site input |
| Exposure factor, Ce | 1 | Table 7.3-1 |
| Thermal factor, Ct | 1 | Table 7.3-2 |
| Importance factor, Is | 1 | Table 1.5-2 |
| Slope factor, Cs | 1 | Fig. 7.4-1 |
| Snow density, γ | 17.9 pcf | Eq. 7.7-1 |
| Settled depth at design load | 14 in | design ÷ density |
- 01Estimated snow density ≈ 17.9 pcf, so 21 psf is roughly 14 in of settled snow.
- 02The §7.6.1 unbalanced (leeward-drift) case is computed above for this gable/hip roof; at roof steps, parapets and walls also check drift loads (§7.7) with the drift calculator, plus sliding snow (§7.9) onto anything below.
- 03Ground snow load Pg is set by your local building department / the ASCE 7 Hazard Tool. Always confirm the value for your exact site before you build or submit.
Reduce the load on an existing roof
Your design roof snow load is 21 psf. If the design load is close to your roof's capacity, the cheapest fix is to keep snow off it. These are the tools that do that.
A long-handled rake pulls snow off from the ground, cutting the load before it builds up. The first thing to buy in a heavy winter.
Shop roof rakesSelf-regulating heat cable along eaves and valleys stops the ice dams that trap meltwater and add weight.
Shop heat cableSome links are affiliate links: if you buy through them we may earn a commission at no extra cost to you. These are starting points, not engineering endorsements. Always confirm a product's rated snow load meets the design load above.
Notes for ground → roof conversion roofs
- The 0.7 base factor already assumes some snow blows or melts off; the exposure, thermal and importance factors fine-tune it.
- On a normal heated, sheltered house the roof load is typically 60–70% of the ground snow load, but exposure and importance can push it higher.
Notes & questions
01How do you convert ground snow load to roof snow load?+
Multiply the ground snow load Pg by 0.7 and by the exposure (Ce), thermal (Ct) and importance (Is) factors to get the flat-roof load Pf, then by the slope factor Cs for a pitched roof. This calculator does it and shows every step.
02Is roof snow load always less than ground snow load?+
Usually, because the 0.7 factor accounts for snow blowing and melting off the roof. But a fully exposed, important or unheated building can have factors that push the roof load close to the ground value, and occasionally above it.