Flat Roof Snow Load Calculator (ASCE 7)
Flat and low-slope roofs carry the full flat-roof load Pf with no slope reduction, and they are the case where the §7.3.4 minimum load and the §7.10 rain-on-snow surcharge most often govern.
The worksheet
ASCE 7-22The balanced load of 21 psf governs (it exceeds the 20 psf minimum).
≈ 31,500 lb total on 1,500 sq ft of roof plan
Flat-roof load Pf = 0.7 × Ce(1) × Ct(1) × Is(1) × Pg(30) = 21 psf. On a flat roof Cs = 1.0, so the balanced load equals Pf.
| 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.
- 02At roof steps, parapets and against walls check drift loads (§7.7) with the drift calculator, and check sliding snow (§7.9) where this roof sheds onto a lower roof, walk or equipment.
- 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 flat roof roofs
- On a flat roof the sloped load equals the flat-roof load Pf, so exposure, thermal and importance factors drive the answer.
- Low-slope roofs in mild-winter areas (Pg ≤ 20 psf) pick up a 5 psf rain-on-snow surcharge: rain soaking into snow that can't drain.
Notes & questions
01How much snow load can a flat roof hold?+
It depends on the structure, but the design value is what matters: enter your ground snow load above to get the ASCE 7 flat-roof load Pf in psf, then compare it to the roof's rated capacity on the structural drawings.
02Why does my flat roof need a rain-on-snow surcharge?+
Where ground snow is light (Pg ≤ 20 psf) and the roof is nearly flat, rain can fall onto snow that has no way to drain, adding weight. ASCE 7 §7.10 adds 5 psf to the balanced load to cover it.