Illinois Roof Snow Load Calculator
Ground snow load across the populated parts of Illinois runs roughly 20 to 30 psf (moderate). The calculator below is pre-loaded with a typical Illinois value of 25 psf. Change it to your confirmed site value and adjust the roof.
The worksheet
ASCE 7-22At 18.4° the §7.3.4 minimum does not apply, so the balanced load of 17.5 psf governs.
≈ 26,250 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 17.41 psf over 8.09 ft · hd 1.75 ft · γ 17.25 pcf
Flat-roof load Pf = 0.7 × Ce(1) × Ct(1) × Is(1) × Pg(25) = 17.5 psf. Sloped-roof balanced load Ps = Cs(1) × Pf = 17.5 psf.
| Ground snow load, Pg | 25 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 | 17.25 pcf | Eq. 7.7-1 |
| Settled depth at design load | 12 in | design ÷ density |
- 01Estimated snow density ≈ 17.25 pcf, so 17.5 psf is roughly 12 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 17.5 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.
Finding your exact Illinois ground snow load
The number above is a planning value, not a permit value. For Illinois, get your site's design ground snow load from the ASCE 7 Hazard Tool or your local building department. Many Illinois jurisdictions adopt a published county or town value.