Roof snow load
Ground snow is not roof snow. Exposure, roof temperature, importance and slope all reduce it — and a drift can add more back than the slope took away. This runs the full ASCE 7-22 Chapter 7 chain from ground snow to the peak load a member is designed for.
| Step | Value | Source |
|---|---|---|
| Ground snow pg | 40 psf | Site value from the hazard map or the authority |
| Exposure factor Ce | 1.00 | Table 7.3-1, partially exposure, category B |
| Thermal factor Ct | 1.00 | Table 7.3-2 |
| Importance factor Is | 1.00 | Table 1.5-1, risk category |
| Flat roof snow pf | 28.0 psf | pf = 0.7 Ce Ct Is pg |
| Minimum 20Is | 20.0 psf | §7.3.4, applied when the formula falls below it |
| Governing pf | 28.0 psf | Formula governs |
| Roof slope | 4.0° | 0.8 in 12 |
| Slope factor Cs | 1.000 | Figure 7.4-1, warm roof |
| Rain-on-snow surcharge | 0 psf | Not applicable |
| Balanced roof snow ps | 28.0 psf | ps = Cs pf + surcharge |
| Settled density γ | 19.2 pcf | γ = 0.13 pg + 14, capped at 30 |
| Balanced depth | 1.46 ft | hc = pf / γ |
| Leeward drift hd | 2.41 ft | §7.7.1, 0.43 lu^⅓ (pg + 10)^¼ − 1.5 |
| Windward drift | 1.81 ft | 0.75 hd |
| Unbalanced windward | 8.4 psf | §7.6.1, 0.3 pf on the leeward-only case |
| Total | 43.4 psf | balanced plus the drift surcharge |
A drift taller than the balanced snow depth is a real surcharge; a shorter one is not a drift at all and the balanced load governs. That check is why the same fetch gives different answers on different roofs.
pf = 0.7 Ce Ct Is pg · Equation 7.3-1.
ps = Cs pf · Equation 7.4-1, with Cs from Figure 7.4-1.
pf ≥ 20 Is · §7.3.4, and it does not apply where a drift governs.
hd = 0.43 lu^⅓ (pg + 10)^¼ − 1.5 · §7.7.1, imperial units throughout.
A clear path from inputs to results.
Ground snow is not roof snow. Exposure, roof temperature, importance and slope all reduce it — and a drift can add back more than the slope took away. This runs the full ASCE 7-22 Chapter 7 chain from ground snow to the peak load a member is actually designed for.
How to use
- Enter the ground snow load for the site from the ASCE 7 hazard map or the local authority.
- Choose the roof exposure and the terrain category — together they set Ce.
- Pick the thermal condition and the risk category, which give Ct and Is.
- Set the roof slope and the fetch available for a leeward drift.
- Read the flat roof load, the sloped roof load and the peak load at the drift.
The formula
Reference: ASCE/SEI 7-22, Chapter 7: Equations 7.3-1 and 7.4-1, Tables 7.3-1, 7.3-2 and 1.5-1, Figure 7.4-1, §7.3.4 minimum and §7.7.1 drift.
Worked example
40 psf ground snow on a low-slope roof
- Ground snow 40 psf, partially exposed, terrain category B.
- Thermal factor 1.0 (heated), Risk Category II, slope 4°.
- 40 ft of fetch available for a leeward drift.
- 1
Partially exposed in category B gives Ce = 1.0.
- 2
Flat roof snow is 0.7 × 1.0 × 1.0 × 1.0 × 40 = 28 psf.
- 3
That is above the 20Is minimum of 20 psf, so the formula governs.
- 4
At 4° the slope factor is still 1.0, so the sloped roof load is also 28 psf.
- 5
Settled density is 0.13 × 40 + 14 = 19.2 pcf, so the balanced depth is 1.46 ft.
- 6
The leeward drift is 0.43 × 40^⅓ × 50^¼ − 1.5 = 2.41 ft, which exceeds 1.46 ft and therefore governs.
- 7
The drift adds 19.2 × 2.41 / 3 = 15.4 psf at its peak, for a total of 43.4 psf.
28 psf balanced, 43.4 psf at the drift.
Interpreting the result
The drift is the number most often missed. A 28 psf balanced load looks comfortable; 43.4 psf at a parapet or a step is a different member. The other thing to notice is that the minimum load is not a safety blanket — it only applies when the formula falls below it, and it does not apply where a drift governs.
Assumptions & limitations
- Ground snow load is an input, not a lookup: the ASCE 7 maps are regional and some jurisdictions publish a higher case-history value that governs.
- The exposure and thermal factors come from the ASCE 7 tables for the stated conditions, with no adjustment for unusual geometry.
- The slope factor follows Figure 7.4-1 with a breakpoint at 30° for a warm roof and 45° for a cold roof, both falling to zero at 70°.
- The rain-on-snow surcharge is 5 psf and applies only for ground snow of 20 psf or less on a roof flatter than about 3°.
- Drift height uses the §7.7.1 leeward expression and is checked against the balanced snow depth.
- Unbalanced loads for gable roofs, sliding snow from a higher roof, and curved-roof cases are reported only where noted.
Frequently asked questions
Why is roof snow less than ground snow?
Wind blows snow off the roof, the roof is warmer than the ground, and the building's importance changes the margin. The 0.7 factor with Ce, Ct and Is captures all three, and together they typically take 30–50% off the ground value.
Why does a cold roof keep more snow?
Because meltwater refreezes at the cold eave instead of running off, so the snowpack is held rather than shed. The slope factor breakpoint moves from 30° to 45°, which on a 35° roof is the difference between Cs = 0.88 and Cs = 1.0.
What about rain on snow?
ASCE 7 §7.10 adds 5 psf for roofs with a ground snow load of 20 psf or less and a slope below about 3°, because rain can saturate a thin snowpack without draining. It is a small number that matters on light-snow sites precisely because the balanced load is small there.
When does the 20 psf minimum apply?
When 0.7 Ce Ct Is pg falls below 20 Is, which happens on light-snow sites with a favourable exposure. It is a floor on the flat roof load, and it does not apply where a drift or a sliding-snow load governs the member.
How is the drift height calculated?
hd = 0.43 lu^⅓ (pg + 10)^¼ − 1.5, in feet and psf. It grows with the cube root of the fetch, so doubling the roof width adds only about 26%. If hd is less than the balanced snow depth there is no drift to add.
Methodology last reviewed 2026-10-21. Results are informational and not financial or engineering advice — see our disclaimer.