Flat roof snow load = 0.7 × Ce × Ct × Is × ground snow load. With a 40 psf ground snow load, a heated house of risk category II in open terrain (Ce 1.0, Ct 1.0, Is 1.0) has a flat roof snow load of 28 psf. On rafters 16 in on center that is 37 lb per foot of rafter, and 37,632 lb of snow on a 1,344 sq ft footprint.
How roof snow load is calculated
ASCE 7 starts with the ground snow load, pg, the weight of snow on the ground at your site. Three factors turn it into a flat roof load, pf: the exposure factor Ce for how much wind sweeps the roof, the thermal factor Ct for how much heat leaks through it, and, in ASCE 7-16 and 7-10, the importance factor Is for the building's risk category.
pf = 0.7 × Ce × Ct × Is × pg (ASCE 7-16) · pf = 0.7 × Ce × Ct × pg (ASCE 7-22)
Worked example: pg = 40 psf, Ce 1.0 (terrain C, partially exposed), Ct 1.0 (heated house), Is 1.0 (category II). pf = 0.7 × 1.0 × 1.0 × 1.0 × 40 = 28 psf. On rafters 16 in on center each foot of rafter carries 28 × 16 ÷ 12 = 37.3 lb. The footprint of 1,344 sq ft carries 28 × 1,344 = 37,632 lb.
The minimum snow load
ASCE 7-16 sets a floor for low-slope roofs. Where the ground load is 20 psf or less the minimum is Is × pg, and where it is more the minimum is 20 psf × Is. The design load is the larger of pf and the minimum. At a ground load of 20 psf, pf is 0.7 × 20 = 14 psf, but the minimum is 20 psf, so the minimum governs and the calculator says so. At 40 psf the flat roof load of 28 psf is already above the 20 psf minimum. The ASCE 7-22 edition changes these rules: the calculator applies the 7-22 flat roof equation only and leaves the minimum load for the engineer.
The factors and their values
| Factor | Condition | Value |
|---|---|---|
| Ce, terrain B | fully exposed / partially / sheltered | 0.9 / 1.0 / 1.2 |
| Ce, terrain C | fully exposed / partially / sheltered | 0.9 / 1.0 / 1.1 |
| Ce, terrain D | fully exposed / partially / sheltered | 0.8 / 0.9 / 1.0 |
| Ct | heated structure | 1.0 |
| Ct | ventilated cold roof, R over 25 | 1.1 |
| Ct | unheated structure | 1.2 |
| Is | risk category I / II / III / IV | 0.8 / 1.0 / 1.1 / 1.2 |
An unheated shed under the same 40 psf ground load has Ct 1.2, so pf = 0.7 × 1.2 × 40 = 33.6 psf. A sheltered roof in terrain B with Ce 1.2 collects more snow than an exposed one because the wind does not blow it off.
Where to get the ground snow load
The ground snow load is the one number the calculator cannot work out for you. It comes from the ASCE 7 ground snow load map or the state tables in the code, and the IBC's contour values run in steps such as 10, 14, 19, 23, 32, 44, 62 and 86 psf. It changes with elevation and can change within a few miles, so a state average is not safe. Ask your building department for the design value, or use the ASCE Hazard Tool with your address, and type it in. Metric users enter kPa: 1 psf is 0.0479 kPa.
What this number does not cover
This is the balanced load on a flat roof. Slope reductions, drifts against walls and dormers, sliding snow, rain-on-snow and unbalanced loads are separate steps in ASCE 7 and often govern the real design. Never use it alone to size a rafter or a beam: it is the load you take to the span tables. Next, the rafter calculator lays out the members, the roof truss calculator counts trusses, and the beam span calculator checks a beam carrying the roof. If you are re-covering, the roofing calculator has the materials, and roof pitch turns your slope into the numbers snow charts use. Everything roof related is under roofing and siding.