Snow Load Calculator (ASCE 7-22)
Calculate the design snow load on your roof using ASCE 7-22 Chapter 7 formulas. Enter your ground snow load, roof slope, exposure factor, thermal factor, and importance factor to get the flat roof load, slope-adjusted design load, equivalent snow depth, and drift surcharge at a roof step. Results update instantly in imperial (psf) or metric (kN/m2) units.
How snow load is calculated under ASCE 7-22
The American Society of Civil Engineers standard ASCE 7-22 uses a two-step process for roof snow load design. First, the flat roof snow load (pf) is determined: pf = 0.7 x Ce x Ct x Is x pg. The 0.7 coefficient accounts for the fact that roofs shed some snow even before pitch is considered. Ce is the exposure factor (how much wind clears the roof), Ct is the thermal factor (how much heat escapes through the roof to melt snow), Is is the importance factor tied to the building risk category, and pg is the ground snow load from the mapped 50-year mean recurrence interval value for your site. The sloped-roof snow load ps is then ps = Cs x pf, where Cs is a slope-reduction factor that depends on the roof pitch and whether the surface is slippery. Steep, slippery roofs can have Cs near zero, meaning almost no design load. Flat roofs always use Cs = 1.0.
Ground snow load: how to find your pg value
The ground snow load pg is the starting point for every ASCE 7-22 snow load calculation. It represents the 50-year mean recurrence interval ground snow load in pounds per square foot (psf) for your location. In the continental United States, pg ranges from 0 psf in Florida and the Gulf Coast to over 300 psf in some Colorado and Sierra Nevada high-altitude locations. The ASCE 7-22 standard provides a map (Figure 7.2-1) and a jurisdiction-specific database, but many jurisdictions publish adopted ground snow load values that supersede the national map. Always check with your local building department or adopted IBC chapter for the controlling value. In Alaska, site-specific studies are often required because the terrain varies dramatically over short distances. In Canada, use the 1-in-50-year snow load from the NBC Climatic Data for Canadian Cities table; a typical conversion is 1 kPa = 20.9 psf.
Drift loads at roof steps and parapets
Snow drift is the accumulation of wind-blown snow at obstructions such as a parapet wall, a lower adjacent roof, or a mechanical equipment curb. Drift loads are often the controlling design case for the structural members near those obstructions. ASCE 7-22 Figure 7.7-1 defines the leeward drift height hd = 0.43 x lu^(1/3) x (pg+10)^(1/4) - 1.5, where lu is the upwind fetch length in feet and pg is in psf. The drift is triangular in cross section, with a peak pressure pd = hd x gamma at the obstruction face and zero load at width w = 4 x hd away from it. The drift height is capped at h minus hb (the existing balanced snow height), where h is the vertical step height. For parapets, a windward drift (from the parapet wall toward the roof) can also form, but it is typically smaller than the leeward drift on the lower roof and should be checked separately.
Rain-on-snow surcharge and minimum roof loads
When the ground snow load is 20 psf or less and the roof slope is shallower than W/50 (where W is the eave-to-ridge horizontal dimension in feet), ASCE 7-22 Section 7.10 requires an additional 5 psf rain-on-snow surcharge. This accounts for the possibility of rain falling onto a thin snowpack, saturating it, and dramatically increasing the load. ASCE 7-22 also sets a minimum roof load: the design flat roof load pf cannot be taken as less than Is x pg for pg values of 20 psf or less. This minimum prevents an unusually favorable combination of Ce and Ct from producing an unrealistically low design load in low-snow regions where the 5 psf surcharge condition is most likely to apply.
ASCE 7-22 adjustment factor quick reference
| Factor | Condition | Value | Note |
|---|---|---|---|
| Ce | Fully exposed, windy ridge | 0.70 | Maximum wind-shedding credit |
| Ce | Partially exposed, open terrain | 0.90 | Low-density suburban |
| Ce | Partially exposed, suburban/wooded | 1.00 | Default for most buildings |
| Ce | Sheltered, forest or dense urban | 1.10-1.20 | No wind-shedding credit |
| Ct | Heated, well-insulated (R >= 30) | 0.85 | Heat loss melts some snow |
| Ct | Heated, standard | 1.00 | Default for most buildings |
| Ct | Unheated or ventilated | 1.20 | Cold roof - no melting credit |
| Ct | Freezer building | 1.30 | Actively refrigerated space |
| Is | Category I - low risk | 0.80 | Agricultural, minor storage |
| Is | Category II - standard | 1.00 | Most residential and commercial |
| Is | Category III - high risk | 1.10 | Assembly, schools, jails |
| Is | Category IV - essential | 1.20 | Hospitals, fire stations |
Common Ce, Ct, and Is values. Full tables in ASCE 7-22 Chapter 7 and the adopted local building code.
Frequently asked questions
What is the difference between ground snow load and roof snow load?
Ground snow load (pg) is the measured or mapped weight of snow on flat ground at a given location, representing what would accumulate during a 50-year return-period storm. Roof snow load is always less than ground snow load because roofs shed snow through wind exposure, heat loss, and slope. ASCE 7-22 accounts for this with the 0.7 coefficient and the Ce, Ct, and Cs factors. The sloped-roof design load ps = 0.7 x Ce x Ct x Is x pg x Cs is typically 50-80% of the ground snow load for a standard heated suburban building.
How do I find the ground snow load for my location?
In the United States, start with ASCE 7-22 Figure 7.2-1 or the online database maintained by ASCE. Many states and municipalities adopt specific pg values that may differ from the national map, especially in mountainous regions. Your local building department or a licensed structural engineer can provide the adopted local value. In Canada, consult the NBC Climatic Design Information tables, which list the 1-in-50-year ground snow load for hundreds of cities. For remote sites, a site-specific snow study by a licensed engineer may be required.
When does the rain-on-snow surcharge apply?
The ASCE 7-22 rain-on-snow surcharge of 5 psf (0.24 kN/m2) applies when the mapped ground snow load pg is 20 psf or less AND the roof slope is shallower than W/50, where W is the eave-to-ridge horizontal distance in feet. The reasoning is that in low-snowfall areas, a small amount of snow can be saturated by rain events, roughly doubling its weight. The surcharge does not apply where pg exceeds 20 psf, because in those areas the snowpack is thick enough that rain typically freezes before penetrating deeply.
What roof slope eliminates the snow load?
For rough surfaces (asphalt shingles, tiles), the slope factor Cs reaches zero at 70 degrees. For slippery surfaces (metal, glass, membrane), Cs starts declining at 15 degrees and reaches zero at 70 degrees. In practice, a metal roof above about 45 degrees will have Cs below 0.15, meaning snow load is minimal. However, you must also account for unbalanced snow loads (wind can pile snow on the leeward side) and drift from adjacent structures, so zero snow load is rarely achievable in practice unless the roof is very steep and isolated.
What is the importance factor and when does it change?
The importance factor Is scales the design load based on the consequence of failure. Most houses and standard commercial buildings are Category II (Is = 1.0). Assembly occupancies with 300 or more people, schools, jails, and facilities storing hazardous materials are Category III (Is = 1.1, a 10% increase). Essential facilities like hospitals, fire stations, emergency operations centers, and certain communication towers are Category IV (Is = 1.2, a 20% increase). Agricultural facilities and minor storage buildings may qualify as Category I (Is = 0.8), reducing the design load by 20%. The risk category must match the applicable local building code adoption.
Do snow guards affect the structural snow load design?
Yes. When snow guards, parapets, or other physical obstructions prevent snow from sliding off the roof, the slope-reduction factor Cs must be set to 1.0, removing any shedding credit from the roof pitch. This is conservative and required by ASCE 7-22 Section 7.4.5 because the guards could allow snow to accumulate to a depth equal to the balanced flat-roof load regardless of slope. If you are adding snow guards to an existing structure, verify the roof framing was designed for Cs = 1.0, or have the structure evaluated by a structural engineer.