Snow Load Calculator

Ground snow load (pg) is the weight of snow on open ground at your site — for example, Chicago's reference value is 25 psf. Roof snow load is what your roof must actually carry, and it is always adjusted down (or occasionally up) from pg. Under ASCE 7-22, the flat roof snow load is pf = 0.7 x Ce x Ct x Is x pg. The 0.7 factor reflects that roofs shed snow through wind and heat even before slope is considered. Worked example: pg = 30 psf on a typical heated suburban home (Ce = 1.00, Ct = 1.00, Is = 1.00) gives pf = 0.7 x 1.00 x 1.00 x 1.00 x 30 = 21.0 psf. Learning how to calculate roof snow load per ASCE 7 starts with getting pg right, then applying the three adjustment factors.

In the US, start with ASCE 7-22 Figure 7.2-1 (the ground snow map) or the ASCE 7 Hazard Tool, then confirm with your local building department — many jurisdictions adopt values that supersede the national map, especially in the mountains. Our ground snow load by city reference table below lists IBC map values for 27 eastern-US cities as a starting point (for instance, Minneapolis 50 psf, Chicago 25 psf). Note that ASCE 7-22 raised many mapped values: Boston went from 40 to 65 psf and Baltimore from 25 to 59 psf. Sites in Case Study (CS) zones and most of Alaska need a site-specific study. Never guess pg — it is the single biggest driver of the result.

These three snow load factor Ce Ct Is multipliers convert ground snow to roof snow. Ce (exposure factor, Table 7.3-1) credits wind scouring: a fully exposed roof in open country gets Ce = 0.80 while a sheltered suburban roof gets Ce = 1.20. Ct (thermal factor, Table 7.3-2) credits heat loss melting snow: heated buildings use Ct = 1.00, unheated structures Ct = 1.20, freezers Ct = 1.30, and continuously heated greenhouses Ct = 0.85. Is (importance factor, Table 1.5-2) scales with risk category: I = 0.80, II = 1.00, III = 1.10, IV = 1.20. A hospital (IV) in the same town as a barn (I) designs for 50% more snow load.

The roof slope factor Cs calculator logic comes from ASCE 7-22 Figure 7.4-1: ps = Cs x pf. For a warm rough roof (asphalt shingles, Ct <= 1.0), Cs stays 1.00 up to 30 degrees, then falls linearly to zero at 70 degrees. Example: at 35 degrees, Cs = 1 - (35-30)/40 = 0.875, so a 21.0 psf flat load becomes ps = 18.4 psf on the slope. Cold roofs (Ct > 1.0, unheated) hold snow longer, so Cs stays 1.00 until 45 degrees; at 50 degrees Cs = 0.80 and ps = 16.8 psf. Above 70 degrees the code treats the slope as shedding all snow.

Use this unbalanced snow load calculator section for hip and gable roofs with slopes between 2.38 and 30.2 degrees (about 0.5:12 to 7:12 pitch). Wind scours the windward slope and piles snow against the ridge on the leeward side. The simplified rule: windward gets a uniform 0.3 x ps, while the leeward side carries the full ps plus a triangular ridge-drift surcharge. Worked gable roof unbalanced snow load example: ps = 21.0 psf, 26.6-degree slope, 30 ft fetch — windward = 6.3 psf, leeward = 21.0 psf uniform plus a drift surcharge peaking at 23.6 psf over 7.0 ft (drift height 1.86 ft). For buildings 20 ft wide or less the code permits a simplified uniform leeward load of Is x pg.

A snow drift load calculator estimates the triangular pile of wind-blown snow that forms against a parapet, a roof step, or rooftop equipment — often the controlling case for nearby beams. ASCE 7-22 computes the leeward drift height from the ground snow load, the upwind fetch length, and a winter wind parameter W2 (typically 0.25 to 0.65). Example: pg = 40 psf with 100 ft of upwind roof and W2 = 0.50 gives a drift height of 3.73 ft, a drift width of 14.9 ft, and a peak surcharge of 71.6 psf at the wall face. The windward drift is smaller — about 75% of the leeward height. If the step is too shallow for the drift to fully form, the height is capped at the clear height above the balanced snow.

Yes — this is the slippery roof snow load reduction. Metal, glass, slate, and smooth membranes let snow slide, so ASCE 7-22 Figure 7.4-1 gives slippery unobstructed surfaces a steeper Cs curve: it starts declining at 10 degrees and reaches zero at 70 degrees. At 30 degrees a slippery roof gets Cs = 0.733 versus 1.00 for rough shingles, so a 21.0 psf flat load drops to ps = 15.4 psf. Try it in a metal roof snow load calculator mode by selecting the slippery surface. One catch: the slippery credit is lost if anything blocks sliding — adding snow guards, a parapet, or rooftop units forces Cs back to 1.0, which is a real snow guards roof load increase you must design for.

Two extras the rain on snow surcharge calculator flag covers. First, the minimum roof snow load pm (Section 7.3.4): for low-slope roofs the design flat load cannot be less than Is x pg when pg is 20 psf or below (Is x 20 psf when pg is higher). Example: pg = 20 psf with Ce = 0.90 and Ct = 1.20 gives pf = 15.12 psf, but the minimum roof snow load pm = 20.0 psf governs. Second, rain-on-snow: when pg is 20 psf or less and the roof slope is shallower than W/50 (W = eave-to-ridge width in feet), add a 5 psf surcharge for rain saturating a thin snowpack.

Multiply psf by 0.0478803 to get kilopascals: 20 psf = 0.9576 kPa. Divide kPa by the same factor (or multiply by 20.8854) to go back: 1 kPa = 20.8854 psf. The converter tab does this snow load psf to kPa conversion both ways, and every result on this page shows both units. Related conversions engineers use alongside it: snow density in pcf (this page's snow density pcf calculator uses gamma = 0.13 x pg + 14, capped at 30 pcf), equivalent snow depth in inches (ps divided by gamma), and the total roof snow weight calculator (ps times roof area — for example 21.0 psf over 30 x 40 ft = 25,200 lbs of snow).

No. Structural snow loads must be determined by a licensed structural engineer following your locally adopted building code — this tool is for education and preliminary estimation only. Real designs must also check partial loading, sliding snow, ponding, seismic combinations, and local amendments this simplified tool does not cover. So what is a safe roof snow load psf for a house? There is no universal number: it depends on your mapped pg, exposure, and framing, which is exactly why an engineer must confirm it. Note the boundary with our sibling tool: the a-frame-size-calculator sizes A-frame cabin rafters from snow load, while this calculator computes code roof snow loads — pf, ps, drift, and unbalanced — for general roofs without sizing any structural members.