Snow Load Calculator

Background.

The number people usually have is the ground snow load, and the number they need is the roof snow load. Those are not the same, and the gap between them is the whole subject of ASCE 7 Chapter 7.

The conversion starts with a flat 30 percent reduction. A roof holds less snow than the ground beside it, because wind scours it, because heat leaks up through it, and because the ground snow load is itself a 50-year statistical value measured in a sheltered clearing. Equation 7.3-1 puts that as pf = 0.7 × Ce × Ct × Is × pg. So a 40 psf ground snow load in ordinary open terrain on a normally heated house becomes a 28 psf flat roof snow load before slope is considered at all.

Then three factors adjust it. The exposure factor Ce, from Table 7.3-1, is the biggest lever on the page: it runs from 0.7 for a fully exposed roof above the treeline to 1.2 for a sheltered roof in wooded suburban terrain, a 71 percent spread. Wind blows snow off an exposed roof and dumps it around a sheltered one, and the same 40 psf ground load gives 22.4 psf in Terrain D fully exposed and 33.6 psf in Terrain B sheltered. The thermal factor Ct, from Table 7.3-2, runs 0.85 for a heated greenhouse through 1.0 for a normal house, 1.1 for a cold ventilated roof, 1.2 unheated, up to 1.3 for a freezer building. The snow importance factor Is, from Table 1.5-2, is 0.8 for a low-hazard farm building, 1.0 for a house, 1.1 for a school and 1.2 for a hospital.

Slope comes last, and it is not linear. The slope factor Cs holds at exactly 1.0 up to a threshold and then falls in a straight line to zero at 70 degrees, and the threshold depends on two things at once: the thermal factor, which picks one of three graphs in Figure 7.4-1, and whether the surface is an unobstructed slippery one. For a warm roof the threshold is 5 degrees if slippery and 30 degrees otherwise; at Ct 1.1 it is 10 and 37.5; at Ct 1.2 or above it is 15 and 45. That is why a 6:12 asphalt-shingled roof gets no slope reduction at all — 26.6 degrees is below the 30 degree threshold — while the identical roof in standing-seam metal drops to Cs 0.668 and sheds a third of its load. It is also why a cold ventilated slippery roof at 12:12 comes out at 12.8 psf where the shingled warm version comes out at 28.

One provision catches people out. On a low-slope roof — monoslope, hip or gable shallower than 15 degrees — Section 7.3.4 imposes a minimum uniform load of Is × pg where the ground snow load is 20 psf or less, and Is × 20 psf where it is more. With a 15 psf ground snow load on a flat roof, Equation 7.3-1 gives 10.5 psf but the minimum gives 15, so 15 governs. The minimum is a separate load case, checked on its own and not combined with unbalanced or drift loading, and it stops a light-snow region from designing a flat roof for almost nothing. Above 15 degrees it does not apply at all, and the crossover between the two on a flat Risk II roof falls at a ground snow load of 28.6 psf.

This page computes the BALANCED case, and that limit matters more than any other caveat here. It does not compute unbalanced snow on a gable roof, which frequently governs the design of the members near the ridge; it does not compute drift against a taller adjacent wall or a parapet, which can be several times the balanced load; and it does not compute sliding snow onto a lower roof, rain-on-snow surcharge or ponding. Every one of those is a separate calculation in Sections 7.6 through 7.11, and a roof designed only for the balanced load is not designed.

The edition is ASCE/SEI 7-16, which is what IBC 2021 and IRC 2021 reference. ASCE 7-22 changed this procedure substantially and is deliberately not implemented: 7-22 publishes ground snow loads per risk category, so the importance factor is no longer a multiplier in Equation 7.3-1; the ground snow load is retrieved per site from the ASCE Hazard Tool rather than read off a national map; the thermal factor for heated structures now comes from a new Table 7.3-3 keyed on roof R-value and ground snow load; and the minimum load changed form and gained a cap. Implementing half of that from memory would produce a confidently wrong design load, so the page states the edition it implements and tells a reader on IBC 2024 that they need the 7-22 procedure instead.

The ground snow load itself is your input and is never assumed for you. Get it from the ASCE Hazard Tool or from your building department, and note that across much of the mountain west ASCE 7 designates case-study regions where no map applies and the local jurisdiction publishes its own value. Local amendments govern, adopted editions differ between jurisdictions, and a licensed structural engineer must confirm the design snow load and sign off before work proceeds.

What is snow load calculator?

The design roof snow load is the uniform downward pressure, in pounds per square foot of horizontal projection, that a roof must be designed to carry from accumulated snow. It is derived from the ground snow load at the site rather than measured on the roof.

ASCE 7-16 builds it in three steps. First, Equation 7.3-1 converts the ground snow load pg to a flat roof snow load pf = 0.7 × Ce × Ct × Is × pg, where Ce accounts for wind exposure, Ct for heat flow up through the roof and Is for the consequence of failure. Second, Equation 7.4-1 applies the slope factor: ps = Cs × pf, with Cs read from Figure 7.4-1 as a function of slope, thermal factor and surface slipperiness. Third, Section 7.3.4 imposes a minimum uniform load on low-slope roofs so that a light-snow region cannot design a flat roof for a trivial load.

Every one of those loads acts on the horizontal projection of the roof, not on the sloped surface. That is a convention worth remembering, because it means the roof area to use is the building footprint and not the shingle area — using the sloped area would overstate the total.

The balanced load is only the first of several cases. Sections 7.6 through 7.8 cover unbalanced and drifted snow, 7.9 sliding snow, 7.10 rain-on-snow surcharge and 7.11 ponding. On many roofs one of those, not the balanced load, sizes the members.

This calculator implements the balanced case of ASCE/SEI 7-16 Chapter 7. It does not compute drift, unbalanced, sliding, rain-on-snow or ponding loads, does not size any member, and does not implement ASCE 7-22.

How to use this calculator.

1. 01Get the ground snow load for your site from the ASCE Hazard Tool or your building department, and enter it. Do not guess it — in case-study regions there is no map value and the jurisdiction publishes its own.
2. 02Pick the terrain category from what is upwind of the building: B for urban, suburban or wooded, C for open country with scattered obstructions, D for flat unobstructed ground or water.
3. 03Pick the roof exposure. Most roofs are partially exposed. Fully exposed means nothing shelters it on any side; sheltered means tight conifers or obstructions stand above roof level nearby.
4. 04Pick the thermal condition. A normal heated house is 1.0. An unheated garage or open carport is 1.2, and a ventilated roof with more than R-25 between the vented space and the heated space is 1.1.
5. 05Pick the risk category — II for houses and ordinary buildings.
6. 06Enter the roof pitch as the rise per 12 inches of run, or 0 for a flat roof, and pick the surface. Unobstructed slippery means metal, slate, glass or membrane with nothing on it to catch snow; shingles and anything with vents or snow guards are "all other surfaces".
7. 07Enter the horizontal projected roof area — the footprint, not the sloped surface — for the total weight.
8. 08Read the design load, then read which case governs. If the minimum governs, your slope is under 15° and the ground snow load is low.
9. 09Read the note beside the answer before using the number, especially the list of load cases this page does not compute. Then have a licensed structural engineer check the drift, unbalanced and sliding cases, which often govern.

The formula.

Three equations and four factors. The flat roof snow load is pf = 0.7 × Ce × Ct × Is × pg (Eq. 7.3-1), where the 0.7 is the ground-to-roof conversion, Ce comes from Table 7.3-1 on terrain and exposure, Ct from Table 7.3-2 on thermal condition, and Is from Table 1.5-2 on risk category. The sloped roof snow load is ps = Cs × pf (Eq. 7.4-1). The slope factor comes from Figure 7.4-1: Cs = 1 for a slope θ at or below a threshold A, Cs = 1 − (θ − A) ÷ (70 − A) between A and 70 degrees, and Cs = 0 at 70 degrees and above. A is 5° for a warm roof with an unobstructed slippery surface and 30° otherwise; 10° and 37.5° when Ct is 1.1; and 15° and 45° when Ct is 1.2 or more. The minimum is pm = Is × pg where pg ≤ 20 psf and Is × 20 psf above that (§7.3.4), and it applies only where the slope is under 15 degrees. Working the example: pg = 40 psf, Terrain C partially exposed gives Ce = 1.0, a normally heated house gives Ct = 1.0, Risk Category II gives Is = 1.0, so pf = 0.7 × 1.0 × 1.0 × 1.0 × 40 = 28.0 psf. The 6:12 pitch is arctan(6 ÷ 12) = 26.565 degrees; the roof is warm with asphalt shingles, so A = 30 degrees, and 26.565 is below 30, so Cs = 1.0 exactly and ps = 28.0 psf. The slope is above 15 degrees, so the minimum of 1.0 × 20 = 20 psf does not apply, and the design load is 28.0 psf. Over a 1,500 sq ft footprint that is 42,000 pounds of snow. Change nothing but the surface to standing-seam metal and A drops to 5 degrees: Cs = 1 − (26.565 − 5) ÷ 65 = 0.6682 and the design load falls to 18.71 psf. The snow density from Eq. 7.7-1 is 0.13 × 40 + 14 = 19.2 pcf, capped at 30, which puts the 28 psf design load at an indicative 17.5 inches of snow — a scale check only, since real snow runs from about 5 pcf fresh to over 30 pcf old and wet. Every intermediate value is carried at twenty significant digits, rounding happens only at the final result, and the comparison between ps and pm that decides the governing case is made on the unrounded values.

A worked example.

Frequently asked questions.

References& sources.

1. [1]American Society of Civil Engineers — ASCE/SEI 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Chapter 7 (Snow Loads). Cited as the edition this page does NOT implement, and as the source of the four changes named on the page: risk-category-specific ground snow loads retrieved from the ASCE Hazard Tool, the removal of Is from Eq. 7.3-1, the new R-value-based thermal factor Table 7.3-3 for heated structures, and the revised minimum load with its cap. Access is by purchase from ASCE; the chapter's provisions are gated to automated retrieval.
2. [2]American Society of Civil Engineers — ASCE/SEI 7-16 Chapter 7, the edition implemented here and the one referenced by IBC 2021 and IRC 2021. Source of Eq. 7.3-1 (pf = 0.7 Ce Ct Is pg), Eq. 7.4-1 (ps = Cs pf), Table 7.3-1 exposure factor, Table 7.3-2 thermal factor, Table 1.5-2 snow importance factor, Figure 7.4-1 slope factor, §7.3.4 minimum roof snow load, and Eq. 7.7-1 snow density. Access is by purchase from ASCE and the provisions are gated to automated retrieval, so every table value shipped here was confirmed against at least two independent published reproductions.
3. [3]The StructEd — "How to Calculate Balanced Snow Loads per ASCE 7-16". Reproduction used to read Table 7.3-1 in full (Terrain B 0.9 / 1.0 / 1.2; C 0.9 / 1.0 / 1.1; D 0.8 / 0.9 / 1.0; above treeline and treeless Alaska 0.7 / 0.8 / N/A), Table 7.3-2 (0.85 greenhouse, 1.0 standard, 1.1 cold ventilated with R above 25, 1.2 unheated, 1.3 freezer), Table 1.5-2 (Is 0.80 / 1.00 / 1.10 / 1.20 for Risk I–IV), and the §7.3.4 minimum rule.
4. [4]Medeek Design Inc. — Snow Loads reference. Independent confirmation of Eq. 7.3-1 as pf = 0.7 Ce Ct Is pg and Eq. 7.4-1 as ps = Cs pf, and source of the snow density relation γ = 0.13 pg + 14 pcf with its 30 pcf maximum, which this page uses for the indicative depth figure.
5. [5]STRUCTURE magazine — "ASCE 7-22 Flat Roof Snow Load Versus Minimum Snow Load". Source for the ASCE 7-22 changes named on the page: that 7-22's Eq. 7.3-1 drops the importance factor, that ground snow loads are obtained per site and risk category from the ASCE Hazard Tool at ascehazardtool.org, and that the minimum load must be evaluated as a separate uniform load case with a pm,max cap.
6. [6]STRUCTURE magazine — "Snow Thermal Factors for Structural Renovations". Independent confirmation of the Table 7.3-2 thermal factor values (0.85 for certain greenhouses, 1.0 for heated structures, 1.1 for cold ventilated roofs, 1.2 for unheated structures, 1.30 for freezer buildings) and of the change to an R-value-based table in ASCE 7-22.
7. [7]American Society of Civil Engineers — ASCE Hazard Tool. The authoritative source for the site-specific ground snow load pg that this page requires as an input and never assumes, including the identification of "case study" regions where no mapped value applies and the local jurisdiction's published value governs.