Snow Load Calculation: Five Worked Examples Compared

The ASCE 7-22 flat-roof snow load formula has only four variables beyond the ground snow load itself, but small changes in exposure, heating, and building importance produce design loads that vary by a factor of two or more between building types at similar locations. The five examples below walk through the full calculation for a house, an unheated garage, an exposed warehouse, a hospital, and a heated greenhouse, with every factor shown and the governing case identified.

Example 1: typical suburban house, Rochester, NY

Ground snow load Pg = 55 psf. Exposure Ce = 1.0 (partially exposed, suburban terrain, Exposure Category B). Thermal factor Ct = 1.0 (continuously heated). Importance factor Is = 1.0 (Risk Category II, ordinary occupancy).

Pf = 0.7 x 1.0 x 1.0 x 1.0 x 55 = 38.5 psf. The roof has a 6:12 slope (26.6 degrees). For a warm, non-slippery roof, the slope factor Cs stays at 1.0 up to 30 degrees, so the sloped balanced load Ps = 38.5 psf. Design load: 38.5 psf.

It's worth pausing on why the house lands where it does. Rochester's 55 psf ground snow load is high by national standards, driven by lake-effect snow off Lake Ontario, yet the design roof load comes in at exactly 70% of that ground value because every one of the three reducing factors, Ce, Ct, and Is, sits at a neutral 1.0. That 70% ratio, the base 0.7 term in the equation with nothing else adjusting it up or down, is close to what a typical suburban heated house sees almost anywhere in the country; the ground value moves a lot by location, but the ratio between ground and roof load for an ordinary house does not.

Example 2: unheated detached garage, same site

Same Rochester site, Pg = 55 psf, Ce = 1.0. This time Ct = 1.2 (unheated structure) and Is = 0.8 (Risk Category I, minor storage). Pf = 0.7 x 1.0 x 1.2 x 0.8 x 55 = 36.96 psf.

Check the minimum: Is x Pg = 0.8 x 55 = 44 psf. The minimum governs. Design load: 44 psf, noticeably higher than the formula result even though the garage is a lower-importance structure.

Example 3: fully exposed warehouse, Boise, ID

Pg = 35 psf. Ce = 0.8 (fully exposed roof, open terrain, Exposure Category C, no obstructions). Ct = 1.1 (partially conditioned warehouse, kept above freezing but not heated to normal interior temperature). Is = 1.0.

Pf = 0.7 x 0.8 x 1.1 x 1.0 x 35 = 21.56 psf. Check the minimum: Is x Pg = 35 psf, but this does not govern since Pg exceeds 20 psf and the applicable minimum floor is lower. Design flat-roof load: 21.56 psf, reduced well below the ground value by the open exposure.

The warehouse example is the clearest illustration in this set of how much a single factor can move the result. Drop Ce from 1.0 to 0.8, and the design load falls by a fifth before anything else about the building changes. That's the value of getting your exposure category right rather than defaulting to a conservative guess: an open, unobstructed industrial site in genuinely exposed terrain is entitled to a real reduction, not just a theoretical one, and claiming Ce = 1.0 out of caution on a site that clearly qualifies for 0.8 or 0.9 means over-designing the roof for a load it will rarely see.

Example 4: Risk Category IV hospital, Des Moines, IA

A hospital is an essential facility, Risk Category IV, giving Is = 1.2, the highest importance factor in ASCE 7. Site conditions: Pg = 25 psf (a moderate-snow location), Ce = 1.0 (partially exposed urban site), Ct = 1.0 (continuously and fully heated).

Pf = 0.7 x 1.0 x 1.0 x 1.2 x 25 = 21.0 psf. Check the minimum: since Pg exceeds 20 psf, the applicable minimum is 20 x Is = 20 x 1.2 = 24 psf. The minimum governs here. Design load: 24 psf. Even in a moderate-snow region, the high importance factor for an essential facility pushes the minimum floor above the formula result, because the minimum scales directly with Is while none of the reducing exposure or thermal effects apply to it.

Example 5: continuously heated greenhouse, moderate-to-heavy snow region

A commercial greenhouse maintained at a warm interior temperature gets a reduced thermal factor under ASCE 7-22, Ct = 0.85, recognizing the significant heat loss through the glazing keeps the roof surface warm enough to shed snow more readily than an ordinary heated building. Site conditions: Pg = 45 psf, Ce = 1.0 (partially exposed), Is = 1.0 (Risk Category II).

Pf = 0.7 x 1.0 x 0.85 x 1.0 x 45 = 26.775 psf, rounding to 26.78 psf. Check the minimum: Pg exceeds 20 psf, so the applicable minimum is 20 x Is = 20 x 1.0 = 20 psf, well below the formula result. The formula governs. Design load: 26.78 psf, noticeably lower than a standard heated building would see at the same Pg, entirely because of the reduced thermal factor.

What these five examples show together

The thermal factor swings the result the most of any single variable: from Ct = 0.85 for a heated greenhouse up to Ct = 1.2 for an unheated structure, a 41% spread on that factor alone before anything else changes. Exposure matters most for large, open, industrial-style roofs, where Ce as low as 0.8 in open terrain measurably cuts the load below the ground value.

The minimum load check is the one most people underestimate, and it does not only affect unheated, low-importance buildings the way example 2 shows. It can just as easily govern a high-importance building like the hospital in example 4, for the opposite reason: not because the formula factors add up to a small number, but because Is is high enough that the minimum floor, which scales with Is directly, climbs above the formula result. The only safe approach is to run both calculations every time and take the larger.

Notice too how little the five design loads resemble their ground snow loads once every factor is applied: the Rochester house keeps 70% of its 55 psf ground value, the Boise warehouse keeps only 62% of its 35 psf ground value, and the Des Moines hospital's design load of 24 psf is actually just under its 25 psf ground value despite carrying the highest importance factor in the code. There is no shortcut that substitutes for running the actual formula and the minimum check for your specific building.

How to apply this to your own building

Start by identifying which of these five profiles your building resembles most closely: a heated house, an unheated outbuilding, an open industrial structure, an essential facility, or a continuously heated specialty structure like a greenhouse or an indoor pool enclosure. That gets you in the right neighborhood for Ct and Is before you even look up your site's Pg.

Then confirm your actual site conditions rather than assuming them. Exposure (Ce) depends on your specific terrain and any sheltering from nearby trees or buildings, which a formula cannot guess for you. Thermal condition (Ct) depends on whether the space below the roof deck is actually heated to normal interior temperature, kept just above freezing, or not heated at all, which is a real distinction with real consequences, as example 5's greenhouse shows. Risk category (Is) is set by the building's use and occupancy under your adopted code, not by its size or cost. Once you have all four factors right for your specific building, run the formula and the minimum, and use RoofHelm's calculator to check your arithmetic against these worked examples.

One habit separates a reliable estimate from a guess: write down each factor as you choose it, with a one-line reason, the way each example above does. If a plan reviewer or an engineer later asks why you used Ce = 0.8 instead of 1.0, or Ct = 1.1 instead of 1.0, you want an answer ready that matches the actual site and building conditions, not a number you picked because it looked conservative.