How to Calculate Roof Snow Load (ASCE 7-22)

A collapsed roof rarely surprises the engineer who ran the numbers. It surprises everyone else. Roof snow load is the design weight of snow your structure has to carry, and ASCE/SEI 7-22 Chapter 7 spells out exactly how to calculate it. The math looks intimidating at first: four factors and a formula with a decimal in front of it. It is not, in practice. Work through it in order, one input at a time, and you get a defensible number in about fifteen minutes. This guide walks through every step, with a full worked example and the mistakes that trip up most first-time users.

Step 1: Find your ground snow load (Pg)

Every roof snow calculation starts with the ground snow load, Pg, measured in pounds per square foot (psf). Pg is the weight of snow that piles up on open, level ground at your site; it has nothing to do with your roof yet. Pull it from the ASCE 7 Hazard Tool, which returns the ASCE 7-22 value for any latitude and longitude in the country. Your local building department is an equally valid source, and often a faster one: many jurisdictions adopt a fixed county or town value in their code amendments, and that adopted number is what a plan reviewer checks against. If the two disagree, the locally adopted value governs your permit.

Pg ranges from 0 psf along the Gulf Coast to more than 100 psf in parts of the mountain West and northern New England. In mountainous states the value depends on elevation as much as location, so a printed statewide number is only a starting point; see our state-by-state ground snow load reference for planning ranges.

Step 2: Convert to the flat-roof load (Pf)

Ground snow does not sit on your roof unchanged. ASCE 7-22 Equation 7.3-1 converts Pg into the flat-roof snow load: Pf = 0.7 x Ce x Ct x Is x Pg. The 0.7 base factor is a blanket reduction that accounts for the snow a typical roof loses to wind and minor melting compared to open ground.

Three factors adjust that number for your specific building. Ce, the exposure factor, describes how much wind scours snow off the roof: as low as 0.7 for a fully exposed roof on windswept terrain, up to 1.2 for a roof sheltered by trees or taller buildings. Ct, the thermal factor, reflects how much heat leaks through the roof deck and melts snow from below: 1.0 for a continuously heated building, 1.2 for an unheated garage or shed. Is, the importance factor, scales the load by how critical the building is, from 0.8 for a minor storage shed up to 1.2 for a hospital or other essential facility. For the full reference tables behind each of these, see our breakdown of the exposure, thermal, and importance factors.

Step 3: Apply the slope factor (Cs) for a pitched roof

Pitched roofs shed some of that load, so the next step multiplies Pf by the slope factor Cs to get the sloped balanced snow load: Ps = Cs x Pf. Cs stays at 1.0 up to a breakpoint slope, then decreases in a straight line to zero at 70 degrees, a pitch steep enough that snow simply cannot hold on. Where that breakpoint sits depends on how warm the roof surface is and how slippery it is. A warm, unobstructed, slippery metal roof starts shedding at a shallow 5 degrees. A cold, unheated roof with an asphalt-shingle surface, which grips snow better, does not start reducing the load until 30 degrees or beyond. Use our roof pitch calculator to convert a rise-and-run measurement into the degree value the slope factor tables need.

Step 4: Check the minimum load and rain-on-snow surcharge

Low-slope roofs, meaning anything under about 15 degrees, get a second calculation: a minimum load check under ASCE 7-22 Section 7.3.4. If Pg is 20 psf or less, the minimum is Is x Pg. If Pg is above 20 psf, the minimum is a flat 20 x Is. Compare that minimum to the Pf or Ps calculated in steps 2 and 3, and use whichever number is larger. This check exists because the 0.7 reduction factor and a shallow slope can otherwise produce a load that is unrealistically low for a nearly flat surface, which behaves more like open ground than a sloped roof.

A related surcharge applies in milder-winter regions. Where Pg is 20 psf or less and the roof slope is under about half an inch per foot, essentially flat, add a 5 psf rain-on-snow surcharge to the flat-roof load. In a mild climate, a flat roof is more likely to see a rain event fall on top of an existing snow layer, and that snow cannot drain the water away the way a sloped roof can, so it holds both the snow and the rain at once.

Step 5: Compare all cases and check for drift or unbalanced loads

Run the calculation in every applicable case, then use the single largest number as your design roof snow load. That is the value your rafters, trusses, and connections have to be sized for. For a simple single-slope roof with no adjacent taller structure, the governing case is usually the sloped load, the minimum, or the rain-on-snow surcharge. For a gable roof, a roof with a step down to a lower section, or a roof near a parapet or rooftop unit, add the unbalanced, drift, and sliding checks from ASCE 7-22 Sections 7.6, 7.7, and 7.9. Any one of those localized cases can exceed the uniform balanced load by a wide margin, and they govern the framing near the affected area even when the rest of the roof is fine.

A worked example: a house near Rochester, New York

Say you are checking a single-family home outside Rochester, NY. The ASCE 7 Hazard Tool returns Pg = 55 psf for the site. The roof is a 6:12 pitch (26.6 degrees), continuously heated, asphalt shingle, in a suburban neighborhood with some tree cover, terrain category B, partially exposed.

Step 1: Pg = 55 psf. Step 2: Ce = 1.0 (partially exposed, terrain B), Ct = 1.0 (heated), Is = 1.0 (ordinary residential occupancy, Risk Category II). Pf = 0.7 x 1.0 x 1.0 x 1.0 x 55 = 38.5 psf. Step 3: asphalt shingle is not a slippery surface, and a warm roof's breakpoint for a non-slippery surface is 30 degrees. At 26.6 degrees the roof is still below that breakpoint, so Cs = 1.0 and the sloped load Ps also equals 38.5 psf. Step 4: the roof is well above 15 degrees, so the low-slope minimum check does not apply.

Design roof snow load: 38.5 psf. That is the number the trusses and connections need to carry, before any local drift or unbalanced checks at roof steps or dormers.

What mistakes do people make calculating roof snow load?

The most common error is using Pg where the equation calls for Pf. Ground snow load is only the starting input; it is not the design load for any roof, and using it directly overstates or misrepresents the actual demand on the structure. A close second is skipping the minimum load check on a low-slope roof; the flat-roof equation with a low Ce can produce a number that looks fine until you compare it to the Section 7.3.4 minimum, which is often the larger and governing value on light-snow, low-slope buildings.

Third, people run the uniform balanced load and stop there, missing the drift surcharge at a roof step, parapet, or rooftop unit. Drift loads concentrate over a narrow strip and can be two to three times the balanced load in that zone; ignoring them is a common cause of localized roof failures, particularly on additions built next to a taller original structure. Finally, some calculations use an outdated Pg instead of pulling a current value for the exact site; ASCE 7-22 ground snow loads sometimes differ from the ASCE 7-16 values at the same location, so always confirm which code edition applies.