Ground Snow Load vs Roof Snow Load Explained

Ask two people what snow load means and you may get two different answers: the weight of snow sitting on the ground outside, or the weight the roof structure has to hold up. ASCE 7-22 treats these as two distinct, related numbers, connected by a single conversion equation. Confusing the two, or worse, designing to the ground value directly, either overbuilds a roof or, in the wrong direction, leaves it dangerously undersized. This guide lays the two values side by side, explains the conversion, and works through three examples that show how much the ratio between them can shift.

Ground snow load vs roof snow load, side by side

The short version: ground snow load (Pg) is a location-based hazard value, fixed the moment you know the site. Roof snow load (Pf, and its sloped version Ps) is a building-specific engineering value, derived from Pg plus three adjustments for exposure, heat, and importance.

Why the roof load is (almost) always lower

Snow does not sit on a roof the way it sits on open ground. Wind scours some of it off, especially at ridges, edges, and exposed faces. Heat escaping through a conditioned building melts some from underneath. Both effects reduce the load compared to a flat, unobstructed patch of ground nearby, and ASCE 7-22 captures that reduction with a base 0.7 factor built into Equation 7.3-1: Pf = 0.7 x Ce x Ct x Is x Pg. On a roof with average exposure, average heat loss, and ordinary importance, the three adjustment factors are all close to 1.0, so the roof load lands close to 70 percent of the ground value before slope reduction lowers it further on a pitched roof.

A quick mental model for the conversion

If the equation feels abstract, picture it as four sequential discounts applied to the ground value. Start with Pg, the full open-ground weight. Apply a 30 percent blanket discount for the fact that a roof is not open ground (the 0.7 factor). Then apply a second discount or surcharge for wind exposure (Ce), a third for how much heat is escaping from below (Ct), and a fourth, which can go either direction, for how critical the building is (Is). Each discount is independent of the others; a building can have a favorable exposure discount and an unfavorable thermal surcharge at the same time, and the equation simply multiplies all four together in one pass. Thinking about the four factors as sequential adjustments, rather than one combined black box, makes it much easier to spot which factor is doing the most work in any given calculation, and which one to double-check first if a result looks off.

Worked example 1: a normal heated house

Take a house in Denver with Pg = 30 psf, terrain category B with some sheltering from nearby trees (Ce = 1.0), continuously heated (Ct = 1.0), and ordinary Risk Category II occupancy (Is = 1.0). Pf = 0.7 x 1.0 x 1.0 x 1.0 x 30 = 21 psf. That is exactly 70 percent of the ground value, the textbook case for an average building. A moderate 6:12 slope with asphalt shingles keeps Cs at 1.0 in this range, so the final sloped load stays at 21 psf.

Worked example 2: an unheated, exposed pole barn

Now take the same Pg = 30 psf site, but for an unheated pole barn sitting in open farmland with no wind shelter. Ce drops to 0.9 (exposed, terrain category C), Ct rises to 1.2 (unheated), and Is drops to 0.8 (Risk Category I, minor storage). Pf = 0.7 x 0.9 x 1.2 x 0.8 x 30 = 18.1 psf, about 60 percent of the ground value. The unheated building's higher thermal factor is largely canceled out by its lower importance factor and better wind exposure, landing it slightly below the average house even though it feels like it should carry more because nothing melts the snow.

Worked example 3: an essential facility

Finally, take a hospital addition at the same Pg = 30 psf site, fully exposed on its roof (Ce = 0.9), continuously and heavily heated (Ct = 1.0), and Risk Category IV, the highest importance tier (Is = 1.2). Pf = 0.7 x 0.9 x 1.0 x 1.2 x 30 = 22.68 psf, about 76 percent of the ground value, the highest ratio of the three examples. Push the importance factor and exposure in the same direction on a different building, a fully exposed, unheated essential facility, for instance, and the ratio can climb close to or even above the ground value itself. That is why the guidance is always to run the actual conversion for your building rather than assuming the common 60 to 70 percent rule of thumb applies universally.

Can roof snow load ever exceed ground snow load?

Rarely, but yes. It takes a combination of a fully exposed roof, an unheated structure (Ct = 1.2), and a high importance factor (Is = 1.1 or 1.2 for substantial-hazard or essential facilities) to push Pf close to or past Pg. In practice this shows up most often on unheated essential-facility structures like emergency equipment shelters, or on structures in very sheltered micro-sites where snow naturally drifts to depths beyond the open-ground average. It is uncommon on ordinary residential and commercial buildings, where the 0.7 base factor keeps the roof load below the ground value in nearly every real-world combination.

Why this distinction matters for permits and inspections

Building departments ask for the roof design value, not the raw ground snow load, when they review structural drawings, because Pf (or Ps) is what actually loads the framing. But they need Pg as the documented starting point, because a reviewer checking your math has to confirm you started from a correct, current site value before checking that you applied the right factors. A permit package that shows only a final roof load number, with no Pg and no factor selections shown, is harder to review and more likely to draw questions. A complete submission shows the full chain: Pg from the Hazard Tool or local amendment, then each factor with its justification, then the final Pf and Ps.

A common misconception: assuming a flat percentage

It is tempting to memorize 70 percent and apply it to every building, but that number is only the outcome of the specific factor combination in the first worked example above; it is not a rule written into ASCE 7. Change any one factor and the ratio moves. An unheated structure with poor exposure could land near 100 percent of Pg once you account for slope reduction working in the opposite direction on a low-slope roof, while a sheltered, heavily importance-weighted building could land under 50 percent. Treat the 60 to 70 percent figure as a sanity check on a finished calculation, useful for catching a data-entry mistake, never as a substitute for running the actual equation.

One more variable: which code edition set your Pg

The ground snow load feeding this whole comparison depends on which ASCE 7 edition your jurisdiction has adopted. ASCE 7-22 generates Pg through a reliability-targeted, site-specific model rather than the older printed contour map used in ASCE 7-16, and the two editions do not always produce identical numbers at the same coordinates. Confirm which edition applies to your project, see our rundown of the ASCE 7-22 changes, before you lock in the ground snow load half of this comparison.

Running your own numbers

The fastest way to see the real ratio for your project is to run it through the free RoofHelm snow load calculator, entering your site's Pg (from the ASCE 7 Hazard Tool or your building department) alongside your roof's actual exposure, heating condition, and risk category. The tool shows every intermediate factor, not just the final number, so you can see exactly how far the roof load sits below, or in rare cases above, the ground value. If you need a documented summary for a permit application, the $29 Pro PDF report packages the same calculation into a permit-ready format, though a licensed engineer still needs to review and stamp the final structural submission in most jurisdictions. Keep a copy of every factor you selected, and why, alongside the final numbers; that record is what turns a calculator output into a defensible engineering input rather than a guess.