Two identical low-slope roofs, one in Nashville and one in Buffalo. Which one carries a rain-on-snow surcharge?

Short answer: Nashville. The lighter snow site gets the surcharge. The heavier one does not, and that is by design.

The provision

ASCE 7-16 Section 7.10, Rain-on-Snow Surcharge Load:

For locations where p_g is 20 lb/ft² (0.96 kN/m²) or less, but not zero, all roofs with slopes (in degrees) less than W/50 with W in ft shall include a 5 lb/ft² (0.24 kN/m²) rain-on-snow surcharge load. This additional load applies only to the sloped roof (balanced) load case and need not be used in combination with drift, sliding, unbalanced, minimum, or partial loads.

All three conditions must hold. The ground snow load is 20 psf or less but not zero. The roof slope, in degrees, is less than W divided by 50, with W the horizontal distance from eave to ridge in feet. And the load lands only on the balanced case.

Why the heavy-snow site is exempt

Nashville sits at a ground snow load of roughly 10 psf. Buffalo is several times that. Under ASCE 7-16 the Buffalo roof is above the 20 psf line, so Section 7.10 does not apply to it at all. The Nashville roof, if it is flat enough to fail the W/50 test, picks up 5 psf on top of its balanced load.

The reasoning is physical. Where the snowpack is thin, a winter rain can weigh more than the snow it lands on. Where the pack is deep, the same rain is a small fraction of a load that already governs.

What ASCE 7-22 changed

ASCE 7-22 Section 7.10 keeps the idea and moves both numbers. The surcharge is now 8 psf. The cutoff is no longer a flat 20 psf; it is the minimum snow load upper limit p_m,max from Table 7.3-4, which depends on Risk Category:

So under ASCE 7-22 the same Nashville roof carries 8 psf instead of 5, and the question of whether a given site is "light snow" now depends on what you are building. A Risk Category IV hospital in a 35 psf town is under its 40 psf cutoff and gets the surcharge. The Risk Category II warehouse across the street, with a 30 psf cutoff, does not.

The slope test people skip

The W/50 condition is easy to forget because it is expressed in degrees against a length in feet. A 60 ft eave-to-ridge run gives a threshold of 1.2 degrees, which is about a quarter inch per foot. Many roofs described as flat are built to a quarter inch per foot for drainage, which puts them right at that line. Run the arithmetic rather than the adjective.

What Prose does

Prose applies the threshold per edition and, for ASCE 7-22, per the structure's Risk Category. It adds the surcharge only to the balanced case and writes the reason on the report whether the surcharge applies or not.

Citations

• ASCE 7-16 Section 7.10, Rain-on-Snow Surcharge Load
• ASCE 7-22 Section 7.10, Rain-on-Snow Surcharge Load
• ASCE 7-22 Table 7.3-4, Minimum Snow Loads for Low-Slope Roofs
• Ground snow loads for specific sites come from the ASCE 7 Hazard Tool. Confirm which edition your jurisdiction has adopted.