How SnowLoadCheck Calculates a Design Roof Snow Load
The formula behind the number
ASCE 7-22's Equation 7.3-1 sets the flat-roof snow load as pf = 0.7 x Ce x Ct x Is x pg. pg is the ground snow load from the code's own contour map; the three factors adjust it for exposure (Ce), heating (Ct) and building risk category (Is). SnowLoadCheck uses the values that apply to the large majority of existing single-family homes: Ce 1.0 for a partially exposed suburban lot, Ct 1.0 for a heated structure, Is 1.0 for ordinary Risk Category II occupancy - ASCE 7-22's own documented defaults, not a guess.
Why the map figure is never the final number
pg alone is a contour value, not a load anyone's roof carries. Aroostook County, ME shows the gap best: its ground snow load is high enough that the flat-roof formula alone would already put it near the top of the national ranking, and the same 0.7 x Ce x Ct x Is formula applies there as everywhere else - the code does not exempt high-snow counties from the math, it just returns a bigger number.
The floor a lot of counties hit
Separately, ASCE 7-22 §7.3.4 sets a minimum load for low-slope roofs: pm = pg (when pg is 20 psf or less) or 20 psf (when pg is higher). Because the flat-roof formula's 0.7 multiplier makes pf smaller than pg, and pm equals pg outright for any county at or under 20 psf, that minimum ends up governing for a large share of the country. Cook County, IL is a working example: the ground-load formula alone would give 17.5 psf, but the §7.3.4 minimum raises the governing figure to 20 psf for flat and low-slope roofs there - the number SnowLoadCheck reports is always the larger of the two, per the code.
Where the snowfall figure comes from
The annual snowfall figure alongside each county's load is a separate source: NOAA's 1991-2020 climate normal at the nearest reporting station, not derived from the ASCE 7 map at all. The two numbers answer different questions - one is a structural design load, the other is how much snow actually falls most years - and SnowLoadCheck keeps them visibly separate rather than blending them into one figure.
Where this plays out
See the range across real counties: Aroostook County, ME, Edgecombe County, NC, Kalawao County, HI. Related: Why Some Counties Have No Snow Load Number, Fresh Snow, Packed Snow and Ice: Why the Same Weight Looks So Different.