Fresh Snow, Packed Snow and Ice: Why the Same Weight Looks So Different
Same weight, three very different piles
A roof's design load is measured in pounds per square foot, but nobody standing on a ladder can eyeball psf. What they can eyeball is depth, and depth depends entirely on how dense the snow is. Fresh, dry powder runs about 5 psf per 11 inches; older, settled snow compacts to roughly 5 psf per 4 inches; solid ice is about 5 psf per inch - eleven times denser than fresh powder for the same weight.
Why this matters more than the psf number itself
Aroostook County, ME's 70 psf design load sounds abstract until it is translated: that is roughly 154 inches of fresh powder (more than most winters bring at once, so fresh snow alone rarely reaches it), but only about 14 inches if it has turned to ice. A homeowner watching a foot of dense, wet spring snow accumulate is watching something much closer to the design limit than the same foot of light January powder.
Why ice is the dangerous case
A slow freeze-thaw cycle - snow melting slightly in the sun, refreezing overnight, repeating for days - packs far more weight into far less depth than a single snowstorm ever does. It is also the least visually alarming: a thin, glassy layer looks minor next to a snowbank, even when it carries several times the load. Dallas County, TX, with a light 5 psf design load, still sees this dynamic on the rare ice storm that does hit it - the low map figure assumes routine snow accumulation, not a heavy ice event.
Using this on your own roof
The gut-check calculator on the homepage runs these same three density figures - enter a rough depth and a snow condition and it returns an estimated psf instantly, no county lookup required. It is a rough estimate, not a substitute for a structural inspection, but it turns "there's a lot of snow up there" into a number worth comparing against a county's actual design load.
Where this plays out
See the range across real counties: Aroostook County, ME, Edgecombe County, NC, Kalawao County, HI. Related: Snow Is Already on the Roof: What Actually Matters, Embed the SnowLoadCheck Snow Load Badge.