SnowLoadCheck

Johnson County, IL Roof Snow Load - 15 psf

What ASCE 7 says about snow load in Johnson County

Moderate
15 psf

That's Johnson County's design roof snow load per ASCE 7-22, for a typical heated single-family home - the number a permit reviewer starts from. §7.3.4's low-slope minimum of 15 psf is what actually governs on a flat or low-pitch roof here - higher than the 10.5 psf the base ground-load formula gives.

Converted to something you can actually eyeball on a roof: about 33 in. of fresh powder, 12 in. once it settles and packs down, or just 3 in. if it's turned to ice. This is a meaningful design load - most modern-code homes handle it fine, though an addition, a carport or a flat-roofed section can be a weak spot.

Already snow on the roof in Johnson County?

in. of

Johnson County, IL: quick facts

Where Johnson County's figure sits nationally

Out of 2,758 rated US counties, Johnson County sits at #1,517, the top 55% nationally - #96 of 102 within Illinois. Illinois counties average 19.3 psf; Johnson County runs 4.3 psf lower than that.

Near 37.4607°N 88.8821°W, FIPS 17087: Johnson County counts 13,320 residents and a 15 psf map value, which the code's own factors turn into 15 psf.

How much it actually snows in Johnson County

Most winters bring Johnson County roughly 10.4 in. of snow - NOAA's normal at ANNA 2 NNE, IL US, 19.4 mi off. (fresh vs. packed vs. ice explains why.)

Read this before you rely on it

Johnson County's number above is ASCE 7-22's default case, not a custom calculation: ordinary residential occupancy, heated, sited on ground that's neither fully sheltered nor fully exposed. Any one of those changing - pitch, exposure, heating, occupancy - moves the real figure.

Nothing here substitutes for an address-level number. Get one free, in about a minute, from the ASCE 7 Hazard Tool using your own coordinates rather than Johnson County's county-wide figure. (How this figure is calculated.)