McDonough County, IL Roof Snow Load - 20 psf
McDonough County, IL: your roof's ASCE 7 snow number, in plain English
McDonough County's roofs are built to hold 20 psf of snow, the ASCE 7-22 figure a permit reviewer checks first. ASCE 7's low-slope-roof minimum (§7.3.4) raises the governing figure to 20 psf - higher than the 14 psf the ground-load formula alone would give, and the code says use the larger one.
In depth, not weight: that's roughly 44 inches of fresh, light snow, 16 inches of old, settled snow, or as little as 4 inches of solid ice - snow that heavy is rare here but ice from a freeze-thaw cycle adds up far faster than the depth suggests. A meaningful design load. Most homes built to modern code handle it, but additions, carports, and flat-roofed sections are worth a second look.
McDonough County in numbers
- Design roof snow load20 psf
- Ground snow load, ASCE 7 map20 psf
- National rank#896 of 2,758
- Illinois rank#47 of 102
- Population26,662
- Annual snowfall, NOAA normal10.5 in.
- FIPS17109
McDonough County by the numbers
Nationally that's #896 out of 2,758 rated counties, the top 32%; within Illinois alone, McDonough County ranks #47 of 102. Illinois counties average 19.3 psf; McDonough County runs 0.7 psf higher than that.
McDonough County, FIPS 17109, is home to 26,662 people near 40.4558°N 90.679°W. Its 20 psf ground snow load figure is the map value; 20 psf is what a roof is actually designed for.
McDonough County's snowfall, most winters
10.5 in. a year: that's McDonough County's NOAA 1991-2020 snowfall normal, measured at MACOMB, IL US (1.6 mi away). See the density guide.
Before you rely on McDonough County's number
Treat the McDonough County figure as a starting point, not a final answer: it assumes standard occupancy, a heated structure and a moderately sheltered site, per ASCE 7-22's own defaults. Exposed terrain, an unheated building or a steeper roof all shift the real number.
For a real application, skip the county number and go straight to the ASCE 7 Hazard Tool - enter your coordinates and it returns a site-specific figure a plan reviewer will recognize. Details: the calculation guide.