Snow load is not one number for the whole country, and it is not one number for the whole of Illinois. It changes by zip code, because it follows the climate, and the climate around Lake Michigan changes over a few miles. The number engineers start from is the ground snow load, pg, and for Chicago and Lake County it is 25 pounds per square foot on the ASCE 7 maps. That is the figure the roof snow load calculator uses for Waukegan, Gurnee, and the rest of Lake County. This guide explains where that number comes from, how it changes across northern Illinois, and what it means for the roof over your head.
Chicago and Lake County: 25 Psf
The ASCE 7 ground snow load for Chicago is 25 psf. That is not an estimate from a weather blog: it is the figure in the standard ASCE 7 worked example for a building in Chicago, Illinois, and it matches the ASCE 7 ground snow load maps for the Chicago area. The Great Lakes region as a whole runs 25 to 40 psf on the same maps, with the higher end in the lake-effect belts where cold air crossing the lakes drops extra snow. Lake County sits at the low end of that band, about 25 psf, with areas right on the Lake Michigan shore running up to about 30 psf in a heavy lake-effect season. A home in Waukegan, a home in Gurnee, and a home in Libertyville are all in the same band, but the home closest to the lake carries the highest number, and that is why the zip code matters.
From Ground Load to Roof Load
The ground load is the starting point, not the answer. The roof load comes from the ASCE 7 formula, pf = 0.7 times Ce times Ct times Is times pg, where Ce is the exposure factor, Ct is the thermal factor, and Is is the importance factor. For an ordinary heated home in a partially exposed spot, all three factors are 1.0, so a Chicago ground load of 25 psf gives a flat-roof load of 17.5 psf. But ASCE 7 also sets a minimum flat-roof snow load of 20 psf, and the minimum governs, so the design load for a typical flat roof in Waukegan is 20 psf. On a 1,500 square foot roof that is 30,000 pounds, or 15 tons.
| Area | Ground snow load (pg) | Flat-roof design load | 1,500 sq ft roof total |
|---|---|---|---|
| Chicago | 25 psf | 20 psf (minimum governs) | 30,000 lb (15.0 tons) |
| Waukegan / Lake County | 25 psf | 20 psf (minimum governs) | 30,000 lb (15.0 tons) |
| Lake shore, lake-effect | Up to 30 psf | 21 psf | 31,500 lb (15.8 tons) |
| Sheltered lot, any area | 25 psf | 21 psf (Ce 1.2) | 31,500 lb (15.8 tons) |
Two rows in that table deserve a closer look. The lake-effect row uses a ground load of 30 psf, the upper end of the Chicago-area band, and the flat-roof load rises to 21 psf, which is 31,500 pounds on a 1,500 square foot roof. The sheltered-lot row uses the same 25 psf ground load, but the exposure factor rises to 1.2 because trees and nearby buildings block the wind, so the snow stays on the roof instead of blowing off, and the flat-roof load is again 21 psf. A sheltered lot in Waukegan carries the same design load as a lake-effect lot, for a completely different reason. That is why a roofer asks about the trees around the house, not just the zip code.
Why the Lake Matters
Lake Michigan is the reason the snow load changes so fast across Lake County. Cold air moving across the relatively warm lake picks up moisture, and when that air hits the shore it drops the moisture as snow. A storm that leaves 6 inches in Libertyville can leave 10 inches in Waukegan, and the snow near the lake is often wetter and heavier, because the lake air is warmer than the inland air. Over a season the difference adds up: the shore towns see more snow days, deeper single storms, and more freeze-thaw cycles, which turn snow into ice and add weight without adding depth. The ASCE 7 maps account for that with higher contours along the shore, and the local building code adopts those contours. If your home is within a mile or two of the lake, use the upper end of the band when you check the load.
What Your Zip Code Does Not Tell You
The zip code gives the ground load, and the ground load gives the design load, but neither tells you the condition of your roof. A roof built in 1975 to an older code may have been designed for less than 20 psf, and sixty years of small leaks can rot the deck and cut the real strength further. The zip code is the starting point for the math, and the roof itself is the rest of the answer. If you want the number for your house, run the roof snow load calculator with your roof size and pitch, and if the roof shows any warning sign, read the guide on signs your roof is overloaded with snow before the next storm.
Local Code Governs
One rule overrides everything in this guide: local code governs. The ASCE 7 maps are the national basis, but the village or city building department adopts the code and sets the number for your address, and that number is the one that matters for a permit, an insurance claim, or a structural check. The figures here, 25 psf for Chicago and Lake County and up to 30 psf on the lake shore, are the ASCE 7 values, and they are the right starting point for a homeowner. If you are building, modifying a roof, or disputing a claim, get the adopted number from the building department in writing. It takes one phone call, and it removes all doubt.
Related reading: roof snow load calculator, plus our guides on how much snow can a roof hold and how to calculate roof snow load, and the roofing calculators hub.
What Is Your Roof Designed For?
Call now for a free roof inspection in Waukegan and Lake County. We will check your roof against the local ground snow load and tell you whether it needs clearing, repair, or nothing at all.
Frequently Asked Questions
What is the ground snow load in Chicago?
The ASCE 7 ground snow load for Chicago is 25 pounds per square foot. That figure comes from the ASCE 7 maps and the standard textbook worked example for a building in Chicago, Illinois, which uses pg = 25 psf. The Great Lakes region runs 25 to 40 psf on the same maps, with lake-effect areas on the higher end. Local code governs, so the building department for your exact address is the final word.
What is the ground snow load in Waukegan and Lake County?
Waukegan and Lake County sit in the same band as Chicago: about 25 psf of ground snow load on the ASCE 7 maps, with lake-effect areas near the Lake Michigan shore running up to about 30 psf. The flat-roof design load for a typical home in this band works out to 17.5 psf from the formula, and the 20 psf minimum governs, so a 1,500 square foot flat roof here is designed for 30,000 pounds, or 15 tons, of snow.
Why does snow load change by zip code?
Snow load follows climate, and climate changes over short distances in Illinois. The lake makes the difference: cold air crossing Lake Michigan picks up moisture and drops it as lake-effect snow on the shore, so a home in Waukegan can see a heavier season than a home twenty miles inland. Elevation matters too, and so does the wind: open ground lets the wind scour snow off a roof, while a sheltered lot surrounded by trees lets it pile up. The ASCE 7 maps draw those differences as contour lines, and your zip code sits somewhere on those lines.
Is the ground snow load the same as the roof snow load?
No. The ground snow load, pg, is the weight of snow on the ground, and the roof snow load is what the roof is designed for after the wind, the heat, and the pitch are accounted for. The ASCE 7 formula is pf = 0.7 times Ce times Ct times Is times pg: for Chicago, pg = 25 psf with all factors at 1.0 gives a flat-roof load of 17.5 psf, and the 20 psf minimum governs. A sloped roof carries less: an 8/12 pitch carries about 15.9 psf. The roof load is always lower than the ground load for a normal heated home, because wind blows some snow off and heat melts some more.
Do I need a structural engineer to check my roof snow load?
For a normal home with no warning signs, no. The building code your house was built to already set the design load, and a roofer can check the roof against it. You need an engineer when the roof has been modified, when you are adding a heavy rooftop unit, when the roof shows sagging or cracking, or when the local code has changed since the house was built and you need a formal number for a permit. If the roof is showing any of the overload signs, call a roofer first: the roofer will tell you honestly whether an engineer is needed.
Zayn Robert is a content specialist focused on roofing, construction, and home improvement topics. He creates easy-to-understand guides that help homeowners learn about roof measurements, materials, maintenance, and repair decisions. His work combines detailed research with practical insights from roofing professionals at Zuniga Roofing Inc.