Chicago’s roofs are engineered to hold 25 pounds of snow per square foot of open ground before that number is even converted to a rafter load, and that ground figure is what a truss drawing, a permit application and a plan reviewer all start from. It is not what actually sits on the shingles: the code cuts the number down before it becomes a design load, and drifting can push isolated spots back up past it. Check with your local building department before assuming a single figure applies to your address, and confirm your county’s ice barrier rule before you sign off on a re-roof.
What is the ground snow load in Illinois?
The design ground snow load at Chicago is 25 psf under ASCE 7-16, the edition that IRC/IBC 2021 makes applicable in most states for a residential permit. This is the number a builder hands to a truss manufacturer when ordering trusses for a new garage or addition, and it is the number a plan reviewer checks against before a permit gets issued. It is not, by itself, what ends up on the shingles.
Ground load vs. roof load
Ground snow load and roof snow load are two different numbers, and mixing them up is where most confusion about this figure starts. The code converts the ground value using exposure, thermal and slope factors specific to the building. For an ordinary heated house with a sloped roof, that conversion lands at roughly 18 psf of actual roof snow load in the Chicago area, and it drops further on a steeper pitch.
The newer edition isn’t a bigger number
A newer edition, ASCE 7-22, maps the same point in Chicago at 51 psf. That is not more snow, and it is not a sign an existing roof is undersized. ASCE 7-22 uses a strength-level basis with a 1.0 load factor, while 7-16 is a nominal value paired with a 1.6 factor, so the two figures sit on different scales rather than measuring the same thing twice. A handful of jurisdictions are starting to adopt 7-22, but the value a Chicago-area permit works from today is still the 7-16 number.
Chicago is the reference point for this figure, and Illinois’s flat but climatically varied terrain means snow load isn’t uniform statewide. A building department downstate, or one closer to Lake Michigan’s snow bands, may work from a different design value than a Chicago-area suburb does. Treat this figure as the starting point for a conversation with your local building department or a licensed engineer, not as a verdict on a specific roof.
How much snow can a roof hold in Illinois?
There’s no single number that answers this, because what a roof is actually carrying depends on what the snow has become, not just how deep it is. The ASCE 7-16 ground value at Chicago converts to roughly 18 psf on an ordinary sloped roof, but that figure assumes an even layer. Real snow on a real roof rarely stays even.
Where drift changes the math
Drift is where roofs actually get into trouble. Snow blown off a taller section piles up against a wall, below a dormer, or in a valley between two roof planes, and in those spots the local load can run well above the flat design figure even while the rest of the roof carries nowhere near its limit. A lower roof set beside a taller one, or a roof tucked behind a parapet, is exactly the geometry a designer is supposed to check separately for drift surcharge.
Why depth alone doesn’t tell you the weight
Fresh, dry snow runs roughly 5 to 7 pounds per square foot for every foot of depth. Let it settle, thaw and refreeze, or let rain fall into it, and that same foot can weigh two to three times as much. An ice layer on top adds still more weight in a much thinner slab. That’s why a foot of light January snow and a foot of wet March snow are not the same load, even though a tape measure can’t tell them apart.
A few signs suggest a roof is being asked for more than it’s comfortable carrying:
- Interior doors that suddenly stick or won’t latch
- New cracks spreading across ceiling drywall
- A ridge line that looks like it’s started to dip
The safe response is to rake snow off from the ground with a long-handled roof rake, working from the eaves. Climbing onto a loaded roof to shovel it adds your own weight to a structure that may already be near its limit, and adds it in exactly the spot most likely to fail.
These figures start from the Chicago reference point. A roof under a heavier Lake Michigan snow band, or one that catches extra drift because of a nearby structure, is carrying a different load than the flat design number suggests. Only your local building department or a licensed engineer can say what a specific roof is rated for.
What wind speed must a roof withstand in Illinois?
The basic design wind speed at Chicago is 107 mph under ASCE 7-16, Risk Category II, the standard used for ordinary houses. Neighboring states go through the same ground-to-roof and wind conversions under their own reference cities, and a reader checking a property near the state line can compare the same categories for Michigan’s snow load and wind rules.
What this number actually measures
That 107 mph figure is a 3-second gust measured at 33 feet up, in open Exposure C terrain, not a sustained wind and not what a weather forecast means when it reports a storm’s wind speed. ASCE 7-22 maps the same Chicago point at the same 107 mph, so this is one of the rare figures where the newer edition doesn’t move the number at all.
What it requires on the roof
Illinois is not in a hurricane-prone region on this map, so the wind-borne debris provisions that require impact-rated glazing or approved shutters do not apply at the Chicago reference point. That’s worth stating plainly, because those provisions are the single most expensive consequence of a high wind map anywhere they do apply, and a reader here isn’t planning around them on that basis.
What the 107 mph figure buys instead is less dramatic but still consequential: it sets the wind rating a shingle needs, the nailing pattern a roofer follows, how the sheathing gets fastened down, and the strength of the uplift connections tying the roof frame to the walls below it. Edges, rakes and ridges fail first in any wind event, because that’s where uplift concentrates, which is why code nailing patterns tighten up in those zones compared with the field of the roof.
This wind speed is anchored to Chicago. It doesn’t swing across Illinois the way elevation drives snow load in mountain states, but a building department elsewhere in the state may still reference a different design value for its own location. A different code entirely governs wind pressure north of the border, which is why Ontario’s snow load and wind rules aren’t a substitute for the Illinois figures even for a property close to the lake. Confirm the design wind speed for your address before ordering shingles rated to a specific wind category.
Does Illinois require an ice barrier under the shingles?
An ice barrier is a self-adhering waterproof membrane installed under the shingles, run from the eave edge up past the interior line of the exterior wall. It isn’t there to stop snow from piling up. It’s there to stop meltwater that backs up behind an ice dam from working its way under the shingles and into the roof deck.
What actually causes an ice dam
Ice dams form when heat escaping from the living space into the attic warms the roof deck enough to melt the underside of the snowpack sitting on it. That meltwater runs down the slope until it reaches the cold overhang past the heated wall line, where it refreezes into a ridge of ice. Water keeps arriving behind that ridge with nowhere to go, so it backs up under the shingles instead. NOAA’s 1991-2020 climate normals put Chicago’s coldest-month mean daily minimum at 18.8°F, cold enough that a snowpack sitting on a roof for days or weeks at a stretch is a realistic scenario most winters, which is exactly the setup an ice dam needs.
The membrane protects the deck once that situation exists. It does not stop the ice dam from forming. Only air sealing the attic floor and bringing Insulation up to level stops the uneven heat loss that melts snow from underneath in the first place. A reader who installs the membrane and stops there has bought protection against the symptom, not the cause.
What the rule actually is here
Illinois has no statewide residential building code, and the Illinois Capital Development Board does not publish one. Since January 1, 2025, state law requires any local residential code adopted by a municipality or county to be at least as stringent as a recent edition of the International Residential Code, but which edition applies, and whether it specifies an ice barrier at the eaves, is decided locally rather than by the state. That means there’s no single ice barrier requirement to state for Illinois as a whole. Check with the building department that issues your permit for the local code edition in force and what it requires at the eaves before you plan a re-roof.
What roofing material suits Illinois best?
The combination that matters here is Chicago’s 107 mph design wind speed, its 25 psf ground snow load, and a state split between IECC climate zone 5A (58 counties) and zone 4A (44 counties). None of that adds up to one obvious winner. It rules some choices in and puts a real question to the others.
| Material | What the local snow load does to it | What the local wind speed does to it |
|---|---|---|
| Architectural asphalt shingles | Adds negligible dead weight of its own, so the design load is mostly the snow itself | Comes with a published wind rating, but performance depends on nailing to that rating, not the shingle alone |
| Standing-seam metal | Sheds snow well, but that means it can release as a slab, so where it lands (a doorway, a walk, a driveway) has to be planned for, not left to chance | Relies on the clip and fastener system for uplift resistance more than on the panel metal itself |
| Slate or concrete tile | Adds substantial dead weight of its own on top of whatever snow load the roof already carries, so the structure has to be sized for both together | Individual units can be dislodged at edges and ridges, where uplift concentrates, if fastening isn’t matched to the wind category |
Fastening and underlayment decide more of the outcome than the material label does, wherever wind is the governing load. A shingle rated for a high wind category performs to that rating only if it’s installed with the fastener count and placement the rating assumes. Installed short of that, it performs like a lower-rated shingle no matter what the packaging says. The same logic applies to metal panel clips and to the fasteners under a tile roof.
None of this substitutes for a design review specific to your address. A licensed engineer or your local building department can say what a given material and fastening scheme needs to meet, once both the ground snow load and the design wind speed for that location are on the table. For the broader steps a re-roof or new build goes through before it gets to material choice, see roofing.