Chicago averages 38.4 inches of snow a year, but the weight on your flat roof depends on the kind of snow, ice and drifting, not on depth alone. The snow load your roof must be designed to carry is set by the Building Code and an engineer, and we do not quote a number here. Check with the Department of Buildings. Watch for sagging, ponding after the melt and ceiling cracks.
Snow load on a flat roof is the weight of snow and ice the roof has to carry. The design figure comes from the Chicago Building Code and a structural engineer, not from a rule of thumb, so for a number on your building, ask the Department of Buildings or an Illinois-licensed engineer. What an owner can do is understand what makes the load heavier and spot the signs of trouble early.
How much snow does Chicago get?
The National Weather Service's 1991 to 2020 normals put Chicago's annual snowfall at 38.4 inches. January averages 11.3 inches and February 10.7 (NWS Chicago). Lake Michigan boosts snowfall, though Chicago gets less lake-effect snow than the east shore because the cold-season wind is mostly from the west or northwest (Illinois State Climatologist).
Those are snowfall totals across a season. They are not the weight on your roof on any given day. Snow melts, compacts, blows off and falls again.
Why depth is not weight
Two roofs with the same depth of snow can carry very different loads. Fresh powder is light. Wet, heavy snow, old compacted snow and ice are much heavier. Rain falling on snow adds weight without adding depth. And wind moves snow, so it tends to pile into drifts against parapets, chimneys and rooftop equipment while other areas blow clear.
On a flat roof there is one more issue: where the water goes when the snow melts. A roof that ponds is carrying water as well as snow, and standing water can overload the deck (ARMA). We cover that in flat roof drainage.
On older two-flats and three-flats, snow drifts into the corner behind the parapet and stacks up around the stair bulkhead or a rooftop unit. Those are the same spots where meltwater sits afterward. The load problem and the drainage problem usually show up in the same place.
What are the warning signs of an overloaded roof?
Do not guess. If you see any of these, have a roofer or engineer look at it:
- A ceiling that sags or has new cracks, especially after heavy snow
- Doors or windows on the top floor that suddenly stick
- A visible dip in the roof where snow or water collects
- Water still standing 48 hours after the melt
- Creaking or popping from the roof structure
- A rooftop deck, planter or equipment added over the years
That last item matters. The City will not issue an Express reroofing permit for a building with an unpermitted rooftop feature such as a roof deck (City of Chicago). If extra weight was added without drawings, nobody checked what the roof can carry. Our flat roof maintenance checklist covers the fall and spring walk-through that catches many of these early.
Should you get the snow off?
That is a judgment call for the specific roof, and the answer depends on the signs above. If the roof shows distress, call before you do anything. Do not climb onto a snow-covered or icy roof yourself, and do not chop at snow with a shovel or pick that can cut the membrane. Clearing a drain or scupper that has frozen over is a better first step than clearing the whole roof.
Any roof work for hire in Illinois must be done by a licensed roofing contractor (Illinois law). If the roof needs structural attention, the contractor may need drawings from an engineer, and the Department of Buildings decides what permit applies.
Ice at the edges of pitched roofs is a related but different problem. See what causes ice dams on Chicago roofs.
What to do now
Walk the roof in the fall, before the first snow. Clear drains and scuppers, note low spots, and photograph anything that looks soft or sagging. Then call if something changes after a heavy storm. Our roofing crew can look at the roof and tell you whether the issue is drainage, age or structure. For the bigger picture, see the Chicago roofing guide.
Common questions
We do not give a single number because it depends on the building's design and the roof structure. The design snow load comes from the Building Code and an engineer. Ask the Department of Buildings or a licensed engineer about your specific building.
Yes. Wet snow, compacted snow and ice weigh much more than fresh powder at the same depth, and rain on snow adds more weight. That is why depth alone is a poor guide to how loaded your roof is.
Only if the roof shows signs of overload, and not by yourself on an icy roof. Digging with metal tools can cut the membrane. Call a roofer first, and keep drains and scuppers clear so melt can leave.
Sources
- National Weather Service Chicago: 1991 to 2020 Climate Normals for Chicago
- Illinois State Climatologist: Climate of Chicago
- Asphalt Roofing Manufacturers Association: Ponding Water Basics
- City of Chicago: Express Permit Program: Reroofing
- Illinois General Assembly: Roofing Industry Licensing Act, 225 ILCS 335/9
Facts last checked: October 7, 2026.
This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.