Thaw leaks in Chicago usually start at one of a few places: a flat roof where water ponds or a seam has opened, flashing at a wall or chimney, a parapet cap or open mortar joints, or a gutter and downspout that dump water at the wall. The stain on your ceiling is where it appeared, not necessarily where it got in. Mark it, photograph it, put out a bucket and call a roofer or mason.
A ceiling stain after the thaw means water found a way in while snow melted. The spot where it shows up is often not where it entered, so the job is to trace it uphill. Chicago averages 38.4 inches of snow a year (NWS Chicago), and all of it becomes water that has to go somewhere.
Where does the water usually get in?
Work from the top down.
The roof surface. On a flat roof, water that stays 48 hours or longer is called ponding, and it can clog drains, wet insulation and wear out the membrane faster (ARMA). Open seams, blisters and a clogged drain are the first suspects. See why Chicago flat roofs leak and finding and fixing a flat roof leak.
Flashing. Flashing is the layer that collects water that got behind a wall and sends it back out, and the brick industry says it belongs at the base of walls, tops of walls, parapets, chimneys, openings and roof edges (BIA). Over windows and doors, steel lintels should have continuous flashing over the angle and weep holes at the lintel level (BIA). Flashing that has lifted, cracked or been caulked over is a common way in.
Parapet caps and brick. The top of a parapet wall and open mortar joints let in meltwater. The City's maintenance standard lists masonry joints that are not weather resistant or water tight among unsafe exterior conditions (Title 14X). See parapet flashing failure for the roof side.
Chimneys. A cracked crown or open joints let water run down the flue and into the wall. See chimney crown repair.
Gutters, downspouts and grade. Water that drops at the foundation is a basement leak waiting. Guidance for basement water says downspouts should discharge at least 4 feet from the wall and the soil should slope away (University of Minnesota Extension). Rain and melt that reaches a below-grade wall can look like a "thaw leak" in a basement.
How can you tell what kind of leak it is?
White crust on an inside or outside wall suggests water is moving through masonry. It often appears in winter, and on a building more than a year old it usually points to water getting in (BIA Technical Note 23A). A stain that grows only after thaws points to melt. A stain that appears after any rain points to a plain roof or wall opening.
What should you do right now?
- Put a bucket under the drip and move valuables.
- Mark the edge of the stain with a pencil and date it. Photograph it every day or two.
- Check attic or top-floor wall cavities for dampness if you can reach them.
- Look from the ground at the roofline, chimney and downspouts. Do not climb onto an icy roof, and see roof raking for why.
- Call a roofer for roof and flashing, a mason for walls and parapets. If you are not sure, call ours.
Drywall that stays wet can grow mold and rot framing, so do not wait for spring to sort it out. Our roofing crew can trace it, and the maintenance calendar lists what to check each season. See also what to check on your building after winter.
Common questions
Melting snow adds a lot of water at once, and it exposes weak points such as ponding, open seams, lifted flashing and blocked drains. The stain shows where water came out, which is not always where it went in. Trace it uphill.
Treat an active drip as urgent: contain it and call a roofer. A dry old stain is less urgent but still worth checking. Water that stays in drywall and insulation can cause mold and rot, so do not wait for warm weather.
Sources
- City of Chicago: Code Modernization Ordinance, including Title 14X Minimum Requirements for Existing Buildings (PDF)
- Asphalt Roofing Manufacturers Association: Ponding Water Basics
- Brick Industry Association: Technical Note 7: Water Penetration Resistance, Design and Detailing (PDF)
- Brick Industry Association: Technical Note 31B: Structural Steel Lintels (PDF)
- Brick Industry Association: Technical Note 23A: Efflorescence, Causes and Mechanisms (PDF)
- University of Minnesota Extension: Moisture in Basements: Causes and Solutions
- National Weather Service Chicago: 1991 to 2020 Climate Normals for Chicago
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.