Finding and Fixing a Flat Roof Leak in Chicago

A flat roof leak is almost never directly above the stain. Water gets in at a weak point, runs along the deck or insulation, and drips where it finds a gap. Fixing the stain does nothing. You have to find the entry point, repair that, and fix whatever made it fail.

Here is how that works on a typical Chicago two-flat, three-flat or commercial building with a low-slope roof, and where the City and the State put rules on it.

Where does a flat roof leak actually start?

Start with the places where the roof changes: anywhere the membrane stops, turns up a wall or gets cut for something else. The open field of a roof is usually the last place to fail. The edges and the penetrations go first.

Check these, in this order:

• Parapet walls and wall flashing. The membrane turns up the wall and is sealed at the top. When that seal opens, water runs behind it. We cover this failure in when parapet flashing fails.
• Drains, scuppers and gutters. A clogged or poorly sealed drain sends water under the membrane at the outlet.
• Penetrations. Vents, pipes, skylights, chimneys, mechanical curbs and antenna mounts all have a seam around them.
• Seams in the membrane itself. Laps between sheets can open, especially at corners and where two sections meet.
• Low spots. Standing water puts constant load on every weakness underneath it.

That last one deserves its own look. The Asphalt Roofing Manufacturers Association defines ponding water on a low-slope roof as water that stays for 48 hours or longer, and says it can clog drains, grow algae and vegetation, wet the insulation and speed up membrane deterioration. If your roof holds puddles two days after rain, read why Chicago flat roofs leak and our guide to ponding water.

How do you trace a leak you cannot see?

Water travels. On a flat roof it can run along the deck, down a joist or through wet insulation before it shows up inside. The inside stain tells you the general area, not the address.

A sensible sequence:

1. Look inside first. Note where the stain is and whether it grows after rain, after snowmelt, or on dry days. A leak that shows up only in heavy rain points to something different than one that shows up in a light drizzle.
2. Go upslope. On a roof with any pitch toward a drain, the entry point is usually uphill from the stain, often near a wall, penetration or seam.
3. Check the roof from the ground first. Look for lifted flashing, torn membrane, bare spots or debris in the drains. You can learn a lot from the ground or from a window.
4. Compare the roof to the interior. Measure from two walls inside to the stain, then walk the same measurement on the roof. It narrows the search.
5. Look for wet insulation. If part of the roof feels soft or spongy underfoot, water is in the layers below the membrane.

What is a flat roof patch, and does it hold?

A patch is a repair to the damaged area of the existing roof. A proper one is cut back to sound material, dried, and rebuilt with compatible materials, then sealed at the edges. A tube of sealant squeezed over a crack is not a patch. It is a delay.

What the right repair looks like depends on the roofing type. NRCA groups low-slope roofing into built-up roofing, modified bitumen, rubber sheets (EPDM), thermoplastic sheets (TPO and PVC), sheet metal and sprayed foam. Each one is repaired differently. Manufacturers also publish how their products are joined, for example that modified bitumen can be torch-applied, self-adhered or mechanically fastened. A patch on the wrong material, or the wrong technique for the membrane you have, tends to fail at the edge of the patch.

If you do not know which membrane you have, that is the first thing a roofer should identify.

Does a flat roof leak repair need a permit in Chicago?

It depends on the scope, and this is a point where you should confirm with the Department of Buildings before work starts.

Two City rules apply, and they point in different directions. The City's list of work that does not need a building permit says roof repair limited to 25 percent of the roof area is permit-exempt on any building, as long as no part of an exterior wall or roof is cut away. But the same City guidance says a permit is required for any structural roof repair and for any work on low-slope roofs, even on residential buildings up to four stories. Only pitched roofs of at least 2:12 get the no-permit treatment for repair, recover or replacement.

A flat roof is a low-slope roof. So do not assume a patch is exempt. Ask the Department of Buildings, or ask your contractor to confirm in writing how they are handling it. If the work grows into a reroof, the City's Express reroofing permit covers repair limited to 25 percent of the roof area, a recover over one existing layer, and replacement with no structural work. Rules change, so confirm on the City's site.

A permit is a separate question from a license. In Illinois, roofing work may be done only by a licensed roofing contractor, with limited exceptions such as work on your own property. If someone offers to "just patch it" with no roofing license, that is a red flag. Our page on the roof permit rules goes into this in more detail.

When is a leak repair not enough?

A repair makes sense when the damage is local and the rest of the roof is sound. It stops making sense when:

• the roof has more than one active leak in different areas
• insulation is wet across a large part of the roof
• the membrane is cracked, shrunken or splitting in the field, not just at a detail
• the roof ponds in several places because the slope or drainage is wrong
• the repair history is a pile of patches

That is the point where repair versus replace becomes the real conversation. We will not tell you a roof needs replacing without showing you why. And where a repair will hold, we will tell you that too.

One more cause worth knowing about: water that cannot leave. A roof with weak drainage keeps loading the same weak spots. Industry guidance is that crickets and saddles redirect water around rooftop equipment and toward drains, and that secondary overflow drains may be required by local plumbing code. If your repair does not address why the water was sitting there, expect to see the leak again.

What about a flat roof leak in the middle of a Chicago winter?

You can still make a roof safer in cold weather, but you often cannot make a lasting repair. Cold, snow and ice make it hard to dry the substrate and get materials to bond.

In winter the goal is to limit the damage:

• Move belongings away from the leak and protect floors and electrical.
• Photograph the stain and its spread so you can show the insurer or the contractor.
• Clear a drain from a safe position if it is blocked, but do not go onto a snow-covered roof.
• Have a roofer look at it, who can tell you whether a temporary measure makes sense until the weather allows a real repair.

Leaks that get ignored through a winter tend to wet the insulation and the structure, and that moves a small repair toward a large one.

What to ask a roofer before they touch the roof

• Do you hold an Illinois roofing license, and can I see it?
• What membrane is on my roof, and how will you repair it?
• Where do you think the water gets in, and how do you know?
• Is the insulation wet, and how will you check?
• Who is pulling the permit, if one is needed, and how are you handling the low-slope rule?
• Does the repair also address drainage, or only the hole?

Akats handles roofing repairs and replacements in Chicago, and we also have the other trades under one roof if the leak has already reached masonry or the structure. For the full picture, see our Chicago roofing guide. If the leak is active, call us at 312-339-9627.

Common questions

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.