Ponding Water on a Chicago Flat Roof: Why It Matters

Flat roof ponding water is water that stays on the roof 48 hours or longer after it rains. That is the Asphalt Roofing Manufacturers Association's (ARMA) definition for low-slope roofs (ARMA). A puddle that dries by the next afternoon is normal. A puddle that is still there on day three means the roof is not draining, and it will wear out faster in the spots where water sits.

Most Chicago two-flats, three-flats and mixed-use buildings have a flat or low-slope roof, and almost every one of them has a low spot somewhere. The useful question is not whether it ponds but how much, where, and why.

What counts as ponding, and what doesn't?

ARMA defines ponding as water that remains on a low-slope roof for 48 hours or longer (ARMA). That gives you a usable test. Look at the roof two days after a rain, with no new rain in between. Water that is still standing is ponding. Damp patches and a thin film that is gone by the next day are not.

Two things are worth saying plainly. First, no roof is perfectly flat, and a membrane that is dry in two days is doing its job. Second, we are not giving an "acceptable depth" number. No verified source we use sets one, so any depth figure you read elsewhere should be checked against the roof manufacturer's own warranty terms.

What does standing water do to a flat roof?

ARMA lists the problems standing water causes: it can overload the deck, grow algae and vegetation, clog drains, accelerate membrane deterioration and wet the insulation (ARMA). Each one deserves a sentence.

• Weight. Water is heavy, and a roof deck is designed with a limit. A large, deep pond puts load where the roof was not planned to carry it. If you see a roof sag where water collects, stop and have it looked at.
• Membrane wear. The membrane sits wet for days and then freezes in winter. Chicago winters add repeated freeze and thaw to wet-and-dry cycles, which is hard on any roofing material.
• Algae and plants. Anything that grows on the membrane holds more moisture against it. Seedlings in a roof are a sign the water has been there a long time.
• Clogged drains. A drain that is blocked with debris creates the pond. A pond that grows organic matter blocks the drain the next time.
• Wet insulation. Water that gets under the membrane soaks the insulation, which then does less and holds the moisture against the deck.

None of this happens in a week. It is a slow failure, which is why it is easy to ignore.

Why does water pond on a Chicago flat roof?

There is rarely one cause. The causes we find most often, from the manufacturer and trade guidance and from what we see on Chicago roofs, are:

1. Not enough slope. ARMA recommends at least 1/4 inch of slope per foot, and the NRCA Roofing Manual recommends positive slope through a sloped deck or framing, tapered insulation and correctly placed drains, scuppers and gutters (ARMA). A roof built dead flat, or one whose deck has sagged, can't shed water.
2. Drains in the wrong place, or blocked. If the drain is not at the low point, or leaves and debris keep it from taking water, the roof holds a pond. ARMA also notes secondary (overflow) drains may be required by local plumbing code (ARMA). Check with the Department of Buildings on yours.
3. No crickets or saddles. ARMA says crickets upslope of rooftop equipment and saddles along the low points between drains reduce localized ponding (ARMA). We cover them in flat roof crickets.
4. Worn insulation or a compressed roof system. Over time, layers settle and low spots form where there were none.
5. Patches and added layers. A repair that sits proud of the roof can dam water behind it.

What are the warning signs?

Go up (or have someone go up safely) two days after the next rain. Look for:

• Water still standing on day three
• A visible ring, stain or line of grit where a pond used to be
• Algae, moss, or weeds growing on the membrane
• Soft or spongy feel underfoot near a pond
• A drain, scupper or gutter with debris around it
• Interior ceiling stains under the same area
• A visibly sagging section of roof

If you can't safely get on the roof, don't. A roofer can do the inspection, and a ceiling stain or a slow sag that you can see from inside is enough reason to call.

How is ponding fixed?

It depends on the cause. The fix has to address why the water sits, not cover it.

• Clear and maintain the drains. A blocked drain is the simplest cause to fix, and keeping drains clear is routine upkeep. See our flat roof maintenance checklist.
• Add slope. Tapered insulation installed during a re-roof is the standard way to build slope into a roof that doesn't have it. The NRCA guidance above names tapered insulation as one of the ways to achieve positive slope (ARMA).
• Add or move drains, add crickets. When the pond is far from a drain, or behind equipment, the roof needs a new path for the water.
• Repair the membrane where it has been damaged. If the membrane has been wet for a long time, a patch is a short-term answer. Our guide to repair versus replace explains how to tell.

Coatings and "ponding fillers" sold at hardware stores sit on top of the problem. They may buy time on a small area. They don't change slope or fix drainage.

Do you need a permit to fix it?

Possibly. The City's Express Permit program covers roof repair limited to 25 percent of the roof area, a recover over one existing layer and replacement with no structural work, and the contractor must hold an Illinois roofing license (City of Chicago). A repair limited to 25 percent of the roof, where no part of an exterior wall or roof is cut away, is listed as not needing a permit (City of Chicago). But the City also says work on low-slope roofs generally needs a permit (City of Chicago). Those rules overlap, so check with the Department of Buildings on yours before work starts.

By state law, roofing work for hire must be done by a licensed Illinois roofing contractor (Illinois Roofing Industry Licensing Act). Ask to see the license.

What to do now

Check your roof two days after the next rain. If you find water, take photos with a date and a reference object for scale. Clear drains and scuppers you can safely reach. Then have a roofer look at the pond, the drain and the slope.

Our roofing crew can walk the roof, tell you why the water sits, and put the fix in writing. For the whole picture on flat roofs, start with the Chicago roofing guide, and read how flat roof drainage works.

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.