Flat Roofs Are A Drainage Problem First

Nothing Is Actually Flat

A flat roof is not flat. It is low-slope, built to shed water toward drains or scuppers at a quarter inch of fall per foot. When that slope is right, water is off the roof in an hour and the membrane spends its life dry. When it is wrong, water sits, and standing water is what kills commercial roofs in Chicagoland.

Ponding does three things, all bad. It loads the deck with weight, roughly five pounds per square foot for every inch of depth. It bakes the membrane under a magnifying lens of water all summer. And in our winters it freezes, expands, and works at every seam it is sitting on. If you can see dirt rings on your roof after a dry week, you have ponding, and no membrane brand is going to save you from it.

Fixing Drainage, Not Just Covering It

Before we quote a new system we figure out where the water is failing to go. Sometimes it is simple: drains clogged with gravel and leaf litter, or a scupper the size of a mail slot serving half a roof. Sometimes it is structural, a deck that sagged between joists years ago and now holds a puddle every time it rains.

Tapered insulation is how we solve most of it. Polyiso board cut in a slope, laid out in a pattern that builds fall toward the drains, so the new membrane sits on a surface that actually sheds. It costs more than flat board on day one. It also stops the roof from dying ten years early, and it usually improves the building’s insulation value at the same time, which shows up on the heating bill in February.

• Tapered crickets between drains and behind curbs, so water gets pushed around rooftop units instead of pooling behind them.
• Drain sumps set properly, with the membrane dropped into the bowl rather than bridged over it.
• Additional drains or overflow scuppers where the original design was short, which is common on older Chicago industrial buildings.
• Deck repair where sag is structural, because insulation cannot fix a broken joist.

Picking A Membrane By How The Building Is Used

The three low-slope systems we install most all work. They fail in different ways, though, and the building tells you which one to use.

TPO is the default for open roofs on warehouses, retail and offices. White surface, heat-welded seams, and the reflectivity knocks real money off summer cooling on a big footprint. It is less happy on a roof with constant foot traffic and dozens of penetrations, because every detail is a weld somebody has to get right.

EPDM is rubber, and rubber moves. On buildings where thermal movement is the enemy, or where the roof has odd geometry and lots of transitions, EPDM handles it without splitting. It is black, so it does not give you the cooling benefit, but for a shaded roof or a building where winter heat retention matters more, that is a fair trade.

Modified bitumen is the choice when people walk on the roof. Restaurants with cooks going up to the exhaust hoods weekly, buildings with service techs on the units every month, roofs with decks and equipment. Multiple plies of asphalt sheet take abuse that a thin single-ply will not. Balcony roofs on Chicago apartment buildings get mod-bit for exactly this reason.

When A Coating Beats A Replacement

Restoration coatings are the most misunderstood product in commercial roofing. Sold wrong, they are paint over a dead roof and a waste of the owner’s money. Used right, they are the best value in the trade.

The test is whether the insulation under the membrane is dry. We take core samples and moisture readings. If the roof is wet underneath, no coating helps, and anyone who tells you otherwise is selling you two years. If the substrate is dry and the membrane is intact but aging, a silicone coating over prepped seams and details gives you real waterproofing, a reflective surface, and often a renewable manufacturer warranty for something in the range of half the cost of a tear-off.

The economics get better on big roofs. A tear-off on a warehouse means disposal of the old system, days of exposure, and a lot of labor. A coating goes on over a weekend with no open roof and no dumpster full of wet insulation. For an owner watching a capital budget, that difference is the whole conversation. We tell you honestly which category your roof falls into, because coating a wet roof comes back on us either way.

What Chicagoland Does To Low-Slope Roofs

Snow load is the obvious one. A flat roof holds every inch that falls, and a wet spring snow can put serious weight on a deck that was engineered decades ago. What does more damage is the freeze-thaw cycle underneath it: meltwater runs to the low spot, refreezes overnight, and pries at seams and flashings a fraction at a time all winter.

Then summer flips it. A black membrane on a July afternoon in Cook County runs well over 150 degrees at the surface. That heat cycling ages asphalt products, dries out sealants and works fasteners loose. A roof here sees a temperature range of roughly two hundred degrees across the year, and every detail on it has to survive that.

Which is why we do not do thin details. Extra plies at the curbs, proper termination bar at the walls, metal edge that is fastened rather than glued, and pipe boots that get replaced instead of caulked. Those are the spots that leak first, and they take five minutes longer to do right.

We work low-slope roofs across the south and west suburbs, regularly in Harvey, Romeoville and Joliet. Flat roof work we have finished is in the gallery, and if you want a specific opinion on your building, send the address and square footage through the contact page.