Flat Roofing
How flat is a flat roof? It is not, and it is not supposed to be. Model codes call for a minimum fall of a quarter inch per foot — about a two percent slope — on new low-slope construction, and require positive drainage on re-roofs, meaning an assembly designed to clear itself within forty-eight hours of rain. "Flat" is a figure of speech for anything too shallow to shed water by gravity alone, and the whole discipline is about managing water that a pitched roof would simply throw off. APC Roofing installs and repairs flat and low-slope roofing on homes and buildings, including TPO and EPDM membrane systems. What follows is what actually decides whether one of these roofs lasts.
Ponding is the defining failure
Water still standing forty-eight hours after the rain stopped is ponding, and it is the classic low-slope enemy. It works at seams, it accelerates membrane aging, it grows algae, and it is heavy — roughly five pounds per square foot for every inch of depth, which on a large area is a load the deck was not necessarily designed to carry indefinitely. Ponding is a drainage fault, not a membrane fault, and no coating or sealant corrects it. Where the fall is genuinely wrong, tapered insulation builds it in, with crickets run between drains and behind curbs so water is directed rather than left to find its own low point.
Drainage is sized for the worst hour, not the average year
This is where regional design actually differs, and it differs by more than people expect. NOAA's rainfall atlas puts the hundred-year, sixty-minute rainfall at roughly 4.2 inches per hour on Florida's east-central coast and about 2.5 inches along Colorado's Front Range — the Florida figure is close to 1.7 times the Colorado one, and its twenty-four-hour depth is well over twice as large. Drain and scupper sizing follows the intense hour, not the annual total, which is why a drainage layout copied from one market to another can be quietly undersized. Every primary drain also needs a secondary path set above the primary flow line, so a blocked drain shows itself as water pouring off the side of the building instead of accumulating on the deck.
TPO, PVC and EPDM
The single-ply membranes solve different problems. TPO is a reflective thermoplastic whose seams are heat-welded into one continuous sheet, which is a real advantage under strong sun and at altitude. PVC works the same way and holds up better against grease and chemical exposure, which matters over a kitchen exhaust. EPDM is a rubber sheet joined with tape or adhesive that stays flexible through hard freeze-thaw cycling and wide temperature swings; its historic weak point is the seam, which is why seam detailing on an EPDM roof deserves close attention. Alongside them sit modified bitumen in torched or self-adhered plies, and traditional built-up roofing in layers of felt and asphalt.
Thickness, in mils
Single-ply membrane is specified by thickness, commonly 45, 60 or 80 mils, and the thickness above the reinforcing scrim is what determines how long the sheet weathers before the scrim shows through. It is also the easiest number to quietly reduce in order to win a bid, and the hardest to notice afterward. Get it written on the proposal. Fleece-backed versions add puncture resistance where foot traffic is a factor, and walkway pads are cheap insurance around any rooftop equipment that gets serviced.
How it is held down
Mechanically attached membrane is fastened through the sheet into the deck on a set pattern — fast, economical, and it flutters more in wind. Fully adhered is bonded across its whole area, which resists uplift better and costs more. Ballasted is held by stone or pavers, cheap and heavy and now uncommon. The attachment method and its fastening pattern, rather than the brand on the roll, is what the wind is going to test.
The perimeter is where it fails
Uplift is not evenly spread across a roof. Pressure is highest at corners, high along the perimeter, and lowest in the field, which is why design standards call for denser attachment in those zones. Most low-slope storm failures start at an edge: a loose termination bar, an unfastened coping, a gutter that peeled and took the membrane with it. Edge metal is among the cheapest components in the assembly and the one most worth specifying properly — the National Weather Service's high wind safety guidance is a reminder that damaging gusts are not a coastal problem, and large low-slope surfaces present a lot of area to them.
Repair, replace, or maintain
Bubbling, open seams, chronic ponding stains and repeated interior leaks all mean it is time for someone on the roof. A moisture survey and core cuts tell you whether water is sitting inside the assembly, which is the finding that separates a patch from a project — saturated insulation carries weight, destroys the R-value the building is paying for, and corrodes a steel deck from above. Beyond that, low-slope roofs reward routine attention more than any other type: clear drains, sound seams, tight equipment curbs. Published construction details for these junctions are available from the National Roofing Contractors Association if you want to compare a proposal against standard practice.
