A flat roof is a roof with a very low slope, often just enough pitch to keep water moving toward drains rather than a slope steep enough to shed it quickly like a conventional shingled roof. True flat roofs are rare, since some minimal slope is needed by design; what people call a flat roof is technically low-slope. Because water lingers on the surface far longer than on a steep roof, flat roofs require a waterproof membrane, such as TPO, EPDM, or modified bitumen, rather than shingles.
What most people call a flat roof is, almost without exception, not perfectly flat. A truly level roof surface would have nowhere for water to go, so even a low-slope roof is built with a slight, deliberate pitch, sometimes just a fraction of an inch per foot, aimed at moving water toward drains or scuppers rather than letting it sit.
That small difference in slope changes everything about how the roof has to be built. A steep, sloped roof relies on gravity to move water off quickly, which is why overlapping shingles work fine, since even a small gap between them rarely has time to let water through before it runs off.
A flat or low-slope roof gives water much more time to find any weak point, so it cannot be shingled at all. Instead it is covered in a continuous, fully adhered or mechanically fastened membrane, materials such as TPO, EPDM rubber, or modified bitumen, designed to form one seamless waterproof layer across the entire surface rather than an overlapping system of small pieces.
Drainage design matters as much as the membrane itself. A flat roof needs properly placed drains or scuppers and often tapered insulation underneath the membrane to guide water toward them, since standing water left on any low-slope surface for extended periods accelerates wear and increases the odds of a leak finding a seam.
Flat roofs show up most often on additions, garages, porches, and commercial-style buildings, and they demand a different maintenance rhythm than a shingled roof: regular clearing of drains and debris, and a periodic look at the membrane's seams and any spot where equipment or vents penetrate it.
For a homeowner with a flat-roofed addition or garage, the first useful fact is that it runs on an entirely different system than the shingled roof beside it, with its own drainage, its own membrane, and its own maintenance rhythm.
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