Why Does Water Pool on My Flat Roof?

Flat roofs aren’t perfectly flat — they’re built with a slight slope (usually around a quarter-inch per foot) toward drains or scuppers. Ponding happens when that slope isn’t doing its job, for one of a few reasons: the roof was installed with insufficient slope to begin with, the deck has sagged or settled over time creating a new low spot, insulation has compressed unevenly, or the drains themselves are blocked or damaged.

A small amount of water sitting briefly after rain is normal. Water that’s still there 48+ hours later, in the same spot every time it rains, is a sign of an underlying slope or drainage issue — not just “how flat roofs work.”

The fix depends on the cause: cleaning or repairing a drain, adding tapered insulation (“crickets”) to redirect water toward existing drains, or in more significant cases, correcting the deck slope during a larger repair or replacement.

The Most Common Slope Issues We Find

Beyond simple drain blockages, the underlying slope problems we most commonly diagnose include: insufficient original slope (built too flat from the start, sometimes to save on tapered insulation costs during original construction), structural sag from age or added rooftop weight over time, and localized settling that creates a new low point that didn’t exist when the roof was installed.

Each of these has a different fix — tapered insulation to correct slope, structural assessment and repair for sag, or in some cases, accepting a manageable amount of ponding on an older roof that’s otherwise sound, paired with more frequent drain maintenance to manage it.