Should water sit on a flat roof?
Small amounts of water may remain temporarily after rain, but persistent ponding can point to inadequate falls, blocked outlets or deformation of the roof. Long-term standing water can place additional stress on some flat-roof systems and should be assessed if it does not drain normally.
Why ponding is a warning sign
Flat roofs are not level; they are laid to a fall so water runs to outlets. Water still sitting on the surface long after rain has stopped means the fall, the deck or the drainage is not doing its job.
Standing water increases loading, keeps the membrane permanently wet and gathers silt and debris, all of which age a covering faster than water that drains away.
What causes it
Common causes are insufficient fall from the outset, deck deflection over time, blocked or badly positioned outlets, and later alterations that changed how water moves across the roof.
Diagnosing which of those applies determines whether the fix is clearing and adjusting drainage or re-forming falls when the covering is replaced.