Should water remain on a flat roof after rain?

Standing water is weight, and it collects where the deck already deflects, which deepens the depression over time. It also keeps seams permanently wet, so any weakness at a lap is under constant pressure rather than occasional.

In cold weather it freezes, and in warm weather it encourages growth and debris to accumulate, both of which restrict drainage further.

Insufficient fall designed into the roof, an outlet positioned away from the low point, an outlet set above deck level, blockage in the outlet or downpipe, or deflection in joists or decking.

Some of these are correctable with drainage work; some require the fall to be rebuilt using tapered insulation or a re-laid deck, which is a bigger piece of work.

Clearing outlets and gutters is the first and cheapest step, and it resolves a fair proportion of cases. Where the fall itself is inadequate, correcting it during renewal is the durable answer rather than repeated repairs.

Have the deck assessed at the same time: persistent ponding and a soft deck together suggest water has already entered the build-up.

Ponding that seems tolerable in summer can become a different problem once temperatures drop, because standing water that freezes expands and puts additional stress on seams and upstands that were already under strain. The same pond in warmer months tends to encourage algae and moss, which then restrict the outlet further and make the underlying problem worse over time.

Checking the roof after both a summer downpour and a winter freeze gives a fuller picture than a single inspection, since the two conditions expose slightly different weaknesses in the drainage and the covering.