A little water sitting on a flat roof for a short time during or just after heavy rain is normal, but water that lingers is telling you something. Rain should run off as a thin film, so a puddle that stays points to a poor fall or a sagging deck.
Standing water collects dirt, leaves and algae, which can wash into the outlets, block them and make the ponding worse. Left there, it wears down the covering over time. If the covering is punctured where the water pools, it gets into the building below faster.
Have a proper look for splits, cracks, blisters or bumps in the covering, and for a damp patch on a ceiling inside. Check from inside and from the ground rather than walking on it, as that can make things worse. After a storm, make sure the gutters and outlets are clear.
Is water still sitting on your roof long after the rain has stopped? Give me a ring at Stratford Roofing and I'll come and look at the fall and the deck properly. There's more on our Flat Roofs page.
