How to drain Retained Pondered Rainwater on flat roof

A lot of flat roofs form bellies due to the felt layers being built up too high when they are laid, especially traditional hot bitumen roofs. However this can be caused as well by sagging boards,especially if the boards have become wet due to a previous leak etc . Rain water puddles in the bellies and can't make it to the gutter. Your builder isn't wrong in saying it's not a problem right away but over a period of time this can cause premature failure of the felt.

if i did the job i would have sloped the roof into the guttering area, but to be honest its not a big problem a new roof will last a long time ,if its built up with underfelt and top coat and the seams are sealed then it should last along time, hope this helps,