From a design perspective that should not. Aged or poorly designed flat roofs will typically suffer with standing water that can exacerbate ingress or create unwanted safety issues during roof access. However, most modern flat roof membranes are unaffected by standing water with minimal to no impact on the supplier’s warranty.
Confirmed at design stage. Normal methods are through roof top outlets that expel rainwater through internal downpipes that pass through the building structure or by external means such as guttering secured to the façade of the building.
Suitable membranes, indefinitely.
Drainage outlets that direct rainwater through an internal downpipe system, drainage outlets that pass-through parapets into a hopper head or over a draining edge directly into external guttering.