Occasionally, you will see garages or small bungalows with flat roofing. While I’m definitely not an expert but a mere hobbyist, I think flat roofs are functionally inadequate.
A flat roof with sagged joists allows rainwater to converge at the center and create a pool. Ideally, it should be draining the water toward the roof edges. This is a common problem and it affects nearly majority of flat roof installations. Having an insufficient roof slope promotes standing pools of rainwater, increasing the likelihood of leaks and cracks as the roof gets further used over time.
I do like flat roofs aesthetically however. They look very nice. Done right, they complement the architecture of the house very well.
Roof drains should be installed at each low point of the roof. Since the joists have sagged, it seems that they were not designed to support a heavy accumulation water. Therefore if you can add drains at the lower points of the roof it could prolong the life of your roof.