Well there are 4 main types of flat roofs available 4 ply tar and gravel, EPDM, invert and modified bitumen.

Tar and gravel is a great system but tends not to last as long as modified bitumen and is almost impossible to identify or pinpoint a leak.

EPDM has the best appearance and is easier to identify and solve a leak right away. Problem with APDM is it stretches and shrinks with temperature changes causing weak or vulnerable points so inadvertently does not last as long as other flat roofing systems.

Invert system is for properly sloped flats roofs with lots of traffic and is a great system but like I have just mentioned it has to be properly sloped.

Modified bitumen is a little more expensive and takes a bit more skill to install then the other flat roofing systems but it lasts longer. With Modified Bitumen it is also very easy to indentify, pinpoint and solve any leak.