In some cases a flat roof is almost impossible to avoid. Buildings over 40' wide get progressively harder and harder to accommodate a sloped roof because of the volume of space that is created. Codes start creating extra costs to meet fire ratings for the roof structure. And many times you might be forced into a more costly HVAC system if there is nowhere to place rooftop equipment.
There are two ways to ensure that your flat roof performs as you hope and expect. Design the flat roof properly, and select an appropriate roofing membrane system.
Flat roof design considerations are: 1/4" per foot slope throughout for proper drainage; adequate roof drains and drainage piping; minimal roof penetrations, and all of them properly flashed. The vast majority of roof leaks occur at penetrations and perimeter flashing. I have only seen one instance in thirty years where the leak was in the field of the roof. It is also wise to specify the maximum warranty offered, since this will usually require the roofer to be certified by the roofing material manufacturer.
Selecting the right roof membrane system is easier, but still requires attention. Flat roofing types include Built-Up, TPO, EPDM, PVC, Roll Roofing, Metal Roofing. This is also my ranking for dependability.
Built-Up roofing - the key to a traditional built-up roof is hiring a top quality roofer, who actually knows how to install the roof properly. I am on the maintenance committee of a non-profit that has a 36 year old built-up roof that is problem-free because of good maintenance, which costs about $1,000 a year for 25,000 SF. Garland Roofing has a top notch evaluation/recommendation service and products to match.
TPO - Thermoplastic polyolefin. The critical thing about TPO is the specific product. Some manufacturers have had problems with shrinkage, cracking, etc. One indirect way to evaluate the best manufacturer is by the length of warranties offered. This alone is not always conclusive. Deep pockets to back up the warranty is even more important than length. The ability to 'weld' the seams of the roof is a major advantage for TPO. Since this welding is the weak link make sure you have a qualified and experienced roofer doing your installation.
EPDM - Ethylene Propylene Diene Monomer. In my opinion EPDM only has one disadvantage ; it is black. Being black is not a problem for the roofing, but it doesn't meet the current vogue for white roofs. Adding a white laminate or coating can solve the color problem, but at the expense of maintainability. EPDM has a good track record, but you still need a quality roofing manufacturer and roof installer to get the most out of the roof.
PVC - Polyvinyl Chloride. PVC membrane is having its third incarnation, the first two having gone down in flames. The easily weldable seams is its strongest feature. The plasticizers that make it flexible are its weak link. Past incarnations have become brittle and shrunk. And not after 20 years. Besides the spotty track record, there is the chloride. PVC manufacturing is a nasty business - environmentally. Being white is little compensation in my opinion.
Roll Roofing is like installing rolls of shingles. I would only use it on a low sloping shed roof with minimal penetrations. Flashing tends to be whatever the installer dreams up. This is a high maintenance product to keep terminations and penetrations weathertight.
Metal Roofing - low sloping metal roofing seems like an obvious problem waiting to happen. Every other roofing system tries to be monolithic. Metal roofing is lapped pieces, that move. I don't get it. High sloping metal roofing is a different story, but this is about flat roofs.