Flat roofs are primarily used on commercial buildings and cover the most space for the dollar. Roof systems in the past used asphalt roofs, and coal tar pitch roofs. The coal tar pitch roofs lasted the longest with life of 50-70 years. They were difficult to install with the fumes being very carcinogenic. The tar had a low melting point and would reseal itself. The roofs could only be used on roofs less then 1/4 inch to the foot. Asphalt roofs were installed with asphalt and paper felt, but are now being used with fiberglass felts. These roofs could be used with more slope but would not last as long as coal tar pitch roofs. They would mostly be covered with pea gravel and would be hard to find leaks when they occurred.
Rubber Roofs or EPDM
Rubber roofs come in large sheets up to 50 ft wide, so there's less seams and less chance of leaks, and have 300% elasticity. They are easy to repair like repairing a bike tire. The cons are they are non-resistant to oil, so can not be used on restaurants. They are also easy to puncture.
PVC Roofs are used for their puncture, fire and oil resistance, and they are heat welded for a strong seal. PVC Roofs come in a wide spectrum of colors, along with white so you can save on air conditioning costs. They have flexibility in low and high temperatures as well.
These roofs can be applied with hot asphalt, torch applied, or self-adhered, making a strong seal, however this type of roof is only made in in 3 ft wide sheets. They are not oil resitant, however are very cheap to install. You can also coat membrane this roofing type to reflect the sunlight to save on electric costs.