Flat Roof Replacement

House built in 1960. Total house size is approximately 30x70 feet. 'Main Roof' area is 30x50 feet. It is a 'Shed' style and has a low pitch slope of 18 inches over 48 feet (1/36). Water drains fine on this area. Problem is on secondary roof area of 30x20, which is completely FLAT and water ponds. This roof area needs to be rebuilt with rafters or trusses to build up a slope. - either to meet the 18 in. 'peak' OR higher - in a similar 'Shed' style as Main, but with a more normal pitch - like 1/4. The entire Roof needs to be re-roofed. But before that happens, I need the FLAT area to be built up for proper drainage. The roofing contractors that have come to analyze and give estimates are proposing building up area with tapered 'ISO poly sheets, but I am looking for a longer term and more robust solution.X