Go with a single ply membrane
Another type of cool roofing material used for low-sloping roofs is a single ply membrane. Single ply membranes are pre-fabricated plastic or vinyl sheets containing solar reflective coatings or materials. The sheets are rolled onto the roof deck, then attached to the structure with some combination of chemical adhesives, mechanical fasteners, or ballast such as gravel or pavers. Single ply membranes don't require any additional coatings or surfacing -- the cool roof properties are integrated within the product itself.
There are several types of single ply membrane, but some of the most common include polyvinyl chloride (PVC), thermoplastic polyolefin (TPO), and ethylene propylene diene monomer (EPDM). Not all single-ply membranes are cool roof products, but the average cost of cool roof single-ply membrane is only slightly higher than the cost of traditional single-ply membrane - about $1.50 - $3 per square foot for cool roof materials versus around $1 to $2 per square foot for standard single ply products [source: Environmental Protection Agency].