Modified Bitumen is a great option for your Colorado flat roof. It has modifiers added to it to give it plastic or rubber-like properties. The most common types of modifiers being used are APP (Atactic Polypropylene) and SBS (Styrene Butadiene Styrene).
APP is added to asphalt to give the asphalt some plastic properties. APP comes in a roll and is applied using a torch. The back of the sheet has extra asphalt on it that when heated will bond to the substrate. This is a great product for a homeowner and Denver commerical roofer to use on more cut up roofs because less room and equipment is needed on site to torch-apply a membrane.