If you are building a carport you are probably trying to decide if you want a flat or sloped roof. Comparing the two may help you to decide.
A flat roof usually consists of plywood decking or other wood material which is covered by rolled roofing or a vinyl coating. These roofs are cheaper to build overall but do require coating about every 3-5 years to prevent leaks. This type of roof tends to warp a bit so puddling can occur in spots. Heat can cause some of the flat roof surfaces to crackle and have the appearance of alligator skin. Also, if the carport is attached to the house care must be taken that the flashing between the roof and the side of the house is installed correctly or water can back up against the side of your house and seep underneath the siding. If you live in an area of heavy snow you will need to remove the snow as the extra weight can cause damage and collapse.
Sloped Carport Roof
A sloped carport roof doesn't have to have a huge slope for it to be effective in draining rain water and snow. Even a minor slope will produce good results. A sloped roof generally will cost a bit more to build since there has to be some trussing built to achieve the slope. This roof, once in place, doesn't require any more maintenance that any other roof.