A flat roof is not entirely flat though. Rain water should still be able to drain off of the roof surface, so as not to enter the construction and cause damage to the roof trusses, but in general a few degrees of roof angle is enough to let this happen. Typical materials used on a so called flat roof include roofing sheets made of plastic or metal, and roofing felt. These are the best choices for a waterproof roof with a shallow angle, as overlaps and seams can be made very tight and closed to water. They are also fairly easy for the average person to use – certainly in the case of roofing sheets. There, it is normally only the case of attaching the roofing material with nails or screws, which is about as easy as roofing will ever get.
Flat roof construction is best when kept simple though. Some desire to maintain the illusion of a completely flat roof, despite the need to drain water off. The solution to obtain this completely “flat” roof, is to build a parapet around the perimeter of the roof. To onlookers, the actual roof surface which is still sloped, will now be hidden, and the parapet will seem to be the actual roof. While this method may seem appealing to some, it quickly becomes complicated to realize, sensitive to leakage, difficult to troubleshoot and tricky to repair. The question remains whether it is worth the extra work to use this method on your garage or home – even though it is an often used technique for commercial roofing. In the opinion of the author, it would be a much better solution to accept the gentle angle of a flat roof as something which is necessary for the roof to properly function. If you must have some of the “flat roof” look, make it so that the roof slopes away from the front of the garage, or whichever side of the garage is the most visible to outside world.
Keep your flat roof garage plans simple and easy to inspect and repair, and it will serve you well for many years to come.