A flat garage roof has the possibility of collecting or pooling water if not designed and constructed properly, as it has no natural drainage. Whilst a pitched roof will have the obvious added benefit of a slope allowing water and other debris to run off the roof. However, this can be quite a costly, time-consuming job. Changing a flat roof to a pitched roof means you will need to have further foundations installed to help support the roof. If you already have one then sticking with a flat roof will probably be the better option.