Roof CAD Software

I am looking for a basic roofing CAD software to outline wet areas on commercial flat roof systems. Any recommendations would be appreciated.

Thanks in advance - Matt

2D or 3D?
Consider Google sketchup

There are many “open source” versions of CAD available, Just search, download a trial or free full version and check out what works for you.
Most are compatible with AutoCAD .dwg etc.