Hello.
I'm in the process of building a 16x20x10 high shed/shop.
I have already framed the floor (2×6 on 16" o.c. ,on blocks with 3/4 t&g plywood floor) and didn't think about the girts taking up 3 inches for the overall size. So now once I add the girts it will be 16'-3"× 20'-3" framed size...