My dad had a sawmill when I was a kid. Two differences in sawmill lumber and home supply store lumber is size and smoothness. Sawmill lumber is rough-sawn and has not been planed (unless, of course, the sawmill has a planer in which case it could be), and home supply store lumber has been planed and is smoother. The lumber at sawmills is usually true in size. A 2"x4" board is actually 2 inches by 4 inches. Lumber you buy at home supply stores is not true in size. I believe a board that is called a 2x4 is actually 1 1/2 inches by 3 1/2 inches.