Choosing the right tree for your beam can be a problem . You don't want it too big so you end up wasting wood , and too small can leave you with waney edges and lots of sapwood . Also is the tree straight enough to get most of the beam out of it ? Most trees have at least one plane that they are quite straight in , use this plane for the widest dimension of your beam . Rest the beam on wooden chocks so that the straightest plane of the log is uppermost . Mark on each end of the log the vertical and horizontal axes using a plumb line and square . Mark vertical lines the required width (shortest dimension) at each end of the log .