by Captain James R. Watson
A deck crown tram is a device used to describe a specifc curve or crown of a deck regardless of the deck’s width. To make a deck crown tram, begin by laying out a baseline on a piece of plywood. Make the line the length of the largest (widest) deck beam. Drive a nail at either end of the baseline to represent the beam ends (points A and B). Draw a centerline between points A and B.
Drive a third nail (point C) on the centerline at the minimum beam height above the baseline. I used a ½” of crown for 12″ of beam. Any less and the deck will look flat. Any more, and you may have difficulty bending the plywood to that shape.
On the layout, position two straight battens—each longer than the overall maximum beam. Both battens should meet at the center nail (point C) and their middles should rest on the nails at each end of the beam (points A and B. With a plywood gusset, join the two battens at the center nail and leave a small gap in the apex of the battens, where you will insert a pencil. You can the move the mechanical tram from one side of the beam to the other, using the two nails as a guide, to make a perfect arc over the entire width of the beam. Use this as your pattern.