How to Make a Center Support Beam for Roof Rafters

Make a ridge board that's one size larger than the rafters. For instance, use a 2-by-6 inch beam for 2-by-4 rafters or a 2-by-8 for 2-by-6 rafters. Use a straight board with no bows or warps. Get a solid board without knots or other imperfections. Ensure that the ends are square by checking them with a framing square.

Splice two boards, if necessary, to make a beam long enough to cover the roof. Make splices of 1/2-inch plywood gussets or 18-inch long pieces of board the same dimension as the ridge. Butt the ends of the two boards together tightly and overlap ends with splices. Space the splices to go between rafters, so each side of the splice will be supported by rafters.

Cut rectangular plywood gussets the depth of the ridge board with a circular saw. Cut board splices 18 inches long with the saw. Fasten gusset splices with a screw gun and four galvanized screws on each side of the joint. Nail board slices in place with 16d framing nails and a hammer. Use a level to ensure the spliced board is level at the top and the splice does not extend above the top of the ridge.

Mark the ridge board for rafter locations, generally 24 inches apart, before installing it. Lift the ridge board to the roof and set it between the tops of the two sets of end rafters, which should be resting lightly against each other. Slide the ridge board in place from the bottom and nail rafters to it with 16d framing nails, two through each rafter. Install other rafters the length of the roof. Keep the ridge board level throughout.

Install vertical post studs on each ridge board end. Cut 2-by-4-inch boards to fit between the bottom of the ridge beam and the end wall top. Toenail these in place with framing nails driven diagonally. Use rafter collar ties for additional ridge board support. These are 2-by-4 inch horizontal braces nailed to the rafters on each side just below the ridge board to reinforce it.