There are two simple ways to build a 12-foot span roof truss. They are similar and use rafter top chords that slope from a peak to outside walls. A king post truss has a horizontal chord across the bottom of the rafters, with a vertical brace between the peak and the bottom chord. A collar tie truss substitutes a horizontal cross beam between the rafters about halfway between the peak and the rafter bottoms.
Things You'll Need
Wood or metal gussets
Galvanized screws or nails
Start either style truss by cutting rafters. Select or determine a pitch or angle of slope; in most short span roofs the pitch is about 5/12, which rises 5 inches per foot. Mark a top or plumb cut by setting the point or heel of a framing square at the bottom of a 2-by-4-inch board. Place the 5-inch mark on the square's thin tongue and the 12-inch mark on the square's wide blade at the top of the board. Mark that angle at the end of the board.
Make rafters for a collar tie by cutting a notch on the end of each rafter. Figure the length of rafter needed from the "length of common rafter" table on the blade of the square. This table shows "13" under the 5-inch mark, meaning a rafter must be 13 inches long for every foot of run; a 12-foot wide truss will have a rafter run of 6 feet. Multiply the figure shown in the table by the run (13 x 6 = 78) and measure 78 inches down the bottom of the board from the bottom of the plumb cut line. Mark that point.
Measure 3 1/2 inches down from that point and draw a vertical line 1 inch up into the board. Draw a line in a triangle to connect those two points to form a birdsmouth, which will fit on top of the wall cap board. Measure for any desired overhang, typically a foot, and mark another line, like the plumb cut but reversed, by setting the heel of the square at the top of the board. Cut that rafter with a circular saw, then use that pattern to cut rafters for all trusses.
Cut rafters for king post trusses basically the same way, but skip the birdsmouth. Make a bottom chord instead, 12 feet long, to sit on top of the wall caps with the truss rafters on top of it. Lay out two trusses on a flat surface with plumb cuts together at the proper pitch and place a 12-foot 2-by-4-inch board between them. Mark the rafter angles on each end of that bottom chord and cut those.
Build the trusses by laying out the components on a flat surface with plumb cuts together at the proper pitch. Fasten that peak with a gusset, either wood or metal plate, which overlaps the joints and is fastened to the wide side of the rafters on both sides. Position the bottom chord for a king post and secure it with gussets. Measure and cut a 2-by-4-inch board to go from the bottom of the peak to the center of the bottom chord -- the king post -- and secure it with gussets.
Finish a collar tie truss in similar fashion. Lay out and fasten the rafters, then measure halfway down from the peak to the bottom of the rafters and cut a 2-by-4 to fit horizontally between the rafters at that point. Secure all the joints with gussets. Brace collar tie rafters on each end of a roof with a king post between the collar tie and the wall cap.
Put gussets on both sides of every truss joint. Use either plywood or metal. Cut 1/2-inch plywood to fit the peak and overlap rafters on both sides and cut rectangular gussets for cross members. Buy appropriately-shaped metal gussets, which usually come with spikes to be driven into the wood to hold them. Fasten wood gussets with 1 1/4-inch galvanized screws and fasten metal gussets with 1 1/4-inch galvanized nails in addition to the spikes.