- Find the pitch of your roof. On a drawing or blueprint this will be labeled on each elevation that depicts the roof as a fraction such as 6/12, also printed 6:12. This indicates that your roof will rise vertically 6 inches for every 12 inches of horizontal run.
- Lay a framing square flat with the larger, or blade arm, pointing away from you and the smaller, or tongue arm pointing to the left. Wrap tape around the blade arm at the rise dimension and wrap tape around the tongue arm at the run dimension. Place a straight edge between the two taped marks and read the angle aligned with the straight edge on the blade of the square.
- Cut the end of two 2-by-4s to that angle and match them up to form the peak of the truss. These are known as the upper cords. Measure and cut a 2-by-4 to the length of the bottom board of your truss as indicated on your drawing or blueprint. This piece should be long enough to reach both walls and overhang by at least 10 inches on each side. This piece is known as the lower cord.
- Lay the lower cord across the bottom, open ends of the upper cords you matched to form a peak. Measure from the peak of the upper cords to the top corner of the lower cord and adjust it until the measurement along each upper cord is the same and the corners are aligned with the top edge of the upper cord on each side. Mark the upper cords along the top edge of the lower cord and cut them at that angle.
- Match the upper cords together at the peak and match the lower cord flush with the newly cut bottom ends. Mark the center of the lower cord and mark 1 3/4 inches to each side of the center mark, using the square to make a vertical line across the board at each of the two points.
- Lay a 2-by-4 on top of the lower cord between these lines, extending up and through the peak of the upper cords. Mark this 2-by-4 where it meets the lower and upper cords and cut to fit between them with a point at the top, equal to the upper cord peak. This is known as the killing post. Fit the cut killing post into the bottom of the upper cord peak and flush with the top of the lower cord, between the lines you marked.
- Cut the ends of two 2-by-4s with two 45-degree angles that meet in the center of the board to make a 90 degree point. Fit the point into the corner formed by the killing post and lower cord, lay the board across the upper cord and mark it where they meet. Cut the board at that angle. Repeat with the second board. These are the webs. Position the cut webs in the angle of the killing post and lower cord, with the angled end resting against the bottom edge of the upper cord.
- Trace the outline of each joint onto 1/2-inch plywood and cut it with a jigsaw to create the plates. Extend the plates 4 inches onto each member at every joint. Use wood glue and box nails to fasten them in place. One to join both upper cords and killing post at the peak. One to join each upper cord to the lower cord at each end of the lower cord. One to join the two web pieces and bottom of killing post to the lower cord, and one to join the top end of the web pieces to each upper cord. Use this truss as a template for all trusses needed for your roof.
How to Build 2X4 Roof Truss
Traditionally, roofs have been supported by two gables, attached to each other by a ridge beam which is further supported by angled beams from the top of the wall to the ridge known as rafters. The rafter ends were then connected to horizontal beams known as joists that held the walls together, tied the rafters down and supported the floor of the attic and ceiling of the room below. Most roofs today are built from triangular units known as trusses that fulfill these same functions and are much quicker and simpler to build.