A wooden roof truss is a prefab triangular frame that replaces one set of rafters and a joist in traditional stick framing. It is made up of four main parts: two upper cords that make up the peak, one lower cord that runs horizontally along the bottom of the triangle and a vertical king post support that runs up through the center. The 4/12 pitch is a roof that rises vertically toward the peak 4 inches for every 12 inches of horizontal run.
Measure the width of your side walls at the top, from outside to outside using a tape measure. Add at least 12 inches to either side for eaves, which are typically 12 to 36 inches depending on the style of the structure.
Use a chalk line to mark a line on your driveway as long as the width of your walls, plus the amount you added for eaves on either side. Find the center of this line and mark it with a piece of chalk.
Measure from the mark to one end of the line using the tape measure and divide by 12 inches. Start from the mark and measure up, perpendicular to the original line, a distance equal to half of the first line divided by 12, times 4 to find your peak. Mark the spot with chalk.
Snap a chalk line from the peak diagonally to both corners of your bottom line. Use an online angle converter to calculate angle from pitch. These angles will be at 18.43 degrees. Set your miter saw as close to that as possible, a small variance of less than a degree will not have a measurable effect on part size or roof pitch.
Cut one 2-by-4 as long as the bottom line, with both ends mitered in at 18.43 degrees, using the miter saw. Measure the diagonal lines from the peak to the ends of the bottom line. Measure the angle cut end of your 2-by-4 and subtract that dimension from the diagonal line length. Cut two 2-by-4s to that length with 18.43 degree mitered ends.
Fit the three pieces into the chalk outline to form your truss. Measure from the center of the top edge of your bottom cord to the inside of the peak. Cut one 2-by-4 to this length. Mark the center of one end using chalk. Cut the corners from that end at 18.43 degrees, so that they meet on the mark to form a point.
Place the king post you just cut into the triangle, with its bottom flat end against the top of the bottom cord and the point against the inside of the peak.
Trace the joints at all three corners and between king post and lower cord onto 1/2-inch plywood using chalk. Cut these out with a jigsaw. Make two for each joint. Glue and staple them to the faces of the truss with wood glue and pneumatic wide crown stapler and 1 1/2-inch staples.