How to Design a King Post Truss

A king post truss is used to form a standard gable roof.

The bottom horizontal, which serves as ceiling and upper floor joist, is known as the lower cord. The king post runs from the top of the lower cord up to support the peak vertically. You will need to know the width of the walls and the proposed roof height to design a king post truss.

Layout

1. Mark a point on a driveway,or other large concrete surface. Measure from this point the width of your walls, plus at least 12 inches overhang for each side and mark a second point. Mark the center point between these two as well.

2. Measure up from the center point as high as the roof peak will be above the tops of the structure's walls and mark that point as the peak.

3. Stretch out a chalk line and snap a line between the two outside corners of the original bottom line you marked. Snap a line from the center mark on this bottom line to the peak. Snap a line from the peak to each of the bottom corners. You should now have a chalk outline of a triangle with a vertical line running up its center.

4. Measure in from the bottom line and both of the upper diagonals 3 1/2 inches, which is the true width of a 2-inch by 4-inch wood stud. Mark this distance at both ends of each line and snap a chalk line parallel to the first line to create a second, smaller triangle centered in the first. This triangle represents the inside edge of the lumber that will make up the actual truss.

5. Create two lines parallel to the center vertical, one on each side, both 1 3/4 inches from the center vertical.

6. Mark the ends of your pieces with a piece of chalk. Extend the top edge line of the bottom cord to the outside edge of the original triangle at both corners. Erase the inside triangle line running down from the top edge of the bottom cord, if there is one. Erase any lines that extend above the bottom edge of the inside triangle at the peak, except for the center vertical line. Erase the center line from the bottom of the inside triangle to the top of the bottom cord.

7. Measure the outlines to determine the exact lengths and miter angles for all of your pieces. The two upper cords will run from the peak to the top of the bottom cord. The bottom cord will run from lower outside corner, to lower outside corner and the king post will run from the top of the bottom cord to the bottom of the peak, where it will be cut at a point.

8. Use an angle finder to measure the angle of the lines at the end of each piece to determine your miter angles. Cut your pieces to fit the pattern.