To determine the width of the building to be roofed, measure the distance between the top edges of the opposite walls, and half this value. Note this value as the roof truss's base boards. To determine the gable height, determine the height difference between the roof's top and the building's top. Note this figure as the roof truss's gable post.

Square half the building width and the gable height, add the results, and find the square root of the figure. This figure is the minimum required length of the roof truss's exterior board.

For example, if the building width is 16 feet, half of this measurement is 8 feet, and its square is 64 feet. If the gable height is 5 feet, its square is 25 feet. The sum of the two squared figures is 89, and its square root is 9.48. Hence, the truss's exterior board length is 9.48 feet.

Next, measure the width of the wooden board that is used as the gable post, and divide the measurement by 2. Subtract this figure from the half building width value. This is the required length of the truss's base boards.

For example, if a 2-by-6-foot board is used for the gable post, its width is 1.75 inches, and half this value equals 7/8 inches. Subtracting this from 8 feet or half the building width value equals 7 feet 11 1/8 inches.