Step 1
Identify or measure the width of the building, the distance that the trusses span. For example, assume a residential home has a footprint of 24 feet wide by 48 feet long.
Step 2
Identify the total height of the building, and then subtract the height of the exterior walls. The remaining measurement is the overall vertical height of the roof system. This is the distance from the top of the walls to the peak of the roof, measured vertically. For example, assume a height of 8 feet.
Step 3
Plug these values into the Pythagorean theorem, which says that for any right triangle, A * A + B * B = C * C, where A and B are the lengths of the two shorter legs of the right triangle, and C is the length of the longest side of the triangle. For example, A is the vertical height of the roof, or 8 feet, and B is half the width of the house, or 12 feet.
Step 4
Calculate the length of the top chord using this formula: (8 * 8) + (12 * 12) = 208, which means that the top chord is the square root of 208, or 14 feet 5 inches.
Step 5
Add to this length to the distance the roof hangs over the edge of the walls. Most soffits are between 16 and 24 inches deep. Using the same equation for a soffit that is 24 inches deep, this calculates a roof plane between the exterior wall and the edge of the roof that adds 2 1/2 feet to the total length of the roof plane. Therefore, the total length of the top chord of the truss is 14 feet 5 inches + 2 feet 6 inches, or 16 feet 9 inches. When the truss is built, it should be built with a top chord that is 17.5 feet to 18 feet long.