How to Design a Barn Truss

Layout

• 1

  Drive one stake in to represent one bottom corner of your truss. Measure across from this the width of your truss, which is typically 1 to 2 feet wider on each side than the walls of the barn. Drive in a second stake at that point. Tie a piece of mason's twine tight between the two posts.

• 2

  Measure between the two posts and wrap a piece of tape around the twine directly in the center. Measure up from this point the height of your roof from the tops of the walls to the roof peak. Drive in a third stake at this point.

• 3

  Mark a point between the peak stake and the center tape on the twine, 1/3 of the way between the two points, by driving in another stake. This will be the height of the pitch break. Measure in from each corner of the twine halfway to the center and add another piece of tape. Measure up from these points to the same distance from the twine as the pitch break stake. Drive in another stake at each point.

Establishing Angles

• 4

  Stretch twine from the bottom corners, around the stakes on the pitch break line to the peak and pull them snug, tying off at the peak stake.

• 5

  Calculate the length of your pieces be my measuring the twine. The upper cords, or top rafter section runs from the peak to the break. The bottom half of the upper cords, or bottom rafter section, will run from the break to the corner and at least 12 inches past, depending on the desired overhang. The lower cord should be 7 inches shorter than the distance between the two bottom stakes, allowing for 2-by-4 upper cords.

• 6

  Figure the angles of your piece ends by using an angle finder at the peak to measure the angle between the two faces and dividing by two to find the top end miter for the upper rafter sections. Measure the angle between the twine sections on each side of the break and cut the top of the bottom section to that angle. Measure the angle between the rafter and the bottom line of the outline to find the angle for the ends of the lower cord.