How to Build Trusses for Post Frame Buildings

Design the Truss

• 1

  Measure the clear span from post to post. This will be the span length for the truss. Trusses are supported on either end by posts, so knowing this measurement is a key to success.

• 2

  Decide on the size, height, slope and style of the roof. Trusses can be built to accommodate almost any structural configuration, but this example will describe a simple, two-sided roof truss. To make the numbers relatively easy to follow, a roof span of 20 feet with a center height of 10 feet will be used. This gives us a slope of 12 feet of rise for every 12 feet of run, or 45 degrees.

• 3

  Draw the design on paper or with a computerized drafting program. It is important to know all the measurements of the pieces before you start cutting. The bottom edge, or chord as they are called, will be a single piece or several pieces bolted together with steel bolts and plates. Similarly, the sloped pieces will also be single pieces or multiple short pieces bolted together. There will be a king post that rises from the midpoint of the bottom chord to the ridge line. At the midpoint between the outer edge of the truss and the king post, a side post will be installed on each side. Finally, a diagonal member, or strut, will be added that connects each side of the bottom of the king post to the top of each side post. This provides lateral bracing for the truss.

Cut the Pieces.

• 4

  Cut the bottom chord of the truss. If possible, use a single piece of wood 20 feet long. For this example, all wooden members are 4 inches by 4 inches in size.

• 5

  Cut the two top chords, which are approximately 14 feet 1 3/4 inches long. This number is calculated by using the Pythagorean theorem. We know that the length of our king post is 10 feet as this is the height of our truss. We also know that the length of our bottom chord is 20 feet, which is our span. A right triangle made by half the bottom chord, the king post and the top chord, provides us with edge length of 10 feet and 10 feet. By taking the two edges, squaring them, adding the results together, and taking the square root of that number, we find the length of the hypotenuse, which is our top chord.

• 6

  Cut the king post to accommodate the ridge height of 10 feet.

• 7

  Cut the side posts, which are 5 feet long. Since the distance from the edge of the truss to the location of the side post is half the distance from edge to king post, and our roof has a slope of 45 degrees, it is simple geometry to see that the length of the side post is 5 feet.

• 8

  Cut the struts. These are approximately 7 feet 1 inch long. Again, using the Pythagorean theorem to calculate the length of the hypotenuse of a triangle with side lengths of 5 feet gives us the 7 feet 1 inch dimension.

Assemble the Truss

• 9

  Drill holes, approximately 1/2 inch in diameter, at the ends of each member. There will typically be 4 to 12 holes at the end of the pieces, depending on the structural situation the truss is being built for. Cut plates from 1/4 inch to 1/2 inch steel, or purchase them premade for your truss design. Drill holes in these plates to align with the holes drilled in the wood members.

• 10

  Lay out all truss pieces in the approximate configuration of the truss. Beginning at the king post, attach the wood members together using the steel plates and bolts. The bolts go through one plate, through the wood piece, through another plate, and then are bolted tightly.

• 11

  Continue assembling the pieces with the same bolted connections until all connections are solid.

• 12

  Attach a truss bracket to the top of the two wood posts onto which the truss is to be installed. This bracket usually fits down over the top of the post, is bolted through with a similar connection as described above, and offers a slot into which the truss will slide.

• 13

  Bolt the truss to the post through the truss bracket. With both ends of the truss installed, the process is complete. Continue in a similar fashion for all trusses in the structural system.