Measure the length and width of your barn to come up with your truss dimensions.
Cut your first two 2 x 6 boards at a 67.5 degree angle using a miter saw at both ends. Each angle should be facing in an opposite direction at both ends if you plan on installing eaves.
Lay the two ends together so the cut sides intersect and form the 22.5 degree angle of a typical barn roof.
Place a metal 22.5 degree angle brace across the intersection of the two boards and fasten with nails or bolts. Use bolts if you are using timber beams thicker than 2 inches wide. Repeat steps two and three for the other side of the truss.
Lay both sides of the roof truss together at the ends to form the half-octagon shape of a barn roof. Fasten them with another 22.5 degree angle brace. Leave room for your center support beam that will attach directly below the peak. It is important to use two braces, one on each side, for the peak of the roof, but since you are going to attach a center support right to the peak, you can use one brace on one side.
Measure across the width of the roof truss, starting from the inside of each side below the first joint and cut another 2 x 6 inch board to this length, for your first cross brace. The angle of cut will be 22.5 degrees, so it lays flush against the two sides.
Fasten this support to both sides of the truss using a flat metal brace.
Find the center of the cross brace, and measure from the top edge to the peak of the roof to get the length of your middle vertical support.
Cut your center vertical support to this length, but be careful to allow for the angled cut at top which allows it to lay flush against the peak roof. The angle should be 133 degrees, or 66.5 degrees times two.
Fasten this center support to the peak of the roof with another metal brace.