The term roof truss refers to large gabled roofs that are installed as pre-assembled trussed rafters. Trusses come in many configurations but basically consist of top chords (rafters), bottom chords (ceiling joists), and diagonal bracing or webbing–all made from either 2-by-4 or 2-by-6 lumber.
The structural members are joined together with toothed steel plate connectors that are pressed into the wood mechanically. Connections can also be made with plywood gusset plates that are glued and nailed at the joint. Roof trusses are usually designed by professional engineers and fabricated by wholesale manufacturers at an assembly plant. From the span, roof pitch and desired overhang supplied by the builder, a truss can be assembled using a pre-engineered design or custom built to meet specific structural and aesthetic requirements. Roof trusses use less lumber to span the same distance and carry the same load as conventional rafters; the diagonal bracing installed at each pair of rafters allows for lighter structural members. Two disadvantages of using roof trusses are they require the use of flat ceilings and they make most attic space unusable. If trusses are preferred to a traditional roof frame, structural members can be assembled into roof trusses as shown in the following steps.
Step 1—Gather the Structural Members
Lay a pair of rafters in position on the ground. You will need a full length joist to span the entire width of the building. 2 x 6 framing lumber will be adequate for most applications. These will function as the top chords and bottom chord of the truss. Divide the bottom chord into thirds and mark it on the board. Measure from the 1/3 point on each side of the bottom chord diagonally up to the midpoint of each rafter. Then measure from the 1/3 point up to the peak connection. Cut a total of (4) 2 x 4 diagonal braces that will join the bottom chord to the top chords. Angle cut the ends of the braces so that you have a flush connection at each joint.
Step 2—Make the Connections
If you have toothed plate connectors, hammer the plates onto each connection so that it grips the end of each member of the joint. You can also make the connections with plywood gusset plates. Using ¼-inch plywood, cut a plywood plate to fit each connection so that it covers at least a foot of each member. Using an adhesive applied with a sealant gun, glue the gusset plates in place. Thoroughly nail each plate to the structural members using #6 nails. Install plates at each connection on both sides of the truss assembly.
Step 3—Install the Truss
Lighter roof trusses can be lifted onto the wall plates and installed by two men; larger trusses usually require heavy lifting equipment. The truss assembly can be toe-nailed onto the walls’ top plates the same as a single rafter (be sure to nail through the structural member, not just the plywood plate). Lateral bracing is provided by purlin equally spaced along each top chord and spanning across each installed truss.