Roof 4: How to Install Roof Trusses

You are going to build a house or add an addition to a house. The house plans are drawn up, the house foundation is about to be started. It's time to order the roof trusses. Most roof truss factories take at least two or three weeks before they even start to make up your roof trusses, depending on their back log. Don't leave it until the walls are framed and then decide to order the roof trusses. You want that roof on and the roofing applied before rain or the elements do harm to the sub-floor and framing.

The house plans usually give the outside measurements of the walls on the Floor Plan. Some framers have these measurements coordinate with the outside of the sheathing, others prefer to go with the outside of the framing, that is the outside of the 2x4 or 2x6 walls. Personally, I go with the outside of the framing.

Determine how the house will be laid out and order the roof trusses accordingly. The main thing is that when the roof trusses are pre-ordered, make sure the framing is accurate and according to the measurements that will coincide with the size of the roof trusses when they come on site.

With delivery of the roof trusses, you should receive a package from the roof truss plant specifying the bracing required for the roof trusses as well as their safe handling and other pertinent information for each roof truss or set of roof trusses. For example, large roof trusses require 1x4 continuous bridging every 10 feet across their span including at the center of the webs. If the webs don't line up with each other, notice the explanation of using "L" or "T" braces nailed to each individual web. Hip girders that are doubled up will show the number and spacing of nails. Hip trusses will show 2x4 strapping required on top of them. Valley sets resting on open roof trusses will be shown. Usually each individual roof truss has a number marked on it and a matching drawing to show their engineering specs and bracing requirements.

Initially, you and a partner will either unload the roof trusses off the truck using the truck crane to place them across the top of the exterior and interior walls of the house, or set them on dunnage on the ground. The walls at this point have been properly secured, braced and laid out for the truss centers. If on top of the walls or laying on dunnage on the ground, place the roof trusses upside down at the opposite end of the building you intend to start erecting them from, if there are no interior walls. If there are interior walls, pile them up on the end of the building across the tops of the walls. Slide or carry the first roof truss, still upside down, to its position that is marked on the top of the double plate. It is handy to get a third person to help swing the roof truss into the upright position. Set the end of the bottom chord flush with the outside wall. When ordering the roof trusses you specified the particular span, whether the span included the framing or the sheathing. This point is where the roof truss bottom chord flushes up. The roof truss is toenailed into position using 3 - 3 1/4" common bright nails through the bottom chord into the double plate. See: Tables 6: Nail Table One nail on one side and two nails on the opposite side on each end. Space the nails out and not too close to the end of the chord.

Diagram of installing trusses on a roof showing plan and front views with top chord, continuous bridging, web and bottom chord.The first roof truss is braced temporarily, usually to a convenient inside wall, or with the use of supports nailed to the gable end wall and attached to the roof truss top and bottom chords, as shown in the drawing.

The second roof truss is then carried over to its designated position as marked on the plate and swung upright and toenailed securely to the double wall plate. A temporary 1x4 is nailed to each roof truss near the ridge to prevent them from falling over. Set the roof truss to the correct spacing when nailing to the 1x4. Continue this procedure. Before coming too close to the end of the building it is best to stand the remaining 2 or 3 roof trusses up, while you still have the room to swing them into their upright position, if hanging upside down. Just lean them against the upright, nailed in roof trusses and start from the opposite end sliding them into position carefully. Use another length of 1x4 to secure the end roof truss to the others at the ridge, tying the entire roof together.

Hurricane clips are installed after all the roof trusses are in place, by nailing them to the face of the double plate and into the sides of the roof truss bottom chord. The continuous bridging of 1x4 is nailed to the bottom chords and webs if specified. Brace the roof trusses accurately before the sheathing is applied. After the sheathing is nailed remove any temporary bracing of the walls and roof trusses.

In most jurisdictions the inspector will require an engineer to inspect the roof trusses for correct assembly. Your roof truss plant should have a name or two of engineers that will come to your site and do this inspection. Have the spec sheets ready to present to him. He will use these to verify that they are installed correctly, with adequate bracing and nailing.


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