Ready-made Trusses to Support the Roof

The fastest and cheapest way to frame a roof for a simple rectangular structure is to use pre-manufactured trusses. They eliminate the need for heavy structural joists and rafters, for the tedious cutting of rafters at complex angles, and for the dangerous job of erecting a ridge beam.

The modern truss, based on building techniques that go back to 2500 B.C., resembles the ones widely used for the wooden bridges of the 19th Century. It is composed of three principal timbers, called chords: two top chords that form the slope of the roof and a bottom chord that spans the distance between the exterior bearing walls. The three corners of a truss are joined with metal or plywood gussets. Additional members, known as webs, are fitted between the top and bottom chords to support the top chord and transfer stress to the bottom chord and to the exterior bearing walls. This design feature helps prevent the walls from bowing outward.

Because the average truss for a 20- to 30-ft. span is made of 2-by-4s, it can be lifted into place with little effort, and a whole roof can be completed in hours—a job that might take days with traditional framing methods. And because these standardized prefabricated trusses can span as much as 40 ft. without center supports, they provide unencumbered interior space by eliminating the need for interior bearing walls.

When ordering trusses, you need to specify the span between the exterior walls of the building, the length of the overhang and the type of end cut— plumb or square—that you desire. You must also specify the pitch of the roof. The standard pitch for trusses spaced at 24” intervals is 4” to 1 ft. but local codes in areas with heavy snowfall may require a greater pitch or more closely spaced trusses. Consult your local building inspector or truss manufacturer for this information. If the pitch of the roof is not regulated by code, trusses that match the pitch and overhang on your house generally give a pleasing appearance to your new structure.

Order the two end trusses with webs spaced 16” apart so that sheathing can be nailed to these trusses before they are put up (Step 6). And have an opening framed in the end trusses so that you can install a ventilator.

Trusses come in a variety of styles. If you plan to build a catwalk above the bottom chords for storage, order a truss that has no center web, rather than the type shown . If you are planning to store heavy objects on such a platform or hang objects from the trusses, order heavy-duty trusses.

To erect trusses, you will need two adjustable scaffolds 6 ft. high, available at most rental agencies. You will also need two framing anchors for each truss and a large supply of galvanized nails. Trusses rely on sheathing rather than the traditional ridgepole for stability. Buy enough 3 tongue-and- groove weatherproof plywood to cover the roof—use 1 to 5 plywood in areas of heavy snowfall—allowing 10 per cent extra for waste. Buy 1-by-6 lumber—two 8-foot-long sections for every three trusses—which you will use to hold the trusses in place until the sheathing is installed and then will reuse to space the bottom chords of the trusses.

You will need at least three helpers to lift and roll the trusses into place. When lifting the trusses, always carry them in a vertical position. With one worker at each end and another in the middle to support the bottom chord.

Preparing the Walls and the End Trusses

118a 1. Marking positions for trusses. Standing on a scaffold, use a steel tape to measure 24¾” from the side wall and make a mark square across the front wall’s top plate. Put an X and a second line on the side-wall side, to indicate the exact position of the truss.

Lay out the rest of the trusses on 24” centers, using the first mark as a starting point. Then lay out the other wall, starting your layout from the same side wall. If the trusses are properly laid out, every second truss should fall directly over a stud in a wall like the one shown here.

119a 2. Fastening the framing anchors. Place the framing anchors on the top plate so that their raised edges align with the marks on the top plate. Nail the framing anchors to the top plate with eight-penny galvanized nails.

119b 3. Laying out the overhang. Overhang is not essential but improves appearance and weather protection. Nail 2-by-4s long enough to provide the desired overhang, to the outside faces of the top plates of the side walls, and flush with the tops of the top plates. Mark the length of the overhang from the front wall on each 2-by-4, then tie a string line tautly between the marks.

120 4. Fastening the nailer. Nail 8-to 10-ft. lengths of 2-by-4 over the top plates of the side walls and flush with the outward edge of the front wall. Space them 1½” from the out side edge, using a 2-by-4 block on edge to maintain the 1½” spacing while nailing.

Prepare the scabs—bracing—for the end trusses by cutting 4-ft. strips of sheathing and nailing them to four 8-ft. studs.

5. Erecting the scabs. Nail two scabs (Step 4) to each side wall, about a third of the way in from the front and back walls. Position each scab so that the top edge of the sheathing is flush with the top plate of the wall and the rest of the scab sticks up above the top plate far enough to provide support for the end truss.

121a 6. Sheathing the end trusses. Lay the end trusses on the ground and cover them with 4-by-8 sheets of plywood, aligning the short edge of the plywood with the heel and bottom chord of the truss. Snap a chalk line along the top chord of the truss and the heel and then cut the plywood along the chalk line. Nail the sheathing to the truss. Similarly finish sheathing the truss. Cut a ventilator opening with a saber saw.

Raising the Roof

121b 1. Hoisting the end trusses. With two helpers, carry an end truss upside down into the building and from a scaffold lift one end until the top chord rests on the wall plate. Then, with a helper standing on another scaffold, set the other end of the truss atop the opposite wall plate.

2. Rolling a truss. With two helpers on scaffolds holding the truss in position, wedge a 2-by-4 into the peak of the truss and rotate the truss into an upright position.

When the peak is upright, walk the truss over to the side wall and set it between the 2-by-4 nailer and the scabs. Align the overhang with the string line on the front wall and nail the scabs to the top chords of the truss.

Roll the second truss up, align it with the string line and secure it to the top plates by nailing it to the framing anchors on the wall plates.

4. Bracing the top chords. Nail a 1-by-6 to the end truss, its end flush with the outside of the top chord. Align the second truss at a distance equal to the truss spacing, mark the brace and nail it to the truss. Nail another 1-by-6 brace on the other side of the ridge.

122 3. Securing the end truss. Standing on a ladder outside the building, drive a 16-penny nail through the sheathing and into the truss and nailer.

123a 5. Bracing the remaining trusses. Roll up, anchor and brace all the other trusses except the last four, bracing them as you go. Attach the remaining end truss. Roll the last three trusses up, then walk them into place and secure them.

Completing the Job on Top

123b 1. Plumbing the end truss. Nail a 16-ft. brace made of 2-by-4s between the ventilator framing of the end truss and a 2-by-4 stake firmly anchored outside the building. Loosen the scabs and adjust the brace while a helper plumbs the end truss with a 4-ft. mason’s level. When the truss is plumb, nail the brace to the stake.

Set up the scaffolds along the side walls and , with a helper, snap chalk lines along the upper chords of the trusses 4 and 8 ft. from the overhang to lay out the plywood sheathing.

124 2. Sheathing the trusses. Tack the corners of a 4-by-8 piece of plywood to the end truss, its edge flush with the outside edge. While a helper re checks the end truss for plumb, tack the corners of the plywood on the center line of the fifth truss. Nail the plywood down with sixpenny galvanized nails spaced every 6”.

After sheathing the bottom 4 ft. of the roof, remove the 1-by-6 braces and sheathe the next 4 ft., starting the first sheet of plywood 4 ft. from the end truss so that joints will overlap.

3. Stabilizing the trusses. Connect the bottom chords of all trusses by nailing to them the 8-ft. lengths of 1-by-6 bracing that you removed from the roof. Align the chords with the marks on the bracing (Step 4), so that the bottom chords are spaced exactly as far apart as the top chords. In high-wind areas, nail additional 1-by-6 diagonal braces to the webs between the top chord of each end truss and the bottom chord of the fifth truss from each side wall.