The most rapid and economical way to frame the roof of a rectangular structure is to install prefabricated trusses. In addition to eliminating the need for heavy structural joists and rafters, trusses save you from having to cut rafters at complex angles and erecting a ridge beam.

Truss Parts.

Trusses typically consist of three chords—the pieces that form the triangular shape—and webs that fit between the chords to support the top chords and transfer stress to the bottom chord and to the exterior bearing walls. The corners of a truss are joined with metal gussets.

Purchasing Trusses.

When ordering trusses, specify the span between the exterior walls, the length of the overhang, and the type of end cut—plumb or square—you desire. Also specify the pitch of the roof. The standard pitch for trusses spaced at 24- inch intervals is 4 inches to 1 foot, but local codes in areas with heavy snowfall may require a greater pitch or more closely spaced trusses. Consult your local building code for this information.

Installing the Framing: Trusses rely on sheathing for stability. In areas with little snow, 1/4-inch plywood is acceptable. In others, 1/2- to 5/8 -inch may be required. Check the code. In either case, bolster the joints with plywood sheathing clips.

You will need at least three helpers to lift, roll, and secure the trusses. When lifting the trusses, carry them in a vertical position with one helper at each end. Trusses can be damaged easily if mishandled.

The final step involves weather-proofing and adding ventilation.

1. Marking the Truss Locations.

If required by your building code, add 2-by-4 fire stops, staggering their heights so you can face-nail them to the studs with 3 1/2-inch common nails.

Standing on a scaffold, measure 24 3/4 inches from a side wall and mark a line across the front wall’s top plate with a carpenter’s square. Mark a second line 1 1/2 inches away to outline the truss location.

Outline the remaining truss positions on 24-inch centers, using the first mark as a starting point (left).

Repeat the process to lay out truss locations on the back wall.

To assist in positioning the trusses, set up a layout line along the front wall. To do so:

Nail a 2-by-4 to the outside face of each side wall top plate with 35-inch common nails so the tops of the board and the top plate are flush. Attach the board so it projects beyond the truss overhang by a few inches.

Mark the overhang on each 2-by-4, then tie a string tautly between the marks (above).

3. Fastening the Nailers.

Secure boards to the side-wall top plates as nailing surfaces for the bottom chords of the end trusses.

Attach 2-by-4 nailers to the top plates with 3-inch nails; hold a 2-by-4 spacer on edge on the top plate while fastening to offset the front edges of the nailer and top plate by 1 1/2 inches (left).

Trim the nailers flush with the front and back walls.

Erecting the Scabs.

Prepare four 8-foot-long 2-by-4 braces— or “scabs”—to align and hold the end trusses even with the edge of the top plates. Since siding is applied to end trusses before they are raised, attach a 4-foot strip ot siding to each scab as a spacer (inset).

Attach two scabs to each side wall with 3-inch common nails (left). Position the scabs about one-third of the way in from the front and back walls; set their height so the top end of the siding spacer is flush with the top plate of the wall.

Install framing in the end trusses for a roof vent.

Set an end truss on the ground and lay a 4-by-8 sheet of plywood siding on it, aligning a corner of the sheet with the heel—or bottom corner—at one end of the truss.

Snap a chalk line across the siding in line with the top chord of the truss (above), then cut the siding along the line.

Nail the cut section of siding to the truss with galvanized box nails long enough to penetrate 1 inch into the truss.

Fasten siding to the rest of the truss the same way, then cut the vent opening with a saber saw.

With two helpers, carry an end truss upside down into the building. Then, standing on a scaffold, lift one end until the top chord rests on the top plate of one side wall (left).

With your helpers on another scaffold, pivot the other end of the truss onto the opposite side wall top plate.

Have a helper wedge a 2-by-4 into the peak of the truss and tilt it upright (above). With another helper on the scaffold, guide the truss so the bottom chord settles between the nailer and the scabs.

Align the front-wall end of the truss with the overhang line along the front wall.

On a ladder outside the building, nail the scabs to the top chord of the truss and drive a 3 1/2-inch galvanized common nail through the siding and the bottom chord of the truss into the nailer every 16 inches.

3. Plumbing the End Truss.

Support the end truss with a 16-footlong 2-by-4 brace, nailing one end of the brace to the vent opening framing and the other to a 2-by-4 stake driven into the ground about 8 feet from the wall.

Loosen the scabs and position a carpenter’s level against the top and bottom chords of the end truss. Meanwhile, have a helper reposition the brace on the stake so that the end truss is plumb (left).

4. Installing Truss Bracing.

Tilt the second truss into position over its outlines on the top plates and align an end with the overhang line.

Toenail the truss to the top plates with 3 1/4-inch galvanized common nails. Attach a multipurpose framing anchor to each end of the truss with the nails recommended by the manufacturer.

Outline the truss spacing on an 8-footlong 1-by-6 brace for each side of the ridge and fasten one end of the brace to the end truss with two 2 1/2-inch nails. Aligning the truss with the first outline, nail the brace to the top chord (right). Repeat on the other side of the ridge.

Position and brace the rest of the trusses except the last four. Install more braces as needed, fastening them to trusses already in place (above).

Install the remaining end truss.

Tilt the last three trusses up to the roof before positioning any of them. Then position and brace them one at a time.

Set up the scaffolds along the side walls. With a helper, snap chalk lines along the top chords of the trusses 4 and 8 feet from the overhang as guidelines for laying plywood sheathing.

Align the top edge of a 4-by-8 sheet of sheathing with the chalk line and center it over the fifth truss.

As a helper slips a plywood clip onto the top edge of the sheathing between each truss, secure the sheet with 2 1/2- inch galvanized common nails every 6 inches along the top chords (right).

After sheathing the bottom 4 feet of the roof, remove the braces fastened to the trusses and cover the next 4 feet, starting the row with a half-sheet in order to stagger the joints from the bottom row.

Before installing the last row of sheets, trim their top edges so the sheathing stops 1 inch short of the ridge. Cover the other side of the trusses the same way.

On each side of the ridge, support the bottom chords of the trusses with the bracing you removed in Step 6. Position a brace across the trusses and, holding the chord in line with its outline, attach the brace to the truss with two 2 1/2-inch nails (left).

If your area experiences high wind, diagonal bracing may be required; check the local building code.