How to Build Roof Truss Rafters

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Roof truss rafters, commonly called simply trusses, have replaced traditional rafter-and-tie framing as the standard for building roofs for houses and other structures. For do-it-yourselfers, simple trusses are easier to build and install than conventional rafter systems, as you can construct the trusses on the ground then simply install them atop the building's walls. While trusses for houses and large outbuildings must be precisely engineered and fabricated by professionals, you can build basic trusses for small sheds and outbuildings using 2-by-4 or 2-by-6 lumber and plywood, provided the completed roof structure complies with the local building code.

Things You'll Need

  • Tape measure
  • Drawing supplies or chalk line
  • 2-by-4 or 2-by-6 lumber
  • Miter saw
  • 1/2-inch ACX or CDX plywood
  • Circular saw
  • Wood glue
  • 1-1/2-inch screws
  • Drill/driver

Design the Trusses

  • Measure the width of building from the outside of one long wall to the outside of the opposite long wall. This dimension is the length of the bottom chord, which is the horizontal cross member of the truss.

  • Determine the pitch, or slope, of the roof. If you plan to use standard asphalt or cedar shingles, it's a good idea to make the slope somewhere between 3-in-12 (about 14 degrees) and 12-in-12 (45 degrees). The steeper the roof slope, the taller the roof will be.

  • Determine the amount of overhang you'd like to have at the eave, along the long walls of the building. The overhang and roof slope determine the length of the top chords -- the angled "rafter" members of the truss.

  • Create a scaled drawing of the truss on paper, or create a full-size template on your garage floor or driveway, using a chalk line. Draw horizontal lines to represent the bottom chord. At the center of the bottom chord, make a perpendicular line going up to the roof peak. Then, draw angled lines from the peak down to the bottom cord, using the desired roof slope; these represent the top chords. Finally, draw a vertical member, called the king post, extending along the centerline between the bottom chord to the roof peak, at the underside of the top chords. Use the drawing to confirm the dimensions and angles for the truss, and make any necessary adjustments.

Cut the Truss Parts

  • Cut the bottom chord to length, using a miter saw or circular saw, mitering the ends at the same angle as the roof slope. The length between the points of the miters should equal the width of the building.

  • Cut the top chords so they meet at matching angle cuts at the roof peak, called plumb cuts, and extend past the bottom edge of the bottom chord by the desired overhang, or eave, dimension. You can make a plumb cut or a square cut at the bottom ends of the top chords.

  • Dry-fit the three chords together so the assembly is perfectly symmetrical and to confirm accurate cuts. Measure between the bottom cord and peak to find the cutting length for the king post. Cut the king post with a square cut at the bottom, which sits on top of the bottom chord, and a symmetrical point at the top to fit into the angle formed by the top chords.

  • Cut gussets from 1/2-inch plywood to cover the mating boards at each joint of the truss assembly, using a circular saw. The gusset at the peak is a triangle, with its top sides matching the roof slope. The gussets for joining the top and bottom chords are squares with the top outside corners cut off to match the roof slope. The gusset for the bottom of the king post is a square. Cut two gussets for each joint, for a total of eight pieces for each truss.

Assemble the Trusses

  • Fit the chords and king post together as before, and measure to confirm symmetry and equal overhangs at the eave ends. Glue the gussets to the lumber pieces, then secure them with 1-1/2-inch screws.

  • Flip the truss over and install the gussets with glue and screws on the other side of each joint. Let the glue dry overnight before working with the truss. Test-fit the truss on the building to make sure everything fits well before you build the remaining trusses.

  • Use the assembled truss as a template to build the remaining trusses. The two end trusses will need additional vertical posts to provide backing for siding at the gable ends of the building. Add one or more of these posts spaced evenly at both sides of the king post, cutting the top ends with a single miter that follows the top chord (roof slope). Install these posts with screws or nails driven at an angle into the top and bottom chords.

Tips & Warnings

  • Use the truss calculator here to assure you have the right dimensions.
  • Consider mixing a few prefabricated trusses with site built ones so that you can ensure you are doing things correctly.
  • Remember not to mix up your end truss and your working truss.
  • Most accidents on a work site happen when trusses are being put up. Make sure that you follow building codes and safety codes strictly!

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References

  • “Graphic Guide to Frame Construction”; Rob Thallon; 2000
  • “Building a Shed”; Joseph Truini; 2002
  • “Ultimate Guide to Barns, Sheds and Outbuildings”; John D. Wagner and Clayton DeKorne; 2005
  • “Better Homes and Gardens Sheds and Gazebos”; Meredith Books; 2005
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