Roof trusses are built for strength and ease of installation. These prefabricated units replace a conventional stick-framed roof, with individual rafters nailed to a central roof ridge beam. Each truss is a triangular frame with a pair of rafters, or upper cords, attached to a bottom horizontal joist, or lower cord. To support the upper cords, vertical and diagonal bracing, known as webbing, is installed in the centre of the truss. This minimises, or even eliminates, space for attic rooms. To make space for attic rooms, you can alter prefabricated trusses without sacrificing strength or safety.
Lay your truss flat on a pair of sawhorses. Use a straight edge to mark along the inside edges of the rafters (upper cords) and the joists (lower cords) everywhere that they are connected to interior webbing (braces). Cut along these lines with a circular saw to remove the braces, leaving an open triangular frame.
Measure the length of the lower cord and divide this measurement by four. Measure in from each corner and mark the lower cord at a distance equal to the divided measurement. This will mark the inside edge of the new braces, leaving one half the width of the truss open in the centre for attic space.
Align a drywall square with the bottom cord on one of the marks you made and mark the upper cord directly above, where the square hits it. Repeat this for the other mark. Lay a 5 x 10 cm (2 x 4 inch) board on top of the truss so that its inside edge is aligned with the marks on the lower and upper cords. Mark the intersection between the bottom edge of the upper cord and upper edge of the bottom cord onto the 5 x 10 cm (2 x 4 inch) board. Mark one 5 x 10 cm (2 x 4 inch) board for each side. Cut the 5 x 10 cm (2 x 4 inch) boards, using a circular saw, on the intersecting lines you marked.
Set the truss on a flat surface, such as your garage floor. Place the 5 x 10 cm (2 x 4 inch) braces you cut so that their inside edges are aligned with the marks on the upper and lower cords. Trace the joints at the top and bottom of each brace onto a 30 cm (12 inch) square of 1.3 cm (1/2 inch) plywood. Cut these out using a jigsaw. Make eight total, two each for the top and bottom of both braces (enough to brace both the front and back sides of the truss).
Apply wood glue to the back of one plywood brace (or gusset) and apply it to the face of the truss. Staple the gusset to the truss with a pneumatic staple gun and 3.8 cm (1 1/2 inch) staples. Repeat these steps for the top and bottom of each brace. Turn the truss over and apply gussets to the back face of each joint for additional stiffness. This completes the alteration.