How to cut roof trusses for an attic room

Dachausbau image by R.R.Hundt from

When planning a truss roof for an attic room, start by creating a slightly steeper pitch for the trusses. This allows the maximum space inside the truss and distributes the weight more to the walls with a more vertical line to remove stress from the upper cords, or rafters. Rearranging the web support for your trusses can also help to leave more open space in the centre of the truss for as much wall height as far from the peak as possible.

Cut your bottom cord, or joist piece, from 2-by-4 lumbar according to the length on the blueprint. Typically, the lower cord runs from wall to wall, extending 10 to 24 inches on the outside of each wall. Cut the ends to the angle indicated on the blueprint. Cut a 45-degree angle at each end for a 12:12 roof (a roof that rises 12 inches vertically for every 12 inches horizontally) to create a really spacious attic. This is the steepest commonly built roof.

Adjust the mitre saw for the cut. Release the lock on the rotating table by pulling up on the trigger or twisting the knob counterclockwise, depending on the saw. Turn the table until the arrow on the indicator is aligned with the 45 on the mitre gauge and lock the table in place. Keep the board against the fence and flat on the table. Use one hand to hold the board in place, kept well clear of the blade, and use the other hand to start the saw. Pull the blade down and make the cut in one fluid motion. Release the trigger and allow the blade to stop before releasing it to spring back up to its original position.

Cut your upper cords with a 45-degree mitre at one end and fit the cut ends together to form a peak. Position the uncut ends of the board over the lower cord, so that the ends overlap it evenly on each end. Mark the upper cords where they intersect the ends of the lower cord. Cut a 45-degree mitre so that the lower ends of the upper cords fit smoothly on the top of the lower cord with the ends flush.

Measure in from each end one-quarter of the width of the lower cord and mark the lower cord. Measure the distance between the top of the lower cord and the bottom edge of the upper cord where the square intersects it with a drywall square, with the top of the T fitted against the bottom edge of the lower cord and the tongue of the square extending straight up from the mark. Cut the 2-by-4 piece of lumbar to fit between the top of the lower cord and the bottom of the upper cord aligned with the inside edge of the 2-by-4 piece of lumbar on the mark. Cut a 45-degree mitre on the top end of each piece. Cut two of these pieces, one for each side. These are your web supports.

Position the web supports with the bottom end on top of the lower cord, aligned with the mark and the upper mitred end against the bottom edge of the upper cord. You should now have a triangle formed with the lower cord as the bottom horizontal and the two upper cords forming the peak, supported by two vertical 2-by-4 pieces of lumbar running up from the lower cord to the upper cord.

Cut four 12-inch squares of 1/2-inch-thick plywood. Fit them under the joints at the peak of the truss between the upper cords, at the lower-left corner between the upper and lower cords and at the top and bottom of the left 2-by-4 web brace. Trace these joints onto the plywood and cut them out to use as gussets, or support plates for the truss. Use this set as templates to make two lower corner gussets for each side, two upper peak gussets and two each for the top and bottom of the 2-by-4 web brace on each side.

Glue and nail these gussets to both faces of the truss to support the joints. Use this truss as a template and cut the same pieces for every truss in your roof, one at each end and every 24 inches in between.

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