How to Alter Existing Roof Trusses for an Attic Room

Trusses must be adapted to allow adequate space for an attic.

While prefab trusses are a serious shortcut to a stronger roof, they don't allow the same uninterrupted interior space as their older cousins, the common rafters. In order to gain space, while still supporting the roof adequately, you will need to build a short support, or pony wall to run along each side of your attic, before removing the the webbing braces in your trusses.

1. Measure the width of your roof trusses, from inside to inside. Measure in from the lower edge of the roof to a point 1/4-the width of the truss on either side of the attic. Use a chalk line to draw a line along the floor, parallel to the lower edge of the roof, from one end of the attic to the other on both sides.

2. Use a plumb bob on a string to find the point on the upper cords, or rafters, of your trusses, directly above the line. Dangle the bob over the line at one end of the attic, with the string against the lower edge of the rafter until it stops moving. Adjust the bob until it is directly over the line and still. Mark the floor along the line in the position of the bob, and the position of the top of the string on the bottom of the rafter. Measure between the two points. This is the height of your pony wall. Repeat the plumbing procedure in each corner of the attic, at the ends of the two lines.

3. Use an angle finder, with one leg against each face of the roof and the hinge pressed against the inside of the roof peak to determine the angle of the roof. Read the angle from the indicator in the center of the finder. Divide this number by two to get the pitch of each face.

4. Cut enough 2-by-4s to make a pony wall stud every 16-inches along both lines on the floor. Cut the top ends of these studs to the roof pitch angle, so that the long end of the miter is 3-inches shorter than the pony wall height from step two.

5. Use the chalk line to snap a line along the faces of the rafters between the two marks you made for the top of the string, at each end of your line on both sides of the attic. Nail 2-by-4s along this line with their top edge on the line. Use two framing nails, from a framing gun in each rafter. Start against the inside of one gable. Butt the ends of the pieces up to the one before, cutting a piece to fit at the end. Repeat for the other side.

6. Lay out enough 2-by-4 to reach from gable to gable along the line on the floor on both sides of the attic. Stand them on edge. Nail a pony wall stud to these 2-by-4s every 16-inches from gable to gable on both sides of the attic. Nail through the long 2-by-4 into the end of the stud, two nails in every stud. Set the studs on edge with the long edge of the miter against the floor, and pointed to the center of the attic, away from the long 2-by-4.

7. Stand the pony wall up, so that the mitered top fits against the bottom face of the horizontal 2-by-4 you nailed to the rafters. Nail the mitered ends to the 2-by-4. Align the base plate you nailed to the bottoms of the studs with its inside edge on the chalk line on the floor and nail it to the floor every 12-inches.

8. Measure up from the floor and mark each center support of each joist at 84-inches. Use a rafter square to mark them straight horizontally. Cut each support off at this mark with a circular saw. Use a hammer to knock the bottom end loose. Cut any diagonal braces in the truss with a reciprocating saw. Cut a 2-by-4 to fit horizontally, from rafter to rafter on each truss. Cut the ends to the same angle you used for the pony wall studs. Nail them to the rafters and angle nail the center support to the top edge of the horizontal brace. This completes the reframing process. Your attic is now ready for finishing.