How to Sister a Roof Rafter
Sister refers to attaching an additional framing member alongside an existing member. With roofs, this is done to improve the load value of rafters when installing roof-mount air conditioning equipment or cutting openings for skylights. In some cases, you may sister rafters to repair a sagging roof. Regardless of the purpose, always consult the local building department or a structural engineer to determine the size of lumber needed. In attics, it’s best to place strips of plywood across the ceiling joist as a work platform. For open ceilings, set scaffolding to sister a roof rafter.
Wear work gloves. Remove any perpendicular 2-inch blocks at either side of the rafter with a nail bar and framing hammer. Tap the forks at the end of the nail bar with the hammer to fit them under the heads of nails, then pry out the nails.
Measure the overall distance of the existing rafter from the ridge board to the outermost point where the rafter attaches to the wall. Note the measurement.
Determine the angle of the plumb cut where the upper end of the existing rafter meets the ridge board with an adjustable builder's square. Mark this angle on one end of a length of 2-inch framing lumber that will be the sister.
Wear safety glasses. Cut the plumb cut on the sister with a circular saw. Similar to a rafter, the longest point of the cut determines the upper edge of the sister.
Determine the angle of the level cut at the lower end of the sister with the builder's square. The level cut is at the lower edge of the sister, and is the portion that rests atop the wall.
Refer to your overall measurement from the ridge board to the outermost point of the wall. Hook the end of the measuring tape at the point of the plumb cut and draw the tape along the face of the sister. Mark the sister to length. Mark the angle of the level cut at this point on the sister. Cut the level cut with the saw.
Position the sister flat alongside the rafter instead of upright like the rafter, with the lower end atop the wall. Swing the upper end away from the rafter until the tip of the plumb cut bypasses the lower edge of the ridge board. Rotate the sister so it's upright and push it against the side of the rafter. Hit the outer edge of the sister at several places with a 1-pound sledgehammer to fit it snug against the rafter.
Attach the upper end of the sister at the ridge board with three 16-penny toenails. Attach the main portion of the sister to the rafter with three nails at 12-inch intervals. The upper and lower nails are 1 1/2 inch from the corresponding edge of the sister. The third nail is at the center of the sister. Attach the lower end to the rafter with a pair of nails through the face of the sister.
- When framing openings for skylights, install the perpendicular head-out lumber before you sister the rafters at each side of the opening.
- In some cases, it's necessary to remove a section of the roofing and sheathing to manipulate the sister into an attic.
William Machin began work in construction at the age of 15, while still in high school. In 35 years, he's gained expertise in all phases of residential construction, retrofit and remodeling. His hobbies include horses, motorcycles, road racing and sport fishing. He studied architecture at Taft Junior College.