How to Sister a Roof Beam


Things You’ll Need

  • Dimensional lumber

  • Tape measure

  • Circular saw

  • Construction adhesive

  • Caulking gun

  • 16-penny nails

  • Framing nailer or hammer

New construction home framing against blue sky New roof construction. Image Credit: leekris/iStock/Getty Images

A typical stick-framed roof consists of rafters that sit on the exterior wall plates and connect at the peak of the roof to a center ridge beam. The sloped rafters usually spaced at 16-inch intervals, which is sufficient to support standard roof decking and shingles. If you want to upgrade to heavier slate or tile shingles or you decide to add dormers to the roof, additional support is necessary. Sistering the beams, or rafters, is one way of adding structural support to the roof. Always consult a qualified building professional when planning any structural alterations of a roof frame.

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Step 1

Select dimensional lumber that's the same size as the existing rafters. If the roof is framed with two-by-eights, use additional two-by-eights.

Step 2

Measure an existing rafter and transfer the measurements to a new board. All measurements must be the same, including the angle where the sloped beam meets the ridge beam and the notch where the rafter sits on the top wall plate.

Step 3

Cut a new sister rafter with a circular saw to match an existing rafter. Test the sister's fit and make any necessary adjustments, then label and keep this rafter as a template for marking the remaining sister rafters.

Step 4

Cut additional sister rafters for every rafter you want to reinforce.

Step 5

Fit a tube of construction adhesive in a caulking gun and apply a 1/2-inch bead of adhesive to the side of the existing rafter where you will be installing the sister rafter. Make a zigzag pattern with the bead of adhesive to cover more surface area.

Step 6

Install the sister rafter by positioning it next to the existing rafter and driving 16-penny nails through the side of the sister and into the existing rafter, using a framing nailer or hammer. Drive four nails in a vertical pattern, about 2 inches apart, every 1 foot along the sister board if you're using two-by-eight lumber.

Step 7

Repeat the process for every rafter you want to sister.


If you’re cutting out a rafter to install a skylight, sister the two remaining side rafters before building a header to bridge the gap between the rafters. You can also sister a broken or split rafter in a similar way. Sandwich the problem rafter with two sisters that extend 4 feet past the break or split in each direction.


Don’t cut or sister roof trusses, which come pre-engineered from a truss manufacturing company. Trusses are designed to carry specific roof loads, and their weight-bearing integrity depends upon smaller rafters in triangular shapes. Consult an engineer to discuss the possibility of altering roof trusses.

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