How to Sister a Roof Beam

Select dimensional lumber that’s the same size as the existing rafters. If the roof is framed with 2-by-4s, use additional 2-by-4s.

Measure an existing beam 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 beams sits on the top wall plate.

Cut a new beam with a circular saw to match the measurements, and keep this beam as a template after fitting it beside the existing beam and ensuring it fits.

Cut additional beams for every beam you want to sister.

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 beam where you will be installing the new beam. Make a zigzag pattern with the bead of adhesive to cover more surface area.

Install the sister beam by positioning it next to the existing beam and inserting 16-penny nails through the side of the sister beam, into the existing beam with a framing nailer.

Insert two nails, vertically, 2 inches apart, and every 1 foot along the sister board if you’re doubling 2-by-4s. If you’re doubling 2-by-6s, use three nails. Use four nails if you’re doubling 2-by-8s.

Repeat the process for every beam you want to sister.