Repair a Damaged Rafter

Rafters fail slowly, then all at once. A crack in a rafter beam might sit dormant for years before a heavy snow or windstorm turns structural compromise into active failure. Most rafter damage shows up as splits along the grain, sags in the roofline, or visible cracks near knots or notches someone cut decades ago for wiring or plumbing. The good news is that most rafter damage can be repaired in place without tearing into the roof deck above. The technique is called sistering — bonding a new board alongside the old one to restore full strength. Done right, a sistered rafter is stronger than the original. Done poorly, it shifts the load without solving the problem. This repair demands precision in layout, solid fastening, and an honest assessment of whether the damage is localized or symptomatic of a bigger structural issue. If multiple rafters show damage in the same area, or if you see rot instead of just cracking, call a structural engineer before you swing a hammer.

1. Identify the full extent of damage. Examine the full length of the damaged rafter from ridge to wall plate. Mark the extent of visible cracking, sagging, or rot with chalk. Measure the rafter dimensions — most residential rafters are 2x6, 2x8, or 2x10. Check the span and note any obstacles like collar ties, electrical runs, or vent stacks that will complicate installation of the sister board.
2. Shore up the broken beam. Set a vertical 2x4 support post under the sagging or damaged section, placed on a solid base like a ceiling joist or load-bearing wall below. Use wooden shims to snug the post tight, lifting the rafter slightly to restore its original alignment. This holds the rafter stable while you work and ensures the sister board bonds to a properly aligned member.
3. Mill the matching sister board. Cut a new board of the same dimension as the existing rafter, running from the ridge board down to the top plate of the wall. If attic access makes full-length installation impossible, cut the sister to extend at least three feet beyond the damage on both sides. Use straight, dry lumber with no large knots in the repair zone.
4. Bond the sister in place. Run a continuous bead of construction adhesive down the face of the damaged rafter. Maneuver the sister board into place alongside the existing rafter, aligning the edges carefully. The adhesive does much of the structural work, so coat generously but avoid so much that it squeezes out and creates cleanup problems.
5. Lock it down with screws. Drive 3-inch structural screws or timber screws in a staggered pattern every 12 inches along the length of the sister, alternating from high edge to low edge. Screws should penetrate both boards fully. Avoid over-driving, which can split the wood, but seat each screw firmly to draw the boards tight together.
6. Brace with plywood gussets. Cut two pieces of half-inch plywood, each about 24 inches long and as wide as the rafter depth. Coat one side of each gusset with construction adhesive and fasten them to opposite faces of the sistered rafter assembly, centered over the damaged area. Use 2-inch screws every 6 inches around the perimeter of each gusset.
7. Verify load transfer works. Pull the temporary post and shims. Check the roofline from outside if possible, or sight along the rafter from inside the attic. The repair should hold the rafter in plane with its neighbors. If the rafter sags again after support removal, the sister isn't doing its job — re-install the post and add more fasteners or a longer sister.
8. Document your structural fix. Write the date and a brief description of the repair on the sister board with a permanent marker. Take a photo showing the sistered assembly and its location in the attic. This documentation helps future inspectors or owners understand what was done and why.