How to Repair a House Rafter

A house roof bears an incredible amount of weight: tons of rafter and sheathing wood (or roof decking), tar paper, flashing and heavy roofing shingles. Add a few inches of snow or ice and some roof rafters may crack or fail under the weight. You will need to jack up a cracked or broken rafter, and "sister" or "scab" it with a section of new wood.

Install the Scab

The rafters bear the weight of the roof
  1. Ask your helper to assist you in hoisting the 8-foot two-by-six scab board over the broken rafter.

  2. Move the scab up or down the rafter until you have 4 feet of scab board on each side of the broken rafter.

  3. Nail the scab board into the broken rafter board with 16d nails.

Install the Support Braces

  1. Go to one end of the scab you just installed. Measure with a measuring tape from the crown of the rafter (up where it meets the roof sheathing) to the very bottom of the ceiling joist below it at a 45-degree angle. Make a mark where you measured.

  2. Go to the other end of the scab you just installed. Measure from the crown of the rafter to the very bottom of the ceiling joist below it at a 45-degree angle. Make a mark where you measured.

  3. Cut the first two-by-six board with the first measurements. Cut one end of the board at a 45-degree angle. Repeat for the second board and the second measurement.

  4. Place the first support brace board into position at the mark you made in Step 1. Line up the brace so that the 45-degree angle cut is pointing downward, at the ceiling joist. Situate the top of the board up toward the crown of the rafter.

  5. Nail in the support brace with 16d nails.

  6. Repeat Steps 4 and 5 for the second board.

Install the Strongback

  1. Take the 10-foot two-by-six board and lay it perpendicular to the ceiling joists, between the two support braces but at the base of the shorter support base. Turn the two-by-six so that it is upright; rest the narrower, 2-inch side of the board on the ceiling joists.

  2. Toenail the 10-foot two-by-six into each ceiling joist with 8d nails. Hammer the nails into the two-by-six at an angle.

  3. Lay the 10-foot two-by-four next to the 10-foot two-by-six, with the 4-inch side of the first board facing down on the ceiling joists.

  4. Hammer the 10-foot two-by-four into the ceiling joists with 8d nails.


  • If you are unsure about any aspect of the project, consult with a structural engineer or your local codes department.
  • Do not pound the nails in too quickly, as this may cause the wood boards to split. You may opt to screw holes at the nail points; be sure to screw holes slightly narrower than the diameter of the nails you will be using.
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