How to repair a house rafter

Written by rebecca mecomber
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How to repair a house rafter
The rafters bear the weight of the roof (roof image by Christopher Meder from Fotolia.com)

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.

Skill level:
Challenging

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Things you need

  • Power saw
  • Power drill
  • Measuring tape
  • Two-by-six wood board for the scab, 8 feet long
  • 2 two-by-six wood boards for support braces
  • Two-by-six wood board for strongback, 10 feet long
  • Two-by-four wood board for strongback, 10 feet long
  • Hammer
  • 16d nails
  • 8d nails
  • Helper

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Instructions

    Install the Scab

  1. 1

    Ask your helper to assist you in hoisting the 8-foot two-by-six scab board over the broken rafter.

  2. 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. 3

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

    Install the Support Braces

  1. 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. 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. 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. 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. 5

    Nail in the support brace with 16d nails.

  6. 6

    Repeat Steps 4 and 5 for the second board.

    Install the Strongback

  1. 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. 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. 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. 4

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

Tips and warnings

  • 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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