How to Repair a Truss Sag

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Sagging trusses are often caused by structural failure due to excess weight. However, old trusses that have been wet or have experienced rot may fail and sag as well. Carpenters have been repairing trusses for hundreds of years in order to avoid costly replacement of trusses. Luckily, these tried-n-true techniques are available to the average do-it-yourselfer. You can expect to repair sagging trusses in two to four hours, depending on the size and scope of the project.

Things You'll Need

  • 2x4 or 2x6 lumber
  • 5-inch wood screws
  • Screw jack
  • 4-foot 2x8 lumber
  • 4 8-inch rubber hose clamps

Cut two 2-by-4s or 2-by-6s to serve as truss patches; one will be placed on either of the flat sides of the truss. Your truss sizes may vary. However, most trusses are either 4 or 6 inches. Cut the truss patches so that they extend 24 inches beyond the center of each side of the truss sag.

Place a 4-foot 2x8 across the ceiling joists just beneath the sagging truss. Temporarily screw a 3-foot 2x4 to the bottom of the sagging truss so that the 4-inch side of the 2x4 is flat against the bottom of the truss and in the center of the sag. Place a screw jack, found at any hardware store, between the 2x8 on the floor of your attic and the sagging truss where the 2x4 is attached. Crank the jack until the truss begins to straighten. Increase the pressure slowly to avoid damage to the sagging truss.

Place one precut patch brace on one side of the truss to gauge whether it is straight. Once the top of the sagging truss aligns with the top of the patch, stop jacking the truss. Place the truss patch over the sagging truss so that it is centered over the previously sagging area. Place another truss patch on the opposite side of the truss, aligning it with the truss and the other truss patch. Insert 5-inch wood screws through the truss patches and truss to secure them together, spacing the screws 4 inches apart.

Release the jack pressure from the sagging truss, and remove the 2-by-4 from the bottom of the truss. Place 4 8-inch rubber hose clamps around the truss and truss patches, spacing them evenly apart. Tighten the clamps so that the metal slightly indents the wood on the patches.

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References

  • "The Home Carpenter & Woodworker's Repair Manual"; William Perkins Spence; 2006
  • "The Repair and Maintenance of Houses"; Ian Alexander Melville, Ian A. Melville, Ian A. Gordon; 1997
  • "Ask the Family Handyman"; Reader's Digest Association, Reader's Digest; 1999
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