How to Repair a Joist With a Sister Board

By Mark Morris

Joists are the floor and ceiling equivalent to studs. They are wooden framing members evenly spaced across the floor. They are typically made from 2-by-8 or 2-by-10 inch lumber. When the joists become damaged, the floor sags or may even fail. One way of repairing them is a with a sister joist, which is a splint placed alongside the joist. The sister board must be cut from the same material as the original joist to provide adequate support. You must reposition the joist before making the repair.

List of Items Needed

  • Hammer
  • Pry bar
  • Twist jacks
  • Level
  • Joist lumber
  • Tape measure
  • Miter saw
  • C clamps
  • Oscillating tool
  • Cutting accessory
  • Drill
  • 1/4-inch bit
  • Lag bolts
  • Socket wrench

    Cutting the Sister

  1. Go underneath the floor if you have a raised floor foundation or remove enough flooring, using a hammer and pry bar, to access the damaged joist.

  2. Set one twist jack underneath each end of the damaged section of the joist. Place a level on the bottom of the joist. Turn the jacks to raise them until they lift the damaged joist up. Raise the joist until it is level and the bubble is centered in the level's indicator.

  3. Measure the damaged section of the joist with a tape measure and cut a piece 2 feet longer than the damage from a piece of lumber the same width and thickness as the original joist using a miter saw.

  4. Mark out any necessary holes or curves for pipes or conduit that may pass through the sister joist as well as the original joist. Cut the holes or curves using an oscillating tool and cutting accessory.

    Installing the Sister

  1. Drill through the original joist every 4 inches with a 1/4-inch diameter drill bit and into the sister joist 1/2 inch. Use a cordless drill. Tap a 1/4-inch thick lag bolt 2 1/2 inches long into each hole using a hammer until the tip pierces into the face of the sister joist.

  2. Tighten the bolts with a socket wrench until the heads are sunken into the face of the joist and the top of the bolt is even with the surface of the original joist. Allow the jacks to release.

  3. Remove the jacks from under the joist and replace any flooring you removed to access the flooring frame. Nail the flooring back in place with a hammer and nails.

Tips and Warnings

  • A sister that spans the entire length of the floor may be necessary for large-scale damage.
  • Long-term damage, such as sagging that affects multiple joists, may require professional help to fix safely.

© Demand Media 2011