How to Sister Joist

Updated February 21, 2017

Joists are the framing boards that support the floors in your home. Floor joists can become weak over time, causing the floor to be bouncy or spongy feeling as you walk over it. It is important to reinforce weakened joists once the problem has been detected. The process of reinforcing floor joists is called "sistering" the joists. Reinforcement boards are fastened to the original joist to strengthen it. You can effectively reinforce your floor joists using 3/4-inch plywood cut down to the dimensions of the original joists.

Measure the width of the joists that need to be strengthened using a tape measure.

Adjust the table saw fence to cut strips that are 1/4-inch less than the measurement from Step 1. Lock the fence in place.

Lay the 3/4-inch plywood flat on the table of the saw with one 96-inch edge flush against the fence. Guide the plywood through the saw to cut it into strips 1/4-inch less than the joist width. Cut as many 96-inch-long strips as needed to reinforce the joists.

Insert the tip of the polyurethane construction adhesive into the build in cutter on the caulk gun. Clip the tip off at a 45-degree angle. Pivot the tube of construction adhesive into the caulk gun tip first. Apply construction adhesive to one side of each joist that needs to be strengthened in a zigzag pattern.

Press the plywood strips against the construction adhesive along each joist that needs reinforcement. Hammer 6d nails every 8 inches along both edges of each plywood strip.


Sister joists as soon as you notice a change in your floor to keep the floor in good condition.


Wear eye protection when operating power tools and hammering nails to prevent possible eye injury.

Things You'll Need

  • Tape measure
  • 3/4-inch plywood, 48-by-96-inch
  • Table saw
  • Polyurethane construction adhesive
  • Caulk gun
  • Hammer
  • Box of 6d nails
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About the Author

Jonah Morrissey has been writing for print and online publications since 2000. He began his career as a staff reporter/photographer for a weekly newspaper in upstate New York. Morrissey specializes in topics related to home-and-garden projects, green living and small business. He graduated from Saint Michael's College, earning a B.A. in political science with a minor in journalism and mass communications.