How to use lag bolts to sister a joist

Updated February 21, 2017

Floor and ceiling joists are the mainstay in home construction. Throughout the life of your home, joists may become damaged, requiring you to make repairs. Reinforcing these weakened joists is the key to minimising the effect of the damage. You can do this by installing a lumber sister joist, which works much like a splint. It is a board that lies alongside the damaged piece to help carry the load and maintain your frames structural integrity.

Cut a piece of treated timber the same size as the damaged joist. Joists are typically made of 5 x 15 cm (2 x 6 inch) or 5 x 20 cm (2 x 8 inch). Cut the sister joist long enough to extend past the damaged area on each side at least 60 cm (24 inches). Use a circular saw or mitre saw to make the cut.

Apply construction adhesive to the face of the sister joist that you will press against the damaged joist. Position the sister joist alongside the damaged frame member. Clamp the sister joist alongside the damaged joist with C clamps. Tighten the clamps, and tap the sister joist into position so that the bottom edges of the two pieces are even down their entire length.

Drill pilot holes through the sister joist. Start with a 1.8 cm (3/4 inch) spade bit, drill 1.2 cm (1/2 inch) holes in two lines, one 5 cm (2 inches) below the top of the joist and one 5 cm (2 inches) above the bottom of the joist. Space the holes every 25 cm (10 inches). Drill 6 mm (1/4 inch) holes into the centre of every 1.8 cm (3/4 inch) hole. Make the holes 5 cm (2 inches) deep. Wrap a piece of tape around the bit 6.2 cm (2 1/2 inches) from the tip to mark the depth.

Drive a 7.8 mm by 5.6 cm (5/16 by 2 1/4 inch) lag bolt into each 6 mm (1/4 inch) hole with a socket spanner. Tighten the bolts down until the head just barely digs in at the bottom of the 1.2 cm (1/2 inch) pilot hole. Allow the construction adhesive to harden overnight before pulling the C clamps off of the sister joist so that it hangs on its own.

Tap shims between the top of the new sister joist and the other framing members at every intersection to help prevent squeaky floorboards. Use cedar shims available from your local hardware store.

Things You'll Need

  • Lumber for sister joist
  • Circular or mitre saw
  • Drill
  • 1.2 cm (1/2 inch) spade bit
  • 6 mm (1/4 inch) wood bit
  • C clamps
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About the Author

Mark Morris started writing professionally in 1995. He has published a novel and stage plays with SEEDS studio. Morris specializes in many topics and has 15 years of professional carpentry experience. He is a voice, acting and film teacher. He also teaches stage craft and lectures on playwriting for Oklahoma Christian University.