How to Repair a Floor Joist Using Plywood

A residential floor structure consists of parallel horizontal wood members, called “joists,” installed at equal spacing to support the floor platform and walls above. Over time, a floor joist can develop a crack or fracture from a concentrated weight or a natural weak point, which can lead to sagging in the floor or other structural problems. An effective and economical repair method is to laminate the joist with a plywood “gusset” to restore the joist’s support strength and halt any further damage.

Tape measure
Chalkline
3/4-inch CDX-grade plywood
Circular saw
Caulking gun
Tube of construction adhesive
Electric drill with screw bit
1 1/4-inch galvanized or plated deck screws

Instructions

- 1
  Access the floor’s crawlspace and locate the damaged floor joist.
- 2
  Measure the length of plywood required to span the damaged area. The plywood gusset should ideally extend beyond the damage by a minimum of 2 to 3 feet on each side.
- 3
  Mark a piece of 3/4-inch plywood to the width of the floor joist by the length determined in the previous step using a tape measure. Connect the measured marks using a chalkline to define the gusset’s cutlines.
- 4
  Cut out the plywood gusset using a circular saw.
- 5
  Spread a continuous bead of construction adhesive around the perimeter edges and zigzagged over the face surface of the plywood on one side.
- 6
  Position the glued side of the gusset against the floor joist and press against the outer surface to spread the adhesive.
- 7
  Secure the gusset in place with deck screws through the plywood into the joist, spaced approximately 6 to 8 inches apart around the edges and across the face, as needed.

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