How to Place a Sister Support on Floor Joists

Updated February 21, 2017

Larger contemporary furnishings and appliances, such as entertainment centres and commercial-grade refrigerator-freezers, can add an increased bearing load to a home's floor frame. A common method of offsetting this increased load in a existing floor is to install a second or even third parallel joist, known as "sister" joists. This basic procedure will strengthen the floor for the changed load and can be adapted to any standard floor assembly.

Define the location of the floor joist(s) to be reinforced with a sister joist. Note the approximate location from above by recording the linear dimensions to points detectable beneath the floor, such as exterior walls, floor vents, the fireplace hearth or any other construction detail discernible from the crawl space below.

Access the crawl space and locate the relevant joist(s) using the demarcations collected in step 1.

Measure the portion of the joist length being sistered. A sister joist essentially doubles the existing joist's load rating by doubling the material support and should extend to bear a minimum of 3 inches on the nearest girder or foundation support at each end, as well as span the full width of the floor area that will hold the increased weight.

Cut a length of joist lumber to the dimension determined in step 3.

Insert the sister joist into position beside the existing joist being reinforced. Remove any joist bracing or blocking that might be in the way for reinstallation once the sister joist is installed.

Align the sister joist's top and bottom edges with the existing joist's edges and secure it in place using 16d nails or flathead deck screws staggered approximately 12 inches apart over the full length of the joist.

Replace any joist bracing or blocks removed to install the sister joist.


The completion of this operation requires manoeuvring lumber and tools within the limited area of the crawl space. Enlisting the assistance of a helper in this project can help this relatively simple procedure go much smoother. When a sister joist is being installed to level a sagging floor joist, the existing joist being corrected should be jacked up to remove the sag, then securely supported during the installation of the sister joist. Additional shear strength can be added to the joist connection by applying a continuous bead of construction adhesive to the adjoining faces of the joists before securing the sister joist in place.

Things You'll Need

  • Tape measure
  • Joist lumber (match existing grade and size)
  • Circular saw
  • Hammer
  • 16d box or sinker nails
  • Power screwdriver (optional)
  • 2-inch flathead deck screws (optional)
  • Caulking gun and construction adhesive
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About the Author

Paul Massey has been writing since 2009, drawing on a 35-year career in the construction industry. His experience includes 15 years as a general building contractor specializing in architectural design, custom homes, commercial development and historic renovations.