How to Make Structural Repairs By Sistering Floor Joists

Repair sagging or broken floor joists by "sistering" on a new joist.

by Brett Martin

From The Family Handyman

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Step-by-Step Instructions and Pictures: How to Make Structural Repairs By Sistering Floor Joists

Q Our house was built in 1849. Some of the floor joists are sagging, cracked and twisted. Should I be concerned about the structural integrity of the joists, and what are some fixes?

Rob Spaulding, Cortland, NY

A Since your house is so old, first call in a structural engineer to inspect the floor and recommend strengthening measures that’ll take out the sags and bring the framing up to code (“Engineers, Structural” in the phone book). The inspection ($300 to $500) will tell if you have structural problems, such as undersized joists, that will require professional repairs. However, you can usually remedy sagging problems yourself by “sistering” new joists alongside the existing ones (this also works for stiffening bouncy floors).

First remove electrical cables, pipes and other obstructions. If this is difficult (it often is!), consult your local building inspector about notching the new joists. Notches will compromise the strength of the new joists, but some notches are acceptable, depending on their location.

Then set new joists (same height as existing ones) alongside the old. Tack a beam under the sagging joists. Nailing two 2x4s together will work to span about three joists, unless the sag is under a weight-bearing wall. Set a hydraulic jack and post under the beam, and jack up the joists about 1/8 in. a day until they’re level. Jacking them up too fast may cause cracks in the walls and floors overhead.