It is important to repair those joists as they’ve lost some ability to handle a proper load. In those cases, sistering one or several joists after jacking up the old joists to the desired height will fix the sag permanently.Īnd finally, if you can do a visual inspection of your floor joists and find that some are rotten in places, or have soft spots due to moisture damage. They may also be spaced too far apart for the span they support. Over time, joists can sag due to being undersized or rotten. The other most common reason for sistering a joist is to fix a sagging floor. Level the floor and reduce sagging joists.Increase floor structural integrity, strengthening weak joists.The reasons for sistering a joist include: A sistered joist in any of these instances will correct a wobbly floor. In most homes, a wobbly floor is caused by an oversized joist span, joists spaced too far apart, or joists that are failing. Sistering is used to fix wobbly floors, sagging floors, or rotten joists. Still, it can also be a fairly laborious process as you may have to remove a finished ceiling, wiring, plumbing, HVAC, or other utilities that pass through the problem joist. Sistering floor joists is an extremely effective method for fixing a broken floor joist of any kind. Plywood, OSB, and even metal flitch plates can also be used as a “sister” to an existing wood joist.

There are many methods for sistering a joist, and not all involve dimensional lumber – but using the same sized lumber is the most common method. Pairing the old joist with a “new” joist is called “sistering” a joist. This additional piece of lumber is then mechanically fastened with nails or bolts to the sagging joist. One of the most common ways to fix a sagging floor joist is to pair it with another piece of dimensional lumber of the same length and width. How Long Does a Sister Joist Have to Be?.Why Do You Need to Sister a Floor Joist?.

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