Repair techniques for floor joists

Written by jessica kolifrath
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Floor joists are the large beams that support the floors in your home. When they are spaced too far apart to handle the floor weight, your floors will begin to squeak and sag as they age. Rot can also develop in floor joists, creating a dangerous situation that could result in a partial floor collapse. Using the right repair techniques for your floor joists will ensure a safe and sturdy living surface.

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Sistering is one of the most common floor joist repair techniques because it can be used on beams that are badly damaged from rot or cracking. This technique involves attaching two similarly sized pieces of lumber on either side of the damaged area, says Hammer Zone. The new lumber must overlap onto a sturdy, undamaged section of the beam by a certain amount or it won't provide any structural support. Wood adhesives and screws should both be used during joist sistering to create the strongest bond possible.


If your floor joists are not properly sized for the floor, they will bend slightly as weight is applied to the floor. This constant bending will lead to cracks in the bottom of the joist. Installing lumber along the bottom of each joist will help stiffen it, according to This Old House, preventing it from flexing and bending. Unlike other repair techniques, this method is not helpful when rot or serious damage has weakened the beam, but is more useful as a preventive repair measure.

Epoxy Wood

Small spots of rot or termite damage can be patched with an appropriate epoxy-based wood aggregate, says Appalachian Log Home Care Products, but this is not an appropriate repair technique for badly damaged joists. Epoxy products will not prevent rot from spreading or kill termites, so treatments to eliminate the problem must be applied first. Applying an epoxy/wood aggregate can remove the need for sistering or complete joist replacement if the termite damage or rot is caught early.

Full Replacement

When a floor joist is completely rotten, cracked or broken in half, it must be fully replaced. This is a serious undertaking that requires floor jacks that can support the weight of the entire floor. Although it might make sense to try and replace the original joist with a piece of lumber of the same size, the United States Environmental Protection Agency says that this is often not possible with older homes. If the floor joist became damaged because it was not large enough to support the floor, a larger replacement beam should be used to prevent the damage from reoccurring.

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