Sistering Floor Joists

How to Remove Floor Bounce by Sistering Floor Joists

By Mark J. Donovan

Sistering floor joists entails fastening additional lumber material to the side of the floor joists.

Sistering floor joists is done to strengthen the floor joist and to reduce flexing and deflection in the floor.

Older homes that have floors that are springy or shake when you walk on them were frequently built with undersized and inadequate floor joists.

Sistering floor joists may also be necessary, if a heavy new load is expected, e.g. the installation of a Jacuzzi tub or a grand piano.

Sistering floor joists can be done in a number of ways. One way is to simply nail or screw the same type and size of lumber to the existing floor joist. Alternatively a smaller piece of lumber can be attached to the existing floor joist.

Also, additional lumber material can be fastened to both sides of the existing floor joist. Plywood, for example, can be attached to either side of the floor joist effectively sandwiching the original floor joist. Normally, however, standard framing lumber is used, such as 2×10’s or 2×12’s or whatever the original floor joist material is constructed out of. In some cases steel or engineered lumber is used.

Sistering floor joists is not always possible, or at least it is not very often simple to do.

Due to existing and completed lower level ceilings, walls, blocking, plumbing pipes and electrical wires sistering floor joists may not be possible or just very expensive to do.

One way around all of these obstructions is to simply sister in smaller lengths of floor joist material. 

Sistering Floor Joists

Though sistering in small sections of additional floor joist material may not be the strongest solution, it does however provide reasonable additional strength, particularly if the joints are strategically placed.

When sistering floor joists with smaller lengths of additional material always avoid joints towards the middle of the span.

It is better to leave the sistered floor joist materials short of the ends of the original floor joists than to splice or join sections of them near the center of the span.

Joining sections of floor joist sistering material near center spans significantly reduces the benefits of the entire floor joist sistering process.

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When sistering floor joists associated with a slightly sagging floor, it is best to first brace the existing floor up just a slight by using a combination of bracing lumber, posts and hydraulic jacks.

Once the floor deflection has been eliminated, you can then sister the floor joists.

After sistering the floor joists you can then remove the temporary bracing, and with any luck the sagging will have been eliminated.

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