Sistering Floor Joists
How to Remove Floor Bounce by Sistering Floor
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 2x10ís
or 2x12ís 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. 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
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.
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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