Floor joists may become the victims of a number of misfortunes that require them to be repaired. Most commonly, floor joists are subjected to water or moisture and begin to rot. They can also be subjected to dry rot, a different kind of damage that doesn't require wetness. Joists can also be damaged by fire, earthquakes and insects.
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Scabbing is an unattractively named but effective technique that can fix minor problems that are affecting floor joists. Scabbing involves taking a piece of wood and permanently affixing it on the side of the joist over the problem area. Scabs can be attached on one or both sides of the joist, the latter option being preferable for greater strength and durability. Scabs can be attached with glue and screws to create a very strong bond. This method may be suitable when a joist has a small rotted area but has not yet deteriorated to the point where it must be replaced.
Sistering is a more extensive version of scabbing. Instead of attaching an extra piece to the side of one part of the joist, in this method you add another entire joist beside the joist that is having problems. The effect is the same as replacing the original joist, but you are spared the hassle of removing the original. Sistering can be used when the floor above the joist is sagging because the house was built with inadequate specs, or when the joist has deteriorated due to insect damage or rot.
Jack posts are heavy-duty metal pipes with a threaded platform at the top that can be raised by turning it with a wrench. You can use a jack post to raise and stabilize the center of a sagging joist by placing the jack post underneath the joist's center and rotating the upper platform so that it is forced up underneath the joist. Jack posts are designed to be left permanently in place, although a permanent post can also be put in their place so the jack posts can be used again.
A joist can be replaced when its problems are severe. Replacement is preferable to sistering when the joist is in a location that doesn't offer enough room for a sistered joist, or when leaving the original joist would not solve the problem, for example, if the joist is infested with carpenter ants. The most difficult part of replacing a joist is often getting the ends of the new joist into the space where the original joist was. Be sure to fully support the surrounding floor with jack posts before removing the damaged joist.