Last edited 14 May 2021

Sistering floor joists

A common problem in older buildings is that floors sag or feel ‘springy’ (that is they 'bounce' when walked on). This can be because the joists are undersized (particularly where older buildings are converted for modern uses where they may be subject to higher loads than was originally intended), or they have been weakened by decay or by having holes or notches cut through them incorrectly. The structural repair of damaged floor joists is known as ‘sistering’.

Floor joists can become damaged due to contact with water which may lead to rot setting in or infestation from termites, woodworm, and so on. These underlying problems should be rectified first before sistering, or the damage will recur.

The typical choice for sistering material is framing timer or engineered timber products such as plywood.

The first step to take is to remove any obstructions, such as electrical cables, pipes, and so on. If this is too difficult the new joists can be notched, although this may mean that are not as strong. The new joists are set alongside the existing ones. Joist material should be chosen that matches the height of what is already installed, as well as being, as closely as possible, the same span.

If necessary, a hydraulic jack and post can be used to jack up the joists until they are level. It is sometimes recommended that jacks are only raised by around 1/8 inch per day to avoid cracking in the walls and floors.

Adhesive should be applied along the length of a joist to strengthen the bond between the existing and its sister, as well as to prevent squeaking. Quick clamps are used to hold the sistered joist in place, and nails driven through the sister and into the existing joist. Shifting and settling can work to loosen and separate the boards over time, therefore, the nails should be used with lag or carriage bolts to lock them together.

The quick clamps can then be released. This process should then be repeated for each joist that needs strengthening or straightening.

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