The term sister stud refers to a secondary stud that is installed alongside an existing stud. It is usually used to reinforce a stud that has been damaged or is bowed in a manner that compromises its load-bearing capacity. Sistering can also be done to reinforce floor joists or rafters. For example, sister joists are sometimes installed to increase the load-bearing capacity of a floor so it will hold more weight, as when installing a large whirlpool tub.
Load-Bearing vs. Non-Load-Bearing Studs
The method of sistering varies depending on whether the wall is load-bearing or non-load-bearing. Load-bearing walls are those that support the weight of the roof or floor above. Non-load-bearing walls are usually interior partition walls that don't carry weight.
With non-load-bearing partition walls, a sister can be simply a length of framing lumber screwed, bolted, or nailed alongside the damaged portion of the existing stud. It serves as a "splint" that simply reinforces the bad spot in the stud. It doesn't need to be a full-length stud. The sister stud (or partial stud) should be anchored to the existing stud at least every 8 to 10 inches.
Sistering with a partial stud is not allowed by most building codes for load-bearing walls. In this case, a bad stud must be sistered by a new full stud that extends from the wall's bottom plate, or sole plate, to its top plate. The sister stud must be anchored to both plates as well as to the damaged stud. In a load-bearing wall, the sister effectively replaces the bad stud by carrying its entire load, from ceiling to floor.
Tips for Sistering
- In non-load-bearing walls, the new piece of material does not have to be as long as the deteriorated section, but it should extend well beyond the deteriorated section in both directions.
- You only need to sister one side of the stud. It's not necessary to sister both sides.
- Be sure the damaged stud has enough solid material to which you can attach the new stud. If the old stud has little structural value or is damaged by rot or insects, it is best to remove it completely and replace it with one or more new studs.
- Make certain that this sistering job has not been made necessary by some larger problem. Serious foundation settling, water incursion, dry rot, and insect damage may be the underlying cause, and such problems need to be corrected before the wall structure is repaired.
- If the wall has horizontal fire-blocking within the stud cavity, the blocking must be removed to make room for a full-length sister stud. The blocking must be cut to fit and reinstalled after the sister is in place.