The term sister stud refers to a secondary stud that is installed alongside an existing stud. It is usually done as a means of reinforcing 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; non-load-bearing walls are usually interior partition walls that don't carry roof 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. Make sure the sister portion is anchored to the existing studs every 8 to 10 inches—the more the better.
This method is not allowed by most building codes for load-bearing walls. Here, a bad stud must be sistered by a new full stud that runs the full distance between sole plate to top plate, and it must be anchored to both, as well as to the adjoining damaged stud In this case, the sister effectively replaces the bad stud by carrying its entire load, from ceiling to floor. If the wall has horizontal fire-blocking within the stud cavity, this blocking must be removed in order to run the sister stud full length from floor to ceiling.
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 deteriorated stud has enough solid material to which you can attach the new material. If the stud is cracked or rotted through its entire length, make sure the sister runs the full length and is anchored to both the top plate and sole plate in the wall cavity.
- 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.