A sister stud is 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 undamaged 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.
Non-Load-Bearing vs. 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 structure weight from above.
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 type of splint that reinforces the bad spot in the stud.
With non-load bearing studs, the sister stud 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. It picks up on some of the remaining structural support left by the damaged stud.
It also allows you to avoid the somewhat messy job of pulling out old studs. Old studs can be attached to the siding by many nails or screws, plus they are nailed into place at the top and bottom.
Tips for Reinforcing a Wall Stud by Sistering
Cut Sister Board Long Enough
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. Aim for about 1 to 2 feet of extension both above and below the damaged area.
Sister One Side Only
You only need to sister one side of the stud. It's not necessary to sister both sides. While you can use a second two-by-four if you wish, any additional structural support afforded by a second two-by-four is negligible.
Support Stud When Nailing
Nailing onto a damaged two-by-four stud can be difficult because the stud is not a solid nailing surface. When you hammer, the stud will vibrate or collapse.
Holding a heavy weight such as the head of a sledgehammer or an anvil on the other side will help control movement as you hammer in the sister stud.
Assess Stud for Replacement
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.
Install the Fire Blocking Again
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 replacement fire block must be cut to fit and reinstalled after the sister is in place. Due to the changed width of the stud bay, the next fire block will be 1-1/2 inches shorter than the previous fire block. Instead of cutting the previous block, use a new two-by-four to cut the replacement fire block.
Exterior wall studs that have been sistered have insulating surrounding them. You'll need to pry off the staples holding the current insulation to the wall studs, carefully remove the insulation, and lay it aside.
If the insulation is in good shape, it can be reused. To accommodate the sister stud, cut away 1-1/2 inches from the side of the insulation. Leave the paper facing material intact.
Look For Larger Problems
While it does sometimes happen, it's rare that damaged wall studs are an isolated problem. 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 causes, and such problems need to be corrected before the wall structure is repaired.