Older homes often have some floor or ceiling joists that have been damaged during the life of the structure. Even some newer homes have floors that are under designed. Adding a second joist to the side of some or all of the existing joists is a way of reinforcing these structures. The second joist is often called a sister joist.
Evaluate your floor or ceiling system. If you have weak or damaged joists or are adding significant weight like a hot tub, adding joists beside the current joists is a good way to reinforce damaged areas or beef up the system to support an additional load.
Determine how you are going to slide the joist into position. Often electrical, plumbing, and heating utilities will hinder easily positioning the joists. You may need to relocate or temporarily remove some of these obstructions. Joists that are slightly shorter than the total span can sometimes be used by securely fastening them to the existing joist.
Purchase the required material after formulating a plan. Get a little extra to cover unforeseen issues. Remember, sister joists perform best when securely fastened to the existing joist. Construction adhesive and screws can be used to make a very effective connection.
Test position the sister joist. While the joist is in position mark the position for fasteners, and any areas where glue should not be applied.
Drill the sister joist for all fasteners. This will make it much easier to install the fasteners once the joist is in position. Use two rows of fasteners located about 1/4 the depth of the joist from each edge. Space the fasteners 12" - 16" on center.
Add construction adhesive to the joist. The combination of screws and construction adhesive forms a secure, squeak free connection.
Install the joist. You may need to force the two joists into alignment. A 2x4 can be driven under the joists to force alignment prior to driving home the screw fasteners.
Drive shims firmly under the sister joist at all bearing locations. Always use the shims in pairs such that a level surface results.
Drive home all of the screw fasteners.