Floor Joist Repair: How to Sister Floor Joists

What You'll Need
Wood similar in length to the pre-existing joist
Construction glue
Drill and bit

If you live in an older home, you may encounter the need for floor joist repair. Older buildings often develop floor problems due to constant foot traffice. Newer homes which were rapidly constructed may also sometimes experience joist weakening or damage. Rather than reinstall your joists, you can create second, or sister, joists alongside the first.

Step 1 - Evaluate Your Situation

Before you begin floor joist repair, you should examine and evaluate your current situation. Consider how weakened or damaged the joists are, and also whether a weight above them could be causing the joists to fail. If you have installed a large bathtub, for example, this may be causing the strain.

However, if you conclude that the culprit is probably age or damaged wood, you can go ahead and support your joists by installing a second pair. While evaluating, consider how you will install the other wood. Electric cables, heating ducts and plumbing can all get in the way of your floor joist repair.

Step 2 - Build the Sister Joist

When building the sister joist, remember that it will work better when you screw it directly into the existing post. If cables and other utilities may get in the way, you can install a slightly shorter joist to help support the existing one. Cut the lumber to the right length for your installation and drill a hole in the top of the joist. You'll use this hole to screw it into the pole. Because you must also screw the bottom of the joist into position, you should make a hole there too.

Step 3 - Install the Sister Joist

You will need two rows of screws at each end of the joist, around 1/4 inch from the top and bottom edges. Before you screw these into place, add a layer of construction glue to the sister joist. Press it into the pole of the pre-existing wood support. Tighten the screws into place with your screwdriver.

Attach the bottom of the joist to the wall. (Use a block of wood or a pre-existing bar above the floor.) Glue the new piece into place and screw it down, as you did for the top. Wipe away any construction glue which squeezes out before you leave your floor joist repair job to dry.