Stiffening a floor

Q: The floors in my new house seem to have been built to minimum code requirements. The floors on the first floor are built with 2x8 joists 16 in. o. c. spanning 12 ft., but the floors are springy because of deflection. I wanted to sister the joists but can’t figure out how to get the lumber in place. One end of the joist is in a pocket formed by the subfloor and sill, and the other end is toenailed into a supporting beam on top of a 2x ledger. Do you have any suggestions?

A: Former associate editor Roe A. Osborn replies: Sistering the floor joists in your house is the right way to go. But because you want just to stiffen your floors and don’t need to upgrade them to meet code, the sisters don’t have to run the entire length of the joist.

There are a couple of ways you can proceed, depending on what you use for a sistering material. The first is to use another 2x8 alongside the first. In this case, make the 2x8 about 2 in. shorter than the existing joist. The sister can then slide into the pocket and back onto the 2x ledger, leaving you with about 2 in. of the sister resting on a typical 2x6 sill. Make sure any crown is facing up; I suggest trimming a little off the top of the first foot or so of the sill end to let the sister slide into the pocket easily.

Once the sister is placed in the joist bay, squirt construction adhesive on the face of the existing joist as well as a bead along the bottom of the subfloor where the sister is going. Slide the sister over to the joist, and clamp the two to hold them together while you fasten them. Next, screw or nail the two sisters together every 2 ft., and then remove the clamps.

If you’re intent on having the sister run the entire length of the joist, you may want to use 1/2-in. or 3/4-in. plywood ripped to the width of the joist instead. In this scenario, two pieces of plywood are sandwiched together with the seams staggered. Start with an 8-ft. length slid all the way into the sill pocket, and glue and nail it to the joist. Cut a shorter piece to the exact length for the balance, and install that shorter piece on the ledger end. Reverse the ends for the second layer, making sure that both layers are glued and nailed well and that you glue the seams between the lengths of plywood.

I’d start by sistering every other joist. You may find that this amount will add enough stiffness to your floor. If not, go back and sister the rest of the joists.