Just like people, older houses can tend to sag here and there. If your house is showing its age in creaking, springy floors, it may be because one or more of the joists below it are starting to sag. Here’s something you can do to strengthen your floors and rejuvenate your poor old house.
A “sister joist” is a second joists installed alongside the original. Often, you’d use framing lumber of the same dimension (or if it’s an older house with full-dimension lumber, a slightly smaller dimension.) You might simply fasten smaller dimension lumber, such as a 2×4″, alongside the upper end of the old joist. (You can also use steel or engineered wood joists.)
If the floor is dipping when you walk, but hasn’t actually sagged, you could install the new joist tight against the floor, and that should do the trick. But if the floor is sagging, or if you intend to finish the ceiling below, and prefer a smooth plane, it’s best to jack up the joist or joists that have dropped, and install a post.
In the ideal situation, you’ll install a single joist across the span, resting either end on a sill plate or top plate. But if new walls or other obstacles have appeared over the years, you may need to use smaller sections. In fact, while you should try and do it if you can, it’s not absolutely necessary that the new joist be borne by a wall or the foundation. The key area you want to reinforce is the middle of the existing joist span – and therefore, if you are using shorter sections which need to butt one other, make the joint as far from the middle as possible.
Next time, we’ll install it.