Here's How: Fix a bouncy floor
Q: I recently bought an older home, and the floors are quite bouncy. What are the techniques for making floors stiffer?
— Karen N.
A floor should be somewhat resilient to make it comfortable to walk and stand on. A bouncy floor is something totally different and can be equally as annoying as a non-resilient floor. A good test to determine if the floor is resilient or bouncy is to walk briskly across it. If items on tables or in cabinets vibrate and rattle, the floor is too bouncy.
A: There are quite a few techniques for making bouncy floors more stable. The best one depends on the design of your home and if you can get beneath the bouncy floor.
It is easiest to work in a deep crawl space so you have room to move. In houses built on a slab, the bouncy floors can only occur on the second floor because the first is supported by concrete.
Stiffening a bouncy floor built over a crawl space with a concrete floor is the simplest project because you do not have to worry about finished headroom, as you do with a basement.
A built-up beam placed over concrete footers creates the strongest floor. The beam is placed perpendicular under the floor joists. Adjustable pole jacks are placed on the footers, and the built-up beam rests on the top of the jacks. When the jacks are extended, they force the beam up against the floor joists. These jacks can be replaced with steel columns, or left in place.
To make the built-up beam, use construction adhesive to nail three pieces of 2-by-10 or 2by-12 lumber together. Joints in the beam should be located over a support column.
An alternative to making your own built-up beam is to use a steel I-beam or laminated veneer lumber. Another option when adequate headroom is an issue is to install a sister joist next to an existing floor joist. This joist is glued and nailed to the joist to double its stiffness.
For extra stiffness, a deeper joist can be used. This is more difficult to install, though, because the ends of the new joist must be notched to fit over the mudsill and existing support beams. If you have some old plywood, two layers of it can be glued and nailed to the joist.
Plywood is strong and stable when assembled to the joist in this manner.
Two other options when space is limited are adding either perpendicular lumber blocking between the existing joists or long metal straps. These straps are attached from the top of one end of the joist, down under it in the middle and back up to the other side.
If you have a bouncy second floor or do not have access to the joists below it, the best option is to add a new layer of plywood over the existing subflooring. Many older homes, such as yours, use diagonal planks for subflooring.
Adding the plywood stiffens it significantly.