Brace against bouncy floors
A floor should be somewhat resilient to make it comfortable to walk and stand on. If you have ever worked in a factory or a store with nonresilient concrete floors, you know how uncomfortable and tiring it can be. A bouncy floor is something totally different and can be equally as annoying as a rigid floor.
A good test to determine if the floor is just resilient or bouncy is to walk briskly across it. If items on tables or in cabinets vibrate and rattle, the floor is too bouncy.
There are quite a few techniques for making bouncy floors more stable. The best one depends upon the design of your specific home and the access beneath the bouncy floor. It is easiest to work in a basement or 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.
To stiffen 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. A strong built-up 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 they can be left in place. To make the built-up beam use construction adhesive and nail three pieces of 2-by-10 or 2-by-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, where 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 existing 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 around the house, two layers of it can be glued and nailed to the joist. Plywood is surprisingly strong and stable when assembled to the joist in this manner.
When space is limited, another option is to add perpendicular lumber-blocking or long, metal straps between the existing joists. The 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 the floor, the best option is 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.