How to Sturdy Up Springy Wood Flooring
Install Sister Joists or Built-Up Beam and Lally Column on a footing
Apr 5, 2009
A certain degree of springy floor deflection is a desirable thing, unless of course the finish floor is ceramic tile or natural stone. Minor give absorbs the impact of walking. But too much bounce feels unnatural and may even cause things to fall off shelving.
What's the best method of shoring up the subfloor? There are a number of methods and the “best” way depends on the situation. This article discusses 3 popular methods.
Use Sister Joists to Stiffen up the Floor
A sister joist is simply an additional new joist of the same dimensions as the existing one. It is attached to the side of the one already in place. It will effectively almost double the horizontal integrity of the span. This is a good method to use when it's important to conserve headroom, as in a finished basement with a suspended acoustical ceiling.
Taller sister joists may be used for additional strength, but they will have to be notched on the ends and elsewhere to fit any beams and the mudsills. If the original timber is sagging visibly, use a floor jack to straighten it while making the repair.
The floor joists can be mated using doubled up 16d nails or bolts with washers. To prevent squeaking in the future, it's a good idea to smear the mating surfaces with a construction adhesive such as Liquid Nails.
Built-Up Beams and Lally Columns
Built-up beams (or steel beams if desired) and lally columns are a good choice if headroom is not an issue. Even some finished and remodeled basements that have been turned into home theaters or game rooms may appropriate for this option.
In this case, there are more things to consider. First, the lally column needs to be set on a concrete support footing. In many cases the existing concrete slab on the floor is not thick enough to support the built-up beam and footing so the floor must be cut so the footing can be poured.
In many cases, the footing only needs to be 1' deep and 2' square, but every case is different. Always check the local building code.
Add Another Layer of Plywood to the Subfloor
Another layer of 3/4” plywood is sometimes all that's needed to stiffen up a springy floor. This is the obvious solution when remodeling a room or two and the plan is to go back with laminate flooring, hardwood, or another material upgrade.
As when repairing a rotten subfloor, the new plywood should be glued down with a construction adhesive and screwed down. Decking screws are a good choice in this situation. Lay the sheets perpendicular to the floor joists for added strength and to distribute the span load.
These are just a few ways to beef up home flooring. Most bouncy situations can be corrected with one or more of these solutions.
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