Removing a Subfloor Under a Wall
Tom Silva explains how to remove water-damaged subfloor from under load-bearing studs
My house took in 10 inches of river water in a recent flood, and the OSB subfloor had to be ripped out up to the walls. Should the OSB left under the studs also be replaced? If so, how is this done, given that the weight of the house rests on these areas?
— Norman Baxley, Columbia, S.C.
Yes, you should get rid of the remaining OSB, or oriented strand board, under the wall, one joist bay at a time, by working in your basement or crawl space. Here's how to do it.
Use a recip saw to slice off the OSB under the wall plate between the floor joists. Then cut pieces of sheathing the same thickness as the OSB to match the width and length of the plate's exposed underside. I strongly suggest that you cut those pieces out of a moisture-resistant floor sheathing, such as AdvanTech. This wax-impregnated OSB won't be affected if your house floods again.
Glue and screw these pieces to the underside of the plate. Now glue and screw 2x6 blocking, cut to fit between the joists, to the underside of the sheathing pieces. Toenail the ends of the blocking to the joists. You want the edges of the 2x6 to project past the plate's edges and form a narrow shelf for attaching your new subfloor. AdvanTech would be your best choice here, too. It's more expensive than regular OSB, but you won't have to rip it out if it gets wet.
Tackle each joist bay, from cutting to blocking, before moving to the next one. That way, the walls won't shift and settle as you work.
Before hanging the drywall, treat the exposed studs and joists with a hydrogen-peroxide mold cleaner, and then spray them with a borate-based mold inhibitor, such as Bora-Care with Mold-Care.
Tom Silva is the general contractor for Ask This Old House.