Jadnashua
Retired Defense Industry Engineer xxx
How close is the drain pipe to the bottom of the subflooring?
If you have enough room, I'd glue and screw a cleat of plywood as wide as I could fit to the bottom, then glue and screw the replacement ply onto it. You want to make it as wide as possible so you get the load spread, and a liquid wood glue would be better than something you caulk out as you can get a full spread...construction adhesive works great on say the top of a joist, but doesn't spread like a liquid glue does when you try to get full coverage over a wider surface.
Ideally, when patching, you want the patch to span two joist bays (three lines of fasteners) so that it's not hanging off of the fasteners at the end. Putting a cleat underneath helps make it stiffer.
If you have enough room, I'd glue and screw a cleat of plywood as wide as I could fit to the bottom, then glue and screw the replacement ply onto it. You want to make it as wide as possible so you get the load spread, and a liquid wood glue would be better than something you caulk out as you can get a full spread...construction adhesive works great on say the top of a joist, but doesn't spread like a liquid glue does when you try to get full coverage over a wider surface.
Ideally, when patching, you want the patch to span two joist bays (three lines of fasteners) so that it's not hanging off of the fasteners at the end. Putting a cleat underneath helps make it stiffer.