How can I support a subfloor patch between joists?

Question

I'd opened up the subfloor in my second-floor bathroom to relocate the plumbing from the floor to the wall. Before I patched the plywood subfloor, I found out that these flat 2x4 joists are engineered floor trusses. The thing is, I want to ensure that the plywood has enough support on the right end, next to the wall, as there is no joist support underneath. I am considering using a :

• square brace: flat 2x4s arranged in a square shape, nailed to the joists on both ends, or
• Adding a 2x4 flat as blocking between the joists.

Would this be an issue with trusses, and if there is a better way to do this?

Answer 1

It's really awkward to try and install framing to the trusses underneath, but that's not warranted. As long as all the joints are tied together properly it'll be solid.

Just float 2x4 blocking, and extend it past the opening a foot or so. Fasten it with 2" screws to the remaining subfloor on the edge of the hole and beyond, staggering screws across the board's width. Reach around or under and screw through the wall plate with 3" screws if you're able.

Also float blocks roughly centered between joists on the perpendicular edges. They'd normally have a tongue-and-groove joint, so you need to replace that support. More 2x4 scrap set parallel to the edges would do well. Gaps between the blocking and the joist of less than about 6" aren't a problem.

Use heavy-duty construction adhesive at all contact points. It'll be as solid as the original floor.

It looks like your opening shape is a bit out of square and maybe wiggly. If that's the case, cut a patch to the largest rectangular dimensions of the hole, position it on the floor and trace it, then re-cut the floor as needed so you have a clean drop-in fit.