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?

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1 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.

enter image description here

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.

  • 3
    For the blocking on the right, OP might be able to reach under and screw it from underneath to the bottom plate. Might be a bit easier than reaching around all the pipes.
    – Huesmann
    Commented Mar 26 at 14:39
  • @Isherwood Wait, so you're saying I don't need to attach the blocking to the joists at all? So then I just need to screw in the subfloor to the blocking underneath... correct? I sure didn't know that was possible. Would it be a strong enough support? Also, I see that you've added a 2x4 on the right parallel to the joist. So does that mean I'd have to lay the 2x4 blocking on its wider side just like the joist(side by side)? Again, is this screwed into the joist at all or just the subfloor?
    – bsong
    Commented Mar 27 at 8:02
  • By "float" I meant not attached to framing. All you really need to do is rebuild the sheet so it spans properly between the original joists. In the case of the long one, running it past does lend some support on the ends as well. No, you don't need to lay the lumber flat, but it's nice to have a wider screw/glue area, which may make things stiffer.
    – isherwood
    Commented Mar 27 at 12:51
  • Got it, thanks for confirming. I’m doing this for the first time so I’d rather ask stupid questions than do something stupid. Clear about the floating blocks now.
    – bsong
    Commented Mar 27 at 17:55
  • As you mentioned.. the cut out is uneven on the top edge.. Also the tongue in groove on the plywood is damaged so I’d probably just have to lay it down and screw it on the floats( leaving less than a 1/4” gap between the new board and the old). That should be okay right? Should I fill that up with a floor patch or leave it as is for any future expansion. PS: a Heavy Vanity will go on top of this board so hoping this is sufficient support.
    – bsong
    Commented Mar 27 at 18:04

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