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I had some water damage. We wanted to remodel anyways, so I ripped everything out, including the subfloor. Now it looks like this:

bathroom photo

Here's the same angle, color-coded and labelled for reference:

angle 1

Reverse angle:

angle 2

I understand that I need to attach the new subfloor along the joists and the perimeter of the room. But I don't see how I can install blocking beneath the wall behind the toilet for two reasons:

  1. Between the first and second joists is a drain pipe (pink) and a vent (purple) that don't allow room for a 2x4 to attach.
  2. Between the second and third joists is an exhaust pipe (blue, running vertical to the wall stud) that doesn't allow room for a 2x4 to attach.

My attempt to illustrate with new blocking in red:

angle 3

I considered cutting the new blocking to get around those but that seems kinda pointless since then it could easily break.

How should I go about this?

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  • I don't think it's supercritical to bridge that purple pipe completely. In that 1st joist bay, I'd just add some blocking to the sides of the joists to project the "top" of the joist as far out as possible. In the 2nd bay I'd put the 2x4 (or bigger) with the longer axis vertically. And there's no reason you can't double or triple up on blocking.
    – Huesmann
    Jan 17, 2023 at 17:33

2 Answers 2

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For the joist bay with the toilet flange; Behind the flange I'd put blocking across anywhere it will fit unimpeded, closer to the toilet flange is likely even better anyway. I'd get another pieces of blocking maybe even a piece of 3/4 ply and screw it into both the 2x6 bottom plate of the wall and screw down from your new subfloor into this piece. The initial piece will hold most of the weight and the blocking on flat will keep the back end from deflecting but doesn't have to go the full width of the joist bay. If you use ply then you'd want to make a couple ledgers for side support on the two joists.

For the joist bay with the vent and the drain pipe; I'd get a length of angle iron. I can't tell the height from the top of the joists to the top of the drain pipe but if it is 2" or 1.5" then I'd get a 2"x2" angle iron and bolt it to the joist running under the bottom plate. I'd then have the subfloor sit on the 2" flange and glue it there.

This is a bathroom the flooring spans aren't crazy and a 2"x2" angle iron at maybe 1/4" thick should provide support to replace 1/2 the load of the small span of your subfloor at this location. What you are looking for is rigid support for your subfloor.

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    Full agreement! Additional thoughts for the OP's consideration: you want the subfloor to be supported so it doesn't flex along the wall. However, right at the wall, in a corner, behind the toilet, nobody's ever going to be standing there, jumping around, etc. so if the support isn't 100% perfect, it'll still be OK. Even if you put up a ladder straddling the toilet with a foot in that corner, you're still only supporting 1/4 the weight of the person on it, and some plywood or angle iron should take that just fine.
    – FreeMan
    Jan 17, 2023 at 13:26
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    I'd put another piece or two of blocking on either side of the toilet flange. Might even use pressure-treated or waterproofed blocking there. Saggy toilets are annoying as heck, and the opportunity to reduce the odds of that happening easily is there to be seized.
    – Ecnerwal
    Jan 17, 2023 at 13:46
  • I've never used angle iron. Do you mean this or this? Jan 17, 2023 at 15:53
  • similar to the first but made of iron. this is an example that is 1/8" thick, I was thinking 1/4 thick. grainger.ca/en/product/p/… Jan 17, 2023 at 16:44
  • What are ledgers? Jan 26, 2023 at 15:21
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I'm of the opinion that no backing is needed in the direction perpendicular to the floor joists -- the sheathing is intended to span between joists after all. It won't hurt to have backing on that edge though, and also Ecnerwal's advocacy in a comment for extra support around the toilet flange is a great suggestion.

The edge that runs parallel to the joists definitely needs support. Cut a strip of OSB/plywood perhaps 5 inches wide. Fasten it to the underside of the existing subfloor and bottom plate so that it provides a ledge for the new subfloor to rest upon. Use screws 2-1/2 or 3 inches long and drive them upward from the floor joist space into the under side of the wall bottom plate. As an alternative, use dimensional lumber (2x6) in the same way and drive the screws from below or from above.

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  • I thought the same thing regarding the perpendicular seams until I stood on the edge of one and could see it flex. I'm not that heavy either. Jan 18, 2023 at 0:11
  • "I'm not that heavy either" says @BigMcLargeHuge... LOL! :) Really, all you need is some sort of block to prevent that flex at the ends. It doesn't need to be full 2x material or to span the entire joist bay or be contiguous through the bay. Some 3/4" plywood/OSB scraps can be cut into small strips to fit in whatever space is available to prevent the edges from flexing. Again, though, nobody walks that close to the wall except when the room is totally empty and you're "testing it out". Mostly because there's stuff in the way.
    – FreeMan
    Jan 18, 2023 at 15:18

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