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I just purchased a ranch house (with basement and garage underneath) and decided to try to renovate the bathroom. The flooring was linoleum, after taking off the linoleum, I discovered that the subflooring was a patchwork job consisting of different subfloor heights overlapped, mortar a couple of inches thick (assuming some kind of leveling compound to account for the different subfloor heights), and old tile on just half of the floor... A few areas of subfloor were spongy and damaged.

I got a bit ahead of myself trying to get rid of this subfloor and ended up cutting the subfloor out of the room up to the walls. It was at this point that I realized that the doorway wall isn't supported by any joists and runs parallel in between two joists. Now that the old 3/8" subfloor integrity is compromised from cutting it, it got me thinking that all of the walls in the room have this old 3/8" subflooring between the bottom wall plate and the joists, and I'm wondering if I just seriously affected the integrity of these walls since all of that subflooring is now cut and not supported across joists as it used to be? It seems like the remnants of the 3/8" old subflooring are just sort of hanging unsupported now around the perimeter - especially on the walls that run parallel to the joists.

Is there a proper protocol for blocking around the perimeter of the room for the new subfloor? What about something to keep the old subfloor around the outside perimeter of the room from sagging?

Do I just make 2x4 boxes and cross braces around the inside perimeter of the room and fasten the new subfloor to that and not worry about the old subflooring around the outside perimeter? Or do I put bracing on both the outside and inside of the room? Confused about the proper protocol here.

I am trying to replace the subflooring in the bathroom with 3/4" tongue and groove plywood.

Here are some images: https://i.stack.imgur.com/You5X.jpg

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You probably want blocking under the doorway wall now. Yes that portion isn't as strong if the subfloor it was resting on is now only 1/2 supported and cantilevered. Walls that are perpendicular to the joists shouldn't be affected. Good news is the door way wall doesn't need a lot of support.

I think process wise you are fine. Remove the rot, add blocking and support where required. If you were having to replace joists that would be a different story. You are going to need blocking to add your new subfloor anyway so you'll pretty much be doubling up the joists under the walls or adding blocking the other way 16" oc whatever is easiest with the plumbing.

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  • Thanks for the reply. So when you say about doubling up the joists under the walls - should I be using 2x10 lumber for this like the joists are? Or are 2x4s sufficient for perimeter blocking?
    – user151128
    Apr 26, 2022 at 23:17
  • I certainly wouldn't be supporting a bathtub and subfloor with 2x4s. The spans are small but the cost difference just isn't that big and I'd never use 2x4 as any kind of floor joist. Maybe I'd think about glueing and screwing 2x6 to an existing 2x10 but I tend to overbuild. I am sure you could get an engineer to sign off or spend some time figuring out the loads but for the cost and knowing it is overbuilt I'd tend to spend the money on the 2x10 and keep it looking standard. Do you want your floor to have a bounce when you walk through your bathroom?
    – Fresh Codemonger
    Apr 27, 2022 at 1:35

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