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I pulled up the tile in my bathroom and discovered that the sub-floor was partially rotted near the bathtub, under the toilet. Based on the tile/mortar, it looks like whoever did this floor last mortared over/into the already damaged subfloor to cover up the fact that it was damaged. It looks, from what I can tell, that the damage goes under the wall.

My questions are:

  1. What's the ideal way to cut out the damaged section? The parts I can get to, I would just use my circular saw with the depth set properly, but for the edges along the tub insert and the wall, (especially the tub edge, which runs perpendicular to the joists), how would I get close enough? I've heard reciprocating saw and toe-kick saw suggested by a few people I know.
  2. Do I need to worry about the damaged subfloor that goes under the wall? If so, how would I go about removing/replacing the damaged subfloor there?

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Your plan to use your circular saw with the table set to depth is good. Even if you were to cut tight to the wall and tub, you still aren't getting all the damage, so a short distance out is fine. Personally I'm not a fan of joints right at walls and tubs anyway. I'd rather have an offset.

When you do the repair, float lumber under the remaining damage, with construction adhesive between, wherever possible to support it.

The fact that the damage extends under the wall probably isn't a serious concern if you haven't seen any significant side effects, such as settling or floor damage in the adjacent room. I'd be fixing whatever you can reasonably access and calling it good. It'll be a far cry better than what you have now.

FYI, a "toe kick saw" is actually called an "undercut saw".

  • If someone recommends that you use an oscillating multi-tool for this, they do technically work for flush cuts, but cutting this much wouldn't be fun and would probably burn a few blades. – JPhi1618 Apr 8 at 17:32
  • @JPhi1618 Interesting. What about my air chisel? That seemed to be very effective in destroying parts of healthy subfloor when misused during tile removal. – michaelgulak Apr 8 at 17:36
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    @michaelgulak, if your goal is to test-to-destruction portions of the floor, then the air chisel will certainly destroy anything that isn't solid and give you some holes to work with (if for some reason you needed holes?). But your goal is to cut out a clean square/rectangle so you can patch it with new subfloor material. You're not going to get a good clean cut with a chisel and you'll tear out material on the back side of the floor. – JPhi1618 Apr 8 at 17:39
  • I would line up my cuts so they end up roughly in the center of the joists. It allows you to screw around the perimeter of the new boards. – Quoc Vu Apr 13 at 2:11

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