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Rotten Subfloor under Bathtub and Plumbing Wall


pontcatalina's Avatar
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07-24-13, 11:32 PM   #1  
Rotten Subfloor under Bathtub and Plumbing Wall

After discovering a rocking toilet and ripping up the tile/particle board underlayment I discovered that the rot around the toilet was the least of my problems. Instead it appears that the subfloor (1/2" plywood) near the front corners of the bath got wet and started to rot.

Below is a picture of the subfloor in the corner with the most damage. The yellow circle is the worst section and this section has actually started to sink. The bottom plate near the door with the 3 studs is actually not touching this piece of the subfloor, there is a 1/8 inch gap. The red lines are the joists (2x10s, 16 OC). I do have underneath access and do not see any rot from below.

So how do I fix this?
  1. I imagine I need to remove the rotted section, but does this mean I need to pull the tub out or can I just cut along the tub edge?
  2. If I can cut along the tub edge can I just install 2x6 blocking between the joists every 16 inches and then install 1/2" plywood over that?
  3. How to I replace the rotted section underneath the wall? I don't think I need to be worried about load bearing questions given the bottom plate isn't resting on that drooping subfloor section.
  4. Lastly should I replace just the rotten section with 1/2" ply or replace the entire bathroom subfloor with 3/4"?


Thank you!

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07-25-13, 05:35 AM   #2  
I would imagine that the particle board took the brunt of the water damage. The area in question is a common place to find water issues. It is more likely from over spray of water that gets around the shower curtain and settles in the corner. The bulk of the plywood is most likely just discolored from moisture and still maintains the integrity. Not seeing the damage from the toilet or the proximity to vanities and other bathroom structures it is harder to give a complete recommendation.

I would open up the floor, install cross blocking as mentioned and then insert blocking into the cross blocking to give a continuous nailing surface for the new subfloor along the tub edge. It is OK to cut close to the tub and the wall, don't worry about what is under the tub unless the tub is no longer sufficiently supported. Replace the 1/2" ply back to at least the next floor joist so that the patch spans at least to 3 nailing surfaces and is orientated so that the strength axis falls perpendicular to the floor joists. Then cover with a second layer of 3/4" T&G flooring, 1/4" Cement Backer in a thinset bed and lastly your tile.

I assume that the whole patch will include the area all the way to the toilet and include that damage. Is the toilet located to the left in the attached picture?

 
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07-25-13, 06:50 AM   #3  
You can use a Toe Kick saw to cut right along the bottom plates and the tub.
Home Depot rents them. Then use an oscillating saw or Sawall to cut out the inside corners.

 
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07-25-13, 10:48 AM   #4  
Thank your responses. I'll check out the toe kick option at my local HD. The bathroom is kitty corner of the damage, as shown below I cut out the rot there but my repair will only span 2 joists. Should I cut further so my repair will hit a 3rd?

What about the subfloor under the bottom plate? It's obviously failing given there's a 1/8" gap b/w the plate and the floor, but seems like a major pain to replace. Another pic showing more detail of the plate follows. I should also note that this is a very small bathroom, ~25 sqft (approximately 5x5), so everything is very close and 3 joists is a good chunk of the floor space.

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07-26-13, 06:13 AM   #5  
When you add the 3/4" T&G you can cut it such that you can tuck a small "ear" under the bottom plate. You may have to break up the bathroom into multiple pieces to get the tuck to work.

Add blocking under each side of the cutout to give the 1/2" ply something to set on to support the span. If you have access from underneath, I would also add cross bracing close to the toilet flange to give you something to attach the flange to that is sturdy.

 
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07-27-13, 06:26 AM   #6  
OK I think I'm almost ready to tackle this. Regarding the blocking near the tub edge, czizzi stated "install cross blocking as mentioned and then insert blocking into the cross blocking to give a continuous nailing surface for the new subfloor along the tub edge." What is the best way to accomplish blocking the cross blocking? Joist hangers? Stagger the blocking which means I'd probably use the wide part of a 2x6? Toe nail them? Seems like as soon as I put one block in then the subsequent blocks' will be more difficult because the first block has covered the face of the joist that I'd like to nail through.

Thanks again!

 
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07-27-13, 07:02 AM   #7  
Are you hand nailing or do you have a pneumatic nail gun? Much easier to toenail with a nail gun. You can shoot through the main beam and into the end grain of the block at an angle from the opposite side of the beam. You said you have underneath access. Might make things easier to do it from the underbelly.

How much of the 1/2" have you removed? Only the area around the toilet flange? or more?

 
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07-27-13, 08:53 PM   #8  
I don't have a frame nailer unfortunately. I could use screws if that would be easier. I think I'm gonna have to get underneath because one of the joists will be under the tub. My wife won't like it (underneath is the kitchen) but not sure I have a choice.

I have only removed the 1/2 inch flooring around the toilet flange, nothing more.

 
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07-28-13, 06:15 AM   #9  
The strongest floor will have some sort of nailing surface along the length of the tub to secure the 3/4" T&G that you are going to put over the 1/2". You can successfully use 3" to 4" screws in lieu of nails for your cross blocking. I would purchase exterior grade decking screw that use a "STAR" bit and not a phillips head bit. You will have way more torque to drive the screws and less stripping of the screw heads. Rub the screw on a bar of soap if you have troubles.

Here is a quick drawing of how you can secure the bracing by driving the screws through the end grain of the wood at an angle from the back side.

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