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Old 07-11-2009, 12:47 PM   #1
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Help repairing damaged subfloor


This all started with one tiny little loose tile in the bathroom.


After pulling the toilet, the tiles and doing some excavating, I found three wet layers.

The top layer looks like 1/4" plywood - swollen and cut out.

Middle layer looks like it once was 1/2" plywood, but is now moldy black and the worst of it (now removed) had the structural integrity of wet cardboard.

Bottom layer is also 1/2" plywood. The top of it is also black and soft, but the bottom is dry and sound so I'm hoping the joists aren't affected.



I really like the floor tile and would like to keep it. Any suggestions on repairing this mess would be appreciated, I really don't want to have to tear up the entire floor.


Where do I go from here? And how do I deal with the flange?


Oh yeah, the joists run parallel with the brick wall, one just in front of the flange.
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Last edited by Blondesense; 07-12-2009 at 02:06 AM.
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Old 07-12-2009, 01:10 PM   #2
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Help repairing damaged subfloor


Do you have access from underneath to assess how much the mold has spread? If not you have to remove the tile. Even then you will probably need access from underneath to install support for the replacement subfloor. Depnding on the adhesive that was used to install the tile, you may be able to use a floor scraper to salvage them. You can rent them.

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Old 07-12-2009, 02:32 PM   #3
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Help repairing damaged subfloor


You stated you have wooden sub-floor, so I'll bet you have a crawl space no matter how small, good. I've done these repairs often. Depending on the total amount of compromised sub floor, you can salvage your floor without having to tear out the entire bathroom floor, IF your floor joist are still good. Depending on the spacing of the floor joist, I would remove tiles and damaged sub floor out to existing floor joist and an equal amount centered from the toilet flange to make a square shaped area. This would be a good time to peek down into the crawl space to do an inspection of how much damage you actually have if you cannot access the crawl space easily. I would then box in this area with new lumber to create floor joists, sistering to the existing floor joist, to nail/screw the new sub floor down to. Having 1/2" + 1/2" + 1/4" plywood is quite unusual to me. I would use two pieces of 5/8" plywood with a layer of roofing felt between the two and alternate the grain of the plywood as it is layed on top of each other. I prefer screws through the first piece layed down and into the floor joist, then the felt, then nail the second piece. You can locate the toilet flange and cut the hole before laying the plywood or cut it out later, it's up to you. I'm not a tile person, but the suggestion of trying to save the tile sounds good. Good Luck, David
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Old 07-13-2009, 11:42 AM   #4
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Help repairing damaged subfloor


Thank you David. I hadn't thought of sistering the joists. The two layers may be 5/8" but considering how they came out, it was difficult to get an accurate measurement. I will double check this before I get the replacement materials.

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Two more questions please.

Would you cut all three layers the same size, or would it be better to cut the top layer(s) slightly larger where possible and so staggering the repair?

Also, I have some 1/4" hardiebacker leftover from another project. I know tile on plywood is not a good idea. Would you recommend it for the top layer under the toilet, or staying with plywood for consistency?

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Last edited by Blondesense; 07-13-2009 at 11:45 AM.
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Old 07-13-2009, 02:41 PM   #5
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Help repairing damaged subfloor


I, me, would not worry about trying to stagger the pieces for this repair as they will not be that large. The important part to me is to make the joist framing support the new pieces to be placed down. I do believe in laying the second piece with the grain perpendicular to the bottom piece for strength, that's the way I was taught and it has always been my practice to do so. As far as using the "hardiebacker", I'm not too familiar with this as I don't lay tile, I just never got into tile, too many tile guys around here. I've heard hardieboard recommended in bath areas and I've heard it the other way also. I've seen tile guys use some type of thinset/mortar over the plywood, then some type of mesh material, then another layer of thinset/mortar to place the tile on. So, I guess it depends on the tile guy as to how they do it. Good Luck, David
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