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Question DetailsAsked on 8/30/2013

Do we need to replace the sub floor in our bathroom if there has been water damage around the toilet?

The sub floor around the toilet is weak and moldy but not a total lost cause. The man doing the repair thinks he can lay Dietra matt and save the sub floor. Is this a good idea. We want it fixed properly.

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2 Answers


Though I can not see the amount of damage I would never do that. I would cut out any rotted wood and replace. Once rot has started any time there is moisture present it will continue to grow, even high humidity during seasonal changes can contribute to it, plumbing pipes tend to sweat even if it is not leaking. If the area this contractor thinks is small enough to bridge (which is questionable) the cost to repair it right should be minimal compared to the cost of your tile floor. Do it right once rather than have to pay more than twice. Sounds like you might need a second opinion from a contractor that may me more well versed in the total house and not just flooring.

Answered 7 years ago by ContractorDon


I totally agree with Contractor Don. This is not the use or place for Dietra mat, which has no structural strength, and this use would not be to code and would leave weakened and potentially rotting flooring right where you need it the most - where it needs to resist toilet rocking. Putting this in will let the toilet rock, so net you will have a wax seal leak (which probably was the initial cause of the leak) and maybe entire toilet base cracking eventually. Also, Dietra mat itself is a rolled, soft flexible product, which would also make for a soft zone under the toilet, again promoting rocking and cracking of the mounting flange or toilet mounting hole base - all around, a very bad idea.

The right way to fix this is to cut out and replace the damaged wood, and alos replace the toilet mounting flange and pipe right below it if they are corroded, to avoid having to get down there again in a few years and eliminate the chance of them breaking in the near future.

For this type of repair I almost proselytize that the right way to fix it is to use marine plywood for the subfloor (pretty much immune to water damage), and when cutting the bad wood out cut only to the near face of the nearest joist(s) on each side (call them A and B) rather than the middle, and appropriate distance in the between-joist direction. Then put in a 2x4 blocking between joists across between joists A and B (or A-B and B-C if two joist spans went bad). Put this blocking flush with the cut edge of the existing good plywood, so all around the hole you have the existing good plywood siting on full 2x4's. Then, inside that cutout, install a prefabricated 2x4 box frame that just fits down into the hole, and nail or screw to the joists and blocking 2x4's at 4 inch centers, giving you doubled 2x4's all around the hole. This gives you a full 2x4 rim to support the new piece of subfloor going in, rather than trying to support the edges of the relatively small cutout piece (which may not span 3 joists as it normally does on only a half-width of 2x4. This gives a much stronger floor with less deflection, which reduces toilet wobble tremendously.

I would guess doinig it the right way would only affect the cost about $100-150, depending on how much subfloor has to be replaced, because the only difference really is removing the damaged wood, and putting in some pieces of 2x4 and new plywood - probably about an hours work additional, assuming the joists are not damaged beyond cleaning up and sanding clean. Plus cost of reoplacing corroded piping and toilet flange if needed, but that is something that should have been in the program anyway once determined they were corroded, if they are.

Answered 7 years ago by LCD

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