How to Repair a Rotten Subfloor

Awhile ago I posted about removing the tile and linoleum on my bathroom floor. Mostly, I took it off because I didn’t want to add yet another layer to an already high floor. I also knew there was rot in the subfloor around the toilet. I had seen the rot on one of my excursions under the house. Sadly, I forgot to take pictures of the worst of the rot, all I have is the surface plywood picture. 

My subfloor is built out of 1.5 inch thick wood planks. For those to rot through there had to be years of wet conditions. 

Repairing the floor isn’t a hard job if the damaged area isn’t to large. If you have damage to floor joists then you may want to get a professional to do the work. 

First you will need to clear the rot by cutting it out. If you can cut back to floor joists then it will be easier to attach the section of plywood that will make up the new floor. 


Once the rot is removed then you may need to add supports between the joists.  The joists in my house are pretty far apart so I had to add quite a network of wood to make a support that I felt would be strong enough for the floor and toilet. You may not need to make add all the supports like I did if you can easily span the joists with your plywood. Another thing you can do is attach a 2×6 directly to the joists and use that as the attachment point for your plywood. 


Next attach plywood to your supports or joists. This step is where I made the biggest mistake. I read up on how to do this before attempting it myself. Everything I read said to use a layer of 3/4th inch plywood. With my sub floor being so thick I should have done more then one layer of plywood. That would have saved me some work and money later. If you are making this repair in a potential wet spot make sure you use screws that will work there. Someone in the past had attempted a repair around the toilet using drywall screws, and they had rusted pretty badly. 

After the plywood is in place, you will need to use floor leveling compound to make the floor level if the plywood isn’t flush with the original floor. This is where not using more plywood hurt me. I had to use a lot of floor leveling compound. Plywood is cheaper and easier to work with. So make sure you get the plywood as close to flush with the original floor as you can. 

Once the floor is level or as close to it as possible then you are ready to start tiling the floor, and that will be another post. 

-Joshua

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3 thoughts on “How to Repair a Rotten Subfloor

  1. Excellent work! I’ll be saving this post because I know I’m going to need to do it. I just re-did our floor and toilet. I had thought about the sub-floor but I think down the line we’ll do a much larger demo of that room so I just put risers on the toilet flange and laid over the old linoleum. Our toilet is over the the cellar so I was able to confirm our floor isn’t as bad as yours. But at least I know what goes to in to fixing it. Glad you showed how to level too! Man I have some rooms where that’ll need to be done. Awesome work!

    Liked by 1 person

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