Should I lay cement backer board directly on existing 1/2″ subfloor or replace it with 3/4″ subflooring before tiling a bathroom floor in a house built in 1964?
Asked by Benita Börner 
6 months ago
Last Updated: May 26, 2024
Hey there! So I’ve got this house from ’64, and I’m converting a small bedroom into a bathroom. Pulled up the oak flooring and now I’m looking at the subfloor situation. It’s 1/2” plywood on 2×10 floor joists at 16”OC.
Now, should I go with option 1: slap some 1/2” cement backer board directly on the existing subfloor, using thinset and screws, and then lay the tile?
Or how about option 2: glue and screw a layer of 1/4” plywood on top of the existing 1/2” plywood, then add the 1/2” backer board, and then tile?
Or maybe option 3: cutting out and removing the existing 1/2” subfloor and replacing it with 3/4” subflooring, followed by cement backer board, and then tile? What do you think?
I’d simply apply thinset backer to it. Adding an uncoupling membrane later would be perfect.
I can share some things based on my own experience. A 1/2″ subfloor won’t cut it for ceramic tile. Cement board is fine, but it might be a bit much. You’ll want to remove the 1/2″ subfloor and check the joists to make sure they’re in good condition. The joists near or under the tub are usually soft due to wet rot. Then, lay down 3/4″ treated plywood followed by a vapor barrier and 1/2″ regular plywood (not necessarily treated) perpendicular to the 3/4″ layer. After that, you can add whatever you like on top.
In converting a bedroom into a bathroom, ensures no soft floors, wet spots, or Joist damage since it’s originally a bedroom.
Honestly, laying tile on plywood is not the best approach, .
I’d recommend taking a close look at the plywood. Honestly, I think 30-year-old plywood is no good.
Please read before commenting, folks! The person is changing a bedroom into a bathroom. It’s unlikely that there are issues with the plywood or rafters underneath. They might just need to add another layer, no need to remove everything down to the rafters unless their pets have been using that corner as a bathroom for two decades.
Is right. The bedroom is above the basement. The floor joists are all dry, solid, and in excellent condition.
What are the dimensions of the joists?
Uses 2×10.
I recently redid my 1964 house and the plywood I used is in better condition than most plywood available for purchase. It’s only 5/8″ thick but incredibly sturdy. I recommend using cement backer board and tile on top. I personally did vinyl glue down and grouted tile on mine, and it’s been crack-free for over 7 years now. 🙂
How about adding another layer of 1/2″ plywood before placing the standard 1/4 inch cement board with thinset? This will provide a sturdy base for the cement board.
I’m not a fan of completely removing the subfloor because the walls depend on it, and creating a weak spot where the new subfloor meets the walls is not ideal. You need a sturdy subfloor for tiling. In my opinion, 1/2″ plywood is too weak, and adding 1/4″ plywood won’t help much. I’d suggest laying 1/2″ plywood on top of the existing subfloor, then adding 1/4″ cement board. This will result in a strong floor. Your tile may end up slightly higher than the rest of the floor, but that’s usually not a problem.
Pointed out that’s exactly what he had in mind. All the walls are supported by the 1/2” subfloor.
Mentioned that there will only be a meeting point in one spot – specifically the doorway from the bedroom to the new bathroom. But he doesn’t have any issue with a transition being there.
On the bright side, 16″oc. Quick question, you removed the hardwood, which I’m guessing was 3/4″ thick. How thick is the tile you plan on installing? Ideally, they would be the same thickness. However, in reality, that’s usually not the case. Hardwood is more forgiving than tile on a thin subfloor. Tiles don’t work well on a bouncy floor. Thankfully, bathrooms don’t usually have heavy point loads, think people standing instead of heavy appliances. Considering the structure and the type of tile, I’d recommend going for option #2. It’s a bit of a gamble. If your joists are smaller due to closer spacing (like 2x8s instead of 2x12s), the tile may not work well regardless. Hopefully, there’s enough space underneath to adjust the plumbing. Many factors to take into account.
Has a spacious basement with easy access and sturdy 2×10 floor joists.
You have the flexibility to decide how you want to design your Fall layout. Get some tile samples to see what will work best for your space, taking into consideration the heights of the flooring and how it will match up with your new bathroom project. As long as it’s sturdy and practical, there should be no need to tear out or replace any of the subflooring. Keep in mind that each project is unique in its own way.
I would suggest gluing and screwing the 1/4 over the existing ply in the opposite direction, followed by adding dura rock. Good luck!
Use glue and screws to attach 1/2 inch durock or hardibacker to the existing half inch that you previously reinforced. Tile installation with thinset is recommended, along with a marble threshold at the door. You could also consider keeping the oak and refinishing it, or covering it with vinyl plank ((: