I could use some subfloor help

[diy-sk]

Hello all, this is my first post on the forum and I could use some help. I am a homeowner taking on my first bath remodel, and feel pretty confident in my skillset but I do not have a lot of experience hence I could use your help.

I have demolished the bathroom with removal of a cast tub, toilet, and floor tile, cement board underlayment, and the wall tile around the tub. On exam of the subfloor, I found 5/8 plywood, and there was 1/2 inch cement board over it. I understand this is too thin for tile so I was wondering which is the better option- (home was built in late 1980's with joists 16" on center and engineered "I-beam" wood joists all in good condition, no rot).

Options are-
1) Remove 5/8 plywood and replace with 1 1/8 plywood? I think this is the strongest, but I have two flights of stairs in my home and carrying these sheets will be difficult for one person.
2) Remove 5/8 plywood and replace with 23/32" (3/4) and then cover with a sheet 1/2" in opposite direction with screw and liquid nail adhesive? Then followed with Ditra or other similar uncoupling membrane over this for tile?
3) Remove 5/8 plywood and replace with 23/32 plywood and cover with 1/2 in cement board on top then followed with Ditra or just thin set and tile over this cement board?

Please let me know your thoughts on this one. I really appreciate any and all help. The current 5/8 plywood has some soft spots one of them I discovered on my own and the other I made while smashing up the cast iron tub with a hammer (d'Oh). Hence, I plan on removing this at a minimum.

Many thanks in advance-SK

[cx]

Welcome, SK.

The nominal 5/8ths" plywood technically meets the minimum requirements for all manufacturers of tile substrate materials. I wouldn't tile over it on a bet.

Since you've already decided to remove it, starting your new subfloor with nominal 3/4" plywood would give you what is usually considered enough for a single layer subfloor over 16" joist centers.

If you want a really good subfloor, you could add a layer of nominal 1/2" plywood over that. Or you could start with the nominal 5/8ths" and add the 1/2" over that. Either would be better than your single layer 1 1/8th" plywood.

The engineered joists should, by design, be sufficient, but you never know. The joist manufacturer's span tables would be the only way to evaluate that.

1. See above.

2. All layers of structural subflooring must be oriented with the strength axis perpendicular to the joists. Here is a good article from our Liberry showing what I think is the best way to install that second layer.

3. If you elect to use a CBU as your tile substrate, all manufacturers recommend their thinner panels, usually 1/4", for floor applications. You would not install Ditra or similar membrane over the CBU, you'd use one or the other.

My opinion; worth price charged.

[diy-sk]

Thank you so much for the advice, this makes sense for sure. One last question, I have read a recommendation or preference for tongue and groove is best, but after I cut and install, I will not be able to use my router to do this, are there other options I could do instead such as cross bracing the joists under the seams of the plywood? Any other ideas or options I could be missing?

Again, many thanks, this advice is like a true north for me.....

[cx]

T&G is mandatory for the first subfloor layer. If not able to use T&G due to your space limitations, use square-edge material and add blocking under the between-joist joints. I prefer 5 or 6-inch rips of plywood for the purpose. Install a sheet. Apply construction adhesive to 1/2 the plywood rip, put that half under the installed sheet and attach with screws through the sheet. Put adhesive on the other half of the rip and install the next sheet on that, fastening the edge into the rip. Pre-drilling the sheet edge helps. Keep the rips short enough that they do not touch the joist at either end.

Takes as long to type as to install.

Second layer, if any, does not require T&G edges.

My opinion; worth price charged.

[diy-sk]

Hello, could I perhaps use a biscuit joiner to join the plywood sheets as long as I leave the 1/8 gap between sheets?

thanks again
SK

[PC7060]

Good morning SK,

No, the edges need to fully supported. And using the scrap plywood as CX recommend it is much simpler than biscuit jointing.

Biscuit joints add very little structural strength, they are primarily used to simplify alignment when gluing up planks. It’s the glue that adds the strength.

Unless your subfloor is rotted or has humps/dips, I’d go with CX 2nd option and add 1/2” plywood over the the existing 5/8” ply. Easy to execute and very strong.

[cx]

I wouldn't be opposed to using a plate jointer, but if you were in a situation where you could use biscuits, you'd be able to use the T&G edges just as easily, would you not?

[diy-sk]

Hello again all, many many thanks for all of the helpful advice. If I could ship a can of beer in gratitude I would...

Having said that I only asked because I do a bit of woodworking so I am familiar with what all of you are commenting on. I think my main concern is that I have not used a handheld router to do tongue and groove edges and I do not have access to a router table. I do not know if that cutter head would possibly "kick back" or risk a kick back while using it to make the tongue and groove on the straight cut edges. Thoughts? Have you made tongue and groove with a handheld router before?

Thanks again-
SK

[cx]

Another wheel that doesn't need reinvented, SK.

While the T&G plywood edges would seem a rather crude sort of engineered joint, some of them are quite well engineered, with tapered edges just right to get tight at just the correct fit, with or without adhesive, and with our without a gap in the surface plies, etc.

Yeah, I guess a fella could biscuit or spline or even a cope and stick if he wanted, but I would hafta axe why? The method I described above requires nothing more than a saw to rip some plywood and a screw driver of some kind, preferably powered. And the joint registers near perfectly and is stronger than even the original T&G. Also allows you to make the fit in even the tightest quarters. But that's:

My opinion; worth price charged.

[diy-sk]

Many thanks again, two final questions.

1) How do I handle the periphery of the field? I.e. when I cut the old subfloor out to replace to 3/4, would I use the same method on the edges? I feel like I would have trouble screwing the side that is against the wall edge since the wall is in the way.
2) I feel like the 3/4 with the 1/2 plywood layer is the way I am leaning towards. I read the article that you sent and it makes sense what it is commenting on orientation of the subfloor vs. underlayment. I feel like the 1/2 plywood will act as more reinforcement than underlayment for my flooring needs. With that in mind, can I use this two plywood layer under where the alcove tub and cabinets will be? Once that is in, I plan on using Ditra or other uncoupling membrane for the area where the tile will be.

again thanks
SK

[cx]

1. If I understand the question, that's generally a place where you'd need to add structural blocking along, or perpendicular to, the existing joist structure. You'll need to determine how to do that once you've cut out the subfloor and see what you have at those perimeter areas.

2. It is unfortunate that they use the term "underlayment" in that article. That second layer is not an underlayment, it's a second layer of structural subflooring.

Originally Posted by SK

...can I use this two plywood layer under where the alcove tub and cabinets will be?

Perhaps I misunderstand again, but I can't imagine why you could not. Was that a trick question?

My opinion; worth price charged.

[diy-sk]

Many thanks- I will raise the old floor and take a look, when you refer to structural blocking I assume this will be 2x4's made into "boxes" that fit between the joists to rest the edge of the plywood? This is what I have seen on YouTube videos regarding subfloor repair and replacement. If I am wrong, please let me know.

As far as the 1/2" under the tub along with the new 3/4 plywood, I only ask because it seemed like it couldn't hurt but I have never done this before. I was thinking along the lines of "More is better" but I also didn't want to make a mistake in good faith that would make me come back and take out that 1/2 inch layer.

Again, many thanks, will get whacking away soon...
SK

[cx]

Don't know what any sort of "boxes" might be in your application, but 2x material of some kind would usually be used to augment your existing joist structure for support of your new subflooring.

I can't think of any problem the extra subflooring might cause and it might even be easier to put the second layer on the entire floor, rather than leaving it out of specific areas. Stronger overall, as well.

My opinion; worth price charged.

[diy-sk]

Hello all, I have been thinking about my original plan and right now I have a 5/8 plywood floor in a smaller bathroom that needs to be replaced. Originally I was thinking about nominal 3/4 plywood as replacement, but with the 1/4 backer board on it, that will just barely pass for TCNA guidelines. Would I be better off to use 2 layers of 5/8 plywood perpendicular to one another followed by the 1/4" backer board or ditra uncoupling? What about two layers of 1/2 inch plywood with the 1/4" CBU on it?

If I go with the two sheets of 5/8 do I screw those into the joist but ensure the backer board does not get screwed into the joist(s)?

thanks for all of your help on this-

[cx]

Welcome back, SK.

Perhaps review post #2?