Bathroom floor 1935 house

My bathroom floor is constructed as follows from the bottom up 2" X 8" joists 16" on center 3/4" X 7" boards running perpendicular to joists 1/2" x 7" boards running parallel to joists (very wavy) 3/16" plywood? ceramic tile
Within a few weeks of buying this house the tiles started cracking and the grout popped out. I just took out most of the tiles by hand.
I was planning to remove the 1/2" boards and put down 5/8" exterior grade plywood thinset 1/4" Hardibacker then tile again
The floor will end up a little higher than it is now which is about even with the hall, but it might still be ok. Would it be better to go down to the joists and start with 1" ply? If I put thicker plywood on the joists can I go directly to cement board or are there two layers for a reason? Is there anything else I should consider? I want to do the job right and make it last.
Thanks Dante
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Dante M. Catoni wrote:

What was there sounds as if it should have been OK. Obviously, it wasn't so step #1 is to determine why...what's flexing and why?
a) Is the ply firmly attached?
b) Ditto the "wavy" boards and what do you mean by wavy?
c) How about the bottom sub-floor? Any movement when you bounce on the floor?
d) How do you know what those layers are? Already ripped up a section?
I'm thinking the fastenings are old/shot/inadequate and that a bunch of 3" x #10 screws through the whole works into the joists might solve your problem. Unfortunately, no way to tell for sure.
dadiOH
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No, there was a drywall screw about every foot.

They are not perfectly flat. I don't think the very thin ply was enough to compensate.

There is a little creaking in different areas.

Yes, I pulled up a small area to see what I was working with.
Thanks for your reply.
Dante
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Dante M. Catoni wrote:

I think you're right. If you put down another layer of ply BEDDED IN MORTAR you should be good to go. If the plank subfloors are sound (no flex) then even 1/4 ply should be OK, 1/2 or 3/4 if you want to be sure. If you do this, you don't want to screw down the ply so tight over low spots that you deform it, you're trying to get a level surface letting the mortar fill in low areas. Same technique if you use cement board instead of more ply.
dadiOH
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dadiOH wrote:

That is, take off old, put down ne in mortar.
dadiOH
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Dante M. Catoni wrote:

Where did the cracking start? Around commode? Something is rotted? What do you mean by "wavy" - the edges aren't cut straight or they are buckled/warped?
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It started mostly in the center of the room. All the wood seems to be in very good condition. I don't see any sign of moisture damage. Last week my 2 year old daughter dumped a pitcher of water from the tub onto the floor so there is a stain there, but everything is solid certainly no sign of long term water exposure. If you put a straight edge across the boards there might be about 1/16" spaces all around.
Thanks Dante
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Dante M. Catoni wrote:

If there were dips in the boards not filled with mortar, the space could cause the tile to crack when pressure applied. See any sign of that on back of tile?
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Dante M. Catoni wrote:

    The movement of the floor is your big problem, and you already have several good suggestions. But I would ask if the floor joists are accessible? For instance is there a basement under this floor? If so, is there bridging for these joists? Just another thing that might help.
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There is an unfinished basement underneath and all the joists are accessible. There are 2" pieces of wood forming "X" braces in a row the whole length of the house. A couple of them are split where the nail is, but they don't move if I push and pull on them.
Thanks Dante
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*It sounds as though you have a decent subfloor. What I would look at is the joists. How long of a span do they have without support?
Something else to consider is that the previous owner may have put down a fast and cheap tile job to sell the house. The tiles may be popping up because they were not installed properly.
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I think you are right about the tile job. No screws were added to the existing subfloor to prepare it for tile. The ply wood is too thin and doesn't have nearly enough screws. Now, the Hardiebacker website says that 5/8" ply is the minimum to put the cement board on top of. Does that mean if there is nothing else underneath it? So if I remove the 1/2" layer then screw the 3/4" layer to the joists every few inches so that it is very solid, could I use 1/2" ply under the cement board? I would still have over 1" of wood underneath the cement board.
I really appreciate everyone's replies. Thanks Dante
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Dante M. Catoni wrote:

I have been told to use minimum 1 1/4" of wood under the backerboard. I added a layer of 3/4" ply under the 3/4" subfloor plywood on a small bathroom to keep the final level even with the other floors. I used 2x4s under the added layer attached to the joists to brace the addition, and lots of screws between the two layers, especially near the joists.
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Bob F wrote:

The additional layer was between the joists, below the normal subfloor level. I didn't make that clear.
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Who told you you needed a minimum of 1 1/4" of plywood under the backer board? I'm not aware of any product that requires more than 3/4".
R
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Please learn to quote correctly and include the poster's name in the quote. It makes it easier for people to follow who said what.
You are thinking along the right lines. There are two options.
Remove the crap, fasten the 1x8 perpendicular-to-the-joists boards securely, install 1/2" plywood with glue and screws, use 1/4" backer board attached to the plywood with thinset and screws or roofing nails, set the tile with thinset.
Second option, which may or may not be preferable depending on the bathroom setup, remove everything down to the joists, 3/4" plywood screwed to joists (don't glue it to the joists as water damaged subfloors happen in bathrooms over the years and pulling up a glued subfloor damages the joists), 1/2" backer board attached to the plywood with thinset and screws or roofing nails, tile in thinset.
The second option would probably turn out to be a bit cheaper, possibly faster and has fewer layers with less chance of movement. The thinset between the backer board and plywood is important - it prevents any movement and makes the whole assembly one unit.
R
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*Yes. Hardiebacker requires a minimum of 5/8" subfloor, but you have much more. If the upper floorboards are not flat I would remove them and put down plywood. Don't use drywall screws. Get deck screws instead.
So if I remove the > 1/2" layer then screw the 3/4" layer to the joists every few inches so that it is very solid, could I use 1/2" ply under the cement board?
*Yes
I > would still have over 1" of wood underneath the cement board.

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John Grabowski wrote:

Had almost the same substructure you have when I did a bathroom tile floor about 10 years ago. If the 1/2" & 3/4" boards are relatively flat you can screw all of them down tight to the joists and even take out some of the humps at the same time. Lay a layer of mud and 1/2" cement board on top and screw it down. While you are screwing it down you can really tweak the floor and level it out as the mud will hold up the cement board in the low places once it dries. I put a latex additive in the mud which supposedly added gripping power and made it less susceptible to cracking. Used the same additive when it came time to put the tile on top and haven't had a bit of problem with cracking. Suspect as someone else said it might have been a quick job just to make it more sellable. Just my 2 cents
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