Flooring Tile - Bathroom Subfloor Worries...

Hello, fellow DIYers.

I’m installing underlayment for ceramic tile in a bathroom, and I’m concerned that I may have made a critical error. This is my first tiling project, and I’m looking for experienced viewpoints on whether I should press on, or start over.

I started by demoing approximately 60 sq ft of 60 year old tile set on a very sturdy old style mortar base with mesh wire underneath. This revealed the sub floor, which is a plank floor (I think that’s the term). Basically, you have floor joists with planks toe-nailed in the spans.

On the advice of a contractor, my plan for achieving the correct floor height (to match the adjoining hallway) was to lay down the following materials in this order.

1. Aquabar vapor barrier (this was my idea, and is just to create an even surface to apply thinset.)
2. Layer of thinset
3. ¼ inch plywood (Luan) attached with 1 1/4 in. backer-on screws every 8"
4. Aquabar “B” vapor barrier (the actual vapor barrier)
5. Layer of thinset
6. ½ inch Hardiebacker 500 attached with 1 1/4 in. backer-on screws every 8"
7. Layer of thinset
8. Ceramic tile & grout

Last Saturday, I got to step 3 (the plywood) when I ran into problems. While troweling thinset for the last piece of plywood, I noticed a little bit of movement in the subfloor right in front of the stall shower. I already had the paper down, so I couldn’t see it, but it was moving just slightly. To add to my troubles, I had underestimated the amount of thinset I would need, and started running out about ¾ to the end of this section of the floor. Therefore, I may not have gotten quite the right amount of thinset under this piece of plywood.

Perhaps against my better judgement, I pressed on, spreading out the thinset as evenly as I could, and screwed down the plywood. It’s been 2 days now, so I figure the thinset is pretty much cured, and when I step on that spot in front of the shower, there’s a little bit of give. The floor is also not completely level.

So here’s my question. Given that my next step involves another layer of thinset (for which I plan to be fully prepared this time) and ½ inch backer, should I be concerned about this slight movement at this stage. I’m not too worried about the level, as I’m pretty sure I can address that with the thinset and backer, but the movement worries me.

So the question is: Press on, or pull up that piece, scrape off the thinset and try again.

If you’ve made it this far into this post, I applaud your perseverance and thank you. Any insight from those with experience would be greatly appreciated.

Thanks in advance.

Sorry to say, but pull it up.

There should be no 1/4" plywood or any other type of wood in a tile installation.

I've also never heard of Aquabar?

You should install at least a layer of 1/2" exterior grade plywood over the planked flooring, then install 1/4" or 1/2" cementboard in a bed of thinset and screwed.

Also, do you know the size of your floor joists, their spacing, and unsupported span?

Hey HotinOKC.

Thanks for the reply. Not what I was hoping for, but I'm not surprised. :)

I'm a little confused though. You said
There should be no 1/4" plywood or any other type of wood in a tile installation.

but then...

You should install at least a layer of 1/2" exterior grade plywood over the planked flooring, then install 1/4" or 1/2" cementboard in a bed of thinset and screwed.


So is it mainly the thickness of the plywood that concerns you? I do plan to install 1/2" Cement backer next.

Not sure about the joist size and spacing. I'll have to figure that one out. I didn't research it because my approach is the same one my contractor used when tiling our kitchen, which has the same original sub-floor.

I had to replace a few planks, and I think I remember cutting them to 14 or 15 inches to fit between the joists. Maybe the spacing is 16 on center like wall studs? Just a very uninformed guess. I'll check.

Aquabar (http://www.fortifiber.com/aquabar_b.html) is just a brand name of vapor barrier they happened to have at my local HD.

Thanks again.

Yes, the thickness is what concerns me. The thinner the plywood, the more deflection between the floor joists you will have. This is the "bouncy" feeling you have over a wooden subfloor.

How thick are the planks? About 3/4" ?

Is the floor flat?

Yes, 3/4 inch thick by about 9.5 wide, and cut into sections as I described. The original floor before was not perfectly level, but close. The overall level is about the same now, but there are minor low spots in the plywood. Probably in places where the planks are slightly lower than the joists. My hope was that the next layer of thinset would address that. It's the movement that has me worried about voids between the plywood and subfloor. I could address that by pulling up that one section, but it sounds like I should start over.

Believe it or not, my instinct was to use 1/2" ply and 1/4 backer. I should have listened to my gut.

Thanks again!

So, the planks are installed between the joists? Why is that? Did you or someone remove a deck-mud base for original ceramic tile floor?

You need to remove everything until you see joists. Install as thick a layer of underlayment plywood as you can, then 1/4" tile backer or a membrane such as Ditra, then the tiles. No vapor barriers on floors, however Ditra is waterproof and the system can be made watertight with an extra step.

Plywood is NOT set into thinset, just fasteners. The backer board is set into thinset. Analyze any floor flatness issues and report back. Tiled floors need to be flat, level is not required, although would be nice.

Jaz

Hey Jazman.

Not sure why the planks were cut that way and put between the joists.

I did remove the original ceramic tile floor. I'm not familiar with the term "Deck mud", but there was a thick mud underlayment with chicken-wire below that. Perhaps that explains it.

I was wondering if the vapor barrier was really neccessary. Is it harmful, or just not needed in floors?

Thanks.

i had a similar assembly in my 200 year old beast. i sledged the old floor out. i was left with 5'x6' floor plate with a 1" difference between the high and low points. the subfloor was very stable. i rolled out some 15# felt to act as a bond break and hold the thckset until it cured. i then elevated a 1/2" galvanized screen on a few stones as reinforcing. I fastened a 1x on opposite walls to act as a guide for a screed (built with a 2x4 with notches at each end). i dumped some mortar mix (3/4" at its minimum) and used the 2x to screed it level (using the guides i had installed). i poured some leveling compound to perfect my novice screeding job and applied thin set and tile on top of that.

that was seven years ago, the house has shifted abit, but the floor is still solid with no cracks.

the deflection you have is no good and the other methods described in this post are certainly appropriate. i just like the concept of allowing the floor to move freely when installing in an older structure. i may have used a reinforcing add mixture to my thick set mortar bed.

i would go back to the original subfloor, stiffen the supporting joists, or add joists to achieve the deflection (as recommended by a reputable tile manufacturer) and consider the method i used. i used a tile manufacturers commercial sales representative as guidance in this assembly and it was thoroughly documented in thier technical literature which should be available on line.

The floor was built that way to accommodate that 'Mud" so the floor would be lower.

There is not need or use for paper moisture barriers on a floor. Ditra is a different matter though.

Jaz

Mskin,
Wow, that sounds like an enormous amount of work. Not sure I'm up to it, but I'm sure that floor will last a lifetime. Thanks for the info.

Jazman,
That explains a lot. I just noticed today that the floor in that bathroom is not the same as the kitchen as I had thought. As you state, the bathroom floor was set up that way to lower it. Never would have occurred to me. :)

Looks like I have a few options to weigh here. Thanks to all for the input.

~ James

mskin,

I'm in a similiar situation as the OP.

Just to clairfy:

-you nailed/screwed felt to the old subfloor joists

-then you put down a metal screen on top of the felt held up by rocks?

How did you secure the rocks?

Are the rocks laying on top of the joists or the planks between the joists?

Thanks!