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Unread 09-22-2016, 01:13 PM   #1
Eldudarino
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Small Floor -> Large Tile and How Flat is Flat?

Hello, long time lurker, first time poster. First, thanks to all everyone on the board for all great information it has been very helpful so far. I am a well above average do-it-yourselfer who is at the tail end of a whole house remodel. Electrical, plumbing, infrastructure you name it. Oddly, in all my years I have not tiled, but am very good at plaster work so I have that going for me.

I have a 74"x74" bathroom floor that needs to be tiled. My wife purchased some so-so 13"x 13"x 5/16" porcelain tile to cover it. The newly replaced 3/4 plywood floor has a hump in the middle where a beam runs. the hump is offset slightly to one side, and probably is something like 1/4 inch in 3 ft. For some time, I have been thinking I can work around the hump when I go to set the tile, but am starting to have doubts. Unfortunately, I already have added a 1/4 inch of underlayment and the thin Kerdi Ditra mat for height and long term stability. I have purchased Tuscan clips and Laticrete 317 to put the tile down.

I don't like most of my options:
- I have been consulting with a local tile store that does have an excellent reputation, they have recommended cutting the Ditra off the hump to gain some space and tile over the underlayment/Ditra.

- Remove the Ditra and underlayment (shoot me now please), use some kind of leveling solution and then replace the Ditra and start over using a medium bed mortar to help out my flatness issues.

- Move forward and try to work with my mortar (Laticrete 317) to make up for some of the leveling issues with some creative backbuttering and Tuscan clips. Pray to the tile gods that the result doesn't go south as it dries?

The whole project is badly behind and I really need to move forward as soon as possible. Any advice is welcome even if it is just start over and do it right. I will post pictures if I can take any that illustrate the hump issue.

Thanks,
Jon

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Unread 09-22-2016, 04:15 PM   #2
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Okay, I think I may be able to answer my own question. I believe I can go in and rip out the underlayment and Ditra in the humped area and then patch in a new area of Ditra with no underlayment and significantly flatten the floor. Be some work, but less than ripping the whole thing out and starting over.

That is the best plan I have come up with to date. Hope it works...
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Unread 09-22-2016, 04:42 PM   #3
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Jon,

Welcome to the forum.

1- Before we get to far - did you check your floor joists on our handy dandy deflecto meter (linked above) to see if the floor will handle tile ?

2- What is the 3/4" plywood over ?

3- What is the grade of 3/4" plywood?
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Unread 09-22-2016, 04:57 PM   #4
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The best way would be to pull up the subfloor and fix it.

Otherwise I would opt for plan #1 but with this twist:

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Unread 09-22-2016, 06:58 PM   #5
Eldudarino
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Thanks for the replies!

The subfloor is 23/32 OSB tongue and groove nailed and glued with urethane adhesive. It has 1/4 OSB over it, screwed every six inches. The subfloor is supported by blocking on the edges and is over 2x8's - 16inch on center. There is a beam just off each side of the room and down the center. The subfloor is one of the few things I had a carpenter buddy do and didn't finish myself. Knowing what I know now I would have shimmed the subfloor to level (flat) or at least done something to bring the 1/4 inch to level.

I have begun cutting into the ditra and once it is removed in the problem areas the straight edge will indeed sit flat. There are two other areas (marked with rectangles where there is an issue but given their odd location I am going to assume the cause is my own poor application of the modified mortar that holds the ditra down.

In the attached picture the marked areas are humps, the main one is just off center to the right where some ditra is cut away. Once the Ditra is out the straight edge sits flat side to side. The hump is that narrow.

P.S. After looking at things more carefully, the straight edge is lifted about a 1/4 at the wall from either side of off center hump. The floor is flat along the other dimension except for the two odd spots that are marked on the left.

P.P.S - I just looked at the deflecto-meter and I am good. 1.5 inch 2x8 (old growth from 1952 I live in PNW) 6ft between beams). The floor has virtually no movement from any load I can put on it, including me holding my very overweight cat.
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Unread 09-22-2016, 10:28 PM   #6
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Update: I am an idiot. We didn't screw the 1/4" underlayment down as well as we thought. My wife and I recalled that we ran out of screws and didn't end up getting one every six or four. Once I got into it I discovered that it was popping up from warpage. Possibly exacerbated from the moisture of putting down the Ditra. The whole thing is coming up. We will rebuild it from the subfloor up, use better tile. Level the floor proper. If you don't have time to do it right when do you have time to do it over. Blah, blah, blah.
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Unread 09-23-2016, 04:10 AM   #7
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Jon, that "disaster" was actually a blessing in disguise. You shouldn't have any 1/4" layers in the subfloor makeup, unless it's 1/4" cement board. I've never seen 1/4" OSB, so if you found some, it's kinda rare.

What you'll want to do now is attach a minimum of 3/8" plywood to the 3/4" subfloor. We typically recommend 1/2" plywood, simply because 3/8" is so thin and difficult to work with (kinda like trying to lay a potato chip flat).

I would recommend following the guidelines in this article to attach the plywood to the subfloor.
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Unread 09-23-2016, 08:53 AM   #8
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I was thinking Tiger Mountain's suggestion looked OK but assumed your 1/4" underlay was cement board. IF NOT I think you have saved yourself much greater aggravation by discovering now that 1/4" wood underlay is a potential disaster.

Yours is a small bathroom like mine that I recently reno'd and I'll give you my thinking. First I think some of the potential expansion issues are diminished in a small room and years ago nothing more than CBU would probably have been used. I did that on my other bathroom 20 years ago and it is still fine.

On my latest bath I deviated a little from standards... I fastened my plank subfloor with many screws and then 1/2" ply with many more screws and then used some floor leveling in my shallowest corner to get within about 1/8" or less over 4'. I used a surface applied crack isolation/membrane over the wood and leveler to keep that thin in my case. It worked great so far and the only watch out I had was it took more than maybe 48 hours for the modified thinset to really dry. You could see the moisture in the stone for 2+ days.

A also had a hump but it was slight and I chose there to start. In that area I went a little lighter on the thinset (angle of trowel) and just back buttered to key in with no excess still getting 90%+ coverage. As I worked away from my slight hump and into my deepest corner I had to put thinset on heavier and backbutter heavier. I thought I might need another deeper notch trowel but didn't. It worked out great and did not see any shrinkage or change with the thinset from lighter to heavier.

I didn't using any leveling system gadgets at all. I simply scooted a small piece of stone back and forth across my tiles as I laid them and you could hear a little click if they were out of plane. I took up quite a few to add more thinset on one side or the other and pushed down on quite a few corners and had to clean thinset out of the grooves there. That was some extra effort but it's a small room with < 50 12" tiles and I bet every joint is <0.010", no kidding, no detectable lippage. If your tiles are not as perfectly flat as stone you don't even need to be that careful. Place various one finished side to finished side, turning 90' a couple times and see how flat they really to one another to get some idea. If the are warped a little you might even use some of that unevenness to your advantage with some planning.
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Unread 09-23-2016, 12:32 PM   #9
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Okay, 1/4" OSB was wrong, it is just sanded ply and it was not primed. I would like to add that the modified thinset and the Ditra had nooooo problem sticking to everything. Both have been very challenging to get up.

I understand that the doulble wood layer is not the way to go, especially the way that I was doing it. That said, adding 3/8 in ply over the base 3/4 isn't possible unless I abandon the Ditra layer all together. I am going to assume that 1/4 cement board or hardiback would probably be a better choice at this point, particularly if some kind of leveling agent is used.
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Unread 09-23-2016, 12:53 PM   #10
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How concerned/likely are you to have water on the floor ? Enough to want water barrier ?

Is your Ditra ruined after taking it up ? You could probably do some leveling with cement-based material then put the Ditra back over that. If you have access from below you could add ply between your joists to stiffen the floor somewhat that way. Not as good as continuous layer above but helpful between the joists.

I studied the Ditra and a few others and decided against it for it's thickness. And again I think less necessary in a smaller space. If you're trying to keep the overall thickness thinner to match a transition to other floors you might do a surface applied membrane instead over the 3/8" ply for water proofing.
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