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Old 07-14-2007, 11:24 AM   #1
jp_fizzer
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How flat does a floor have to be?

I was about to start tiling my bathroom floor today but when I started laying down a few of my tiles in a "dry run" I noticed some of them would wobble or rock. I hadn't thought the concrete floor would be a problem since it feels flat to walk on it. After putting my 24" level all around I would say the worst areas (2 of them) have about a 1/16" gap under the level. I am curious if the thinset will just fill this space and provide a solid underlay for the tile or will I end up cracking tiles? I am using 16" x 16" porcelain tile and Megalite crack prevention mortar. If this will not be okay should I go pick up some self leveling cement?

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Old 07-14-2007, 11:42 AM   #2
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Do you have a longer straight edge than that? 2 ft just isn't enough to check for flat, 4' is better, 10' is to code. It doesn't have to be a level, just a straight board will do fine. Tile don' care about level, just flat, and those 16's will really want flat. That being said, if your within 1/8" or so, thinset will easily take of that as it's being set, although if you screeded it first with thinset to get it flat, the setting will go much easier and quicker.
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Old 07-14-2007, 12:32 PM   #3
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Agree w/ Jay,

A 2' edge is almost no edge at all. Not really very definative. Your goal is no more than a 3/16" variation in ten feet,

Suggest not to use a straight edge. They bow easily and give very false readings. 8' foot levels are available at most better lumber yards, forget the Homer route.

If you are very careful you may get away with using a string. Just watch that nothing contacts the string between holding points. The string will not rock.

Also using a good sized piece of glass check that the tile is not warped.

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Old 07-14-2007, 12:47 PM   #4
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Unfortunately the bathroom is only 7' x 7' and with the vanity and shower stall it doesn't leave much room for checking with large straightedges. With a 4' straightedge moving all around the room there is nowhere I can fit my 1/8" spacer underneath. I brought my 8' straightedge in and although I can only go diagnonally in one place that does happen to go right over the worst part. I can just barely fit my 1/8" spacer under this in about a 6" section of the 8' length.

The floor does not feel uneven when walking on it which is why I had never checked. I was shocked to feel a couple of the tiles wobble when I laid them on the floor.

In addition to the cracking concern I imagine having an uneven floor is going to make it much more difficult to make sure every joint (particularly corners) at the exact same height. I suspect this would be important or you would definitely feel the edge on your bare feet.

Jay, you mentioned "screeding" with thinset first. Does that mean lay down a layer of thinset in the low spots sort of the same as using self leveling cement? If that is an accepted practice that sounds like it would be my best route? I guess I use the flat side of the trowel
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Old 07-14-2007, 12:52 PM   #5
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Well, technically, the usual industry standard is no more than 1/4" variation in ten feet and no more than 1/16th" variation in any one foot.

The bigger the tile, the flatter you want the floor for ease of installation.
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Originally Posted by Tom Tee
Suggest not to use a straight edge. They bow easily and give very false readings.
Not sure what you mean by that, Tom.

I have eight and ten foot straight edges in my trailer that are 'bout as straight and rigid as any level I've used.

My opinion; worth price charged.
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Old 07-14-2007, 01:07 PM   #6
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Thanks CX, so what you are saying is that by industry standards I am okay?

Does that mean that the thinset will take care of the uneveness? I did buy the most expensive thinset I could hoping that it would pay off. Should I put in on a little thick to help fill the gaps? The instructions say to use a 3/8" notched trowel but I imagine I could make it a little thicker by combing the thinset at a 90 deg angle instead of at the usual 45 deg angle?
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Old 07-14-2007, 01:09 PM   #7
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Based on what I've read so far you are better off with the thinset than SLC.
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Old 07-14-2007, 01:11 PM   #8
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Jeff,
I had a similar situation when I dry ran my 13x13 porcelain (http://johnbridge.com/vbulletin/show...t=47647&page=5)

I set them last night and was able to get everything flat with just the thinset. I used a 1/4 x 3/8 trowel to give me a thicker mortar bed. I had to pull up a few tiles and spot on some thicker thinset in a corner when it was too low, but if you only spread a small area of mortar at a time (I only spread for one tile at a time) it's not too bad. Once I got going I would lay a tile down before spreading the mortar to see how it compared to the tile next to it, this gave me an idea of where to spread the thinset thicker.

It takes a little longer this way, but I figured given my lack of experience w/ SLC it was probably much quicker in the long run. I also found the Tavy Tile Puck very helpful in getting everything flat.

Good luck!
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Old 07-14-2007, 01:20 PM   #9
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Thanks Stephanie, since this is my first tile job ever, 1 tile at a time sounds like a good pace anyways.

Would using Ditra help at all or would the uneveness just translate right up through that?

Sounds like my best bet would be to make up a little thinset (it will be good practice anyways) and use it to fill the low spots a bit. Then, after it dries I will do what Stephanie says.

thanks everybody!
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Old 07-14-2007, 05:32 PM   #10
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As always, the industry standard is a minimum, Jeff. Flatter is always better.

Filling in the low spots with thinset to make the floor flatter will pay off come time to set tile.

All leveling is to be done prior to installation of Ditra. And your guess is correct, the Ditra won't do a thing to level the floor, it's only purpose it to provide some isolation from the normal movements in the subfloor, be it concrete or wood.

My opinion; worth price charged.
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Old 07-14-2007, 05:47 PM   #11
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Thanks, I have already started leveling. I made up a batch of thinset and spread it around. What seemed like a lot in the bucket didn't go to far once it was spread on the floor. I guess I will wait until this dries and then put on another layer. I am surprised how long it takes to set, I put it down 4 hours ago and it is still wet enough I could mark it with my finger. At least this gives me a good indication of how long I will have to make adjustments to the tiles. Since the instructions say to only make as much as you can use in 15 minutes I kind of assumed that meant it would be set quickly.

thanks.
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Old 07-16-2007, 10:54 AM   #12
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I thought the standard was 1/8" over a 10 foot span, but what do I know? the bottom line is the flatter the better. If your variations were less than 1/8" I think i would have opted to just Level as you go and build up any imperfections with the thin-set, but there is more than one way to skin a cat.
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Old 07-16-2007, 12:11 PM   #13
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how much Thinset should be under the tile???

So, after somewhat leveling the floor with thinset I decided it was time to lay the 9 field tiles. I made up what I thought was a fairly large batch of thinset to do at least 4 or 5 tiles and went to work. I am pretty sure I made it to the right consistency because when I loaded the margin trowel and turned it over it did not fall off. I laid the thinset down, combed it with the 3/8" notched trowel, placed a tile and kind of squished it into the thinset. I then repeated the same on the next tile and ran out of thinset. This kind of surprised me because I thought it would go alot further. Plus, with all this talk of needing a flat floor I guess I was expecting to have a much thinner layer of thinset under the tile. With as much thinset as I was using I could have a crazy floor and it seems the thinset would compensate. Which makes me wonder if I am using WAY too much thinset. Now that it has set overnight I measure the thickness of the thinset to be 3/16" thick. The tile itself is 5/16" thick 16" x 16". Does this sound about right or should I be putting down less thinset?

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Old 07-16-2007, 12:43 PM   #14
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you just want to ensure that you are getting good coverage (minimum 80%). my guess is your thin-set could be a little looser and still be ok, but id opt for too thick over too thin any day. did you pull up any of the tiles to check the coverage? Ideally you want to trowel in one direction (left to right) and set the tiles across the lines and compress those lines by pushing down then sliding the tiles back and forth. this will ensure you get the coverage you are looking for. a thin layer of thinset really wont help you achieve this good coverage, which is the reason for the sized notched trowel you are using. And according to industry standards u shoould be back buttering, but a good sized trowel and a really flat floor can eliminate that step.
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Old 07-16-2007, 02:11 PM   #15
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1/4" in ten feet, no more than 1/16th" in any one foot is what I think you'll find is the current standard, Robert.

My opinion; worth price charged.
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