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mark942 02-02-2008 05:40 AM

Bump in underlayment
I laid 2 layers of 1/4" dura board over 3/4"rough sawn sub floor. Used thin set between each layer.After laying out the initial pattern I noticed a slight unevenness(1/8" to 3/16") in a few 18"x 18" tiles I am installing.Can I just go with thin set to level it all out with out any cracking of tiles in the future? Thank you in advance.

JazMan 02-02-2008 10:31 AM


Too bad you didn't ask questions before you started instead of now.

What you've done so far is wrong. :(

First you NEVER install any CBU direct over plank subfloors. You should have install plywood first then your 5/16" thick Durock.

The substrate has to be very flat for tile, especially with large tiles like that. You're always going to have some lippage, but 1/8 or 3/16" is a lot! Thinset can be used to fix a small area, but not an entire floor. Depends if you have one small depression or a peak.

If you can get the floor flat by using SLC's or Portalnd based patching cements, AND if your subfloor framing is up to speed for tile, I might say just go ahead anyway in spite of the lack of proper underlayment? (at your risk of course)

The basic standard for floor flatness is within 1/4" in 10 ft, AND 1/16" in 12" of required plane. With large tiles like yours I would like within 1/8" in 10 ft, especially if the tiles are smooth and shinny. You also need special thinset, not your basic stuff.

Go measure and tell me: the type, size, species, grade and the unsupported span of the joists. Also when was this house built and where is it?


mark942 02-02-2008 01:18 PM

Figures I did it wrong. I was told to do it this way in a home depot seminar. When I was told this I had to ask twice just to make sure I understood. My own thoughts were to use 5/16 plywood, then use the good old cement board or ( Wonderboard) for my underlayment. As to my Joists. 12" OC rough cut 2" x 12" # 1 Pine. The span to bearing wall is 12`. House was built in 1981,,,,,,Hence the #1 Pine. The bump I am talking about is just one tile. The rest of the tiles seem to be laying flat. My question has been answered by you with the mention of @ own risk. The thin set I am going to use is #2 modified. Longevity I guess will be time telling. Thank you again. (Pennsylvania)

JazMan 02-02-2008 02:54 PM


Attending a 'How-To Class' at one of those big box stores was your first mistake. You have to know what you're talking about to teach others.:censored: This is true in MOST cases with tile, not too sure about other classes?

I don't know what #2 thinset is. You should be using a medium-bed thinset mortar with a large notched trowel, maybe 1/2x1/2 or larger? What brands of thinset can you get? Are you going to the orange place again? Aren't there any real tile shops near you?


Bud Cline 02-02-2008 03:29 PM

I suspect the "#2 thinset" he is referring to is the Custom brand thinsets labeled for Home Depot's use. #1 is prep goods, #2 is thinsets, #3 is grouts, some dumb stuff like that!!!:)

JazMan 02-02-2008 06:14 PM

You're right Bud, I remember the numbers now.:thumbsup: Thought maybe he escaped from the orange place and went to the red-vest place and was using UltraFlex 2? :laughing:


mark942 02-07-2008 05:18 AM

Flexbond thin set which is fortified is what was suggested for me to use.Dura board is the underlayment I was also directed to use.All of which is new to me.Cement board and medium bed thinset is what I am familiar with.Thats use with 2 tile floors I have done in the past.Thought I would get past the "Teach a old dog a new trick" statement.Framer by trade here gents,by the time tile gets in a building I am 1/2 way through another job........:laughing: So now I am sitting here with thinset not worthy,underlayment that doesnt level out the floor,1/8 to 1/4 notched trowels,Which are to small and a Wife that is wondering when I am going to finish the bathroom................:censored:

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