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Old 01-01-2008, 11:23 AM   #1
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Tile underlayment - mortar bed question


Hi and happy new year everyone!

I am going to be putting down a tile underlayment in the next couple days. It's my understanding that I need to lay down a mortar bed beneath the underlayment. My question is, when I screw down the underlayment, isn't the mortar going to want to squeeze out from the screwed area leaving the mortar bed uneven throughout the floor causing low spots? I'll be putting screws every 8 inches.

Also, should I try to avoid kneeling or standing on the underlayment while I'm screwing it down? It seems this this would also cause uneven distribution of the mortar.

First time doing this so I don't know what to expect.


Thanks!

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Old 01-01-2008, 11:53 AM   #2
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Tile underlayment - mortar bed question


The backerboard sheets are large enough that it won't be a problem. Make sure you use an unmodified thinset below the backerboard.

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Old 01-01-2008, 11:59 AM   #3
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Tile underlayment - mortar bed question


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The backerboard sheets are large enough that it won't be a problem. Make sure you use an unmodified thinset below the backerboard.
Why do you use a unmodified thinset under the backer board?
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Old 01-01-2008, 12:04 PM   #4
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Tile underlayment - mortar bed question


Because the manufacturers of the backerboard say so. (and I figure they are a lot smarter than I am)
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Old 01-01-2008, 12:57 PM   #5
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Tile underlayment - mortar bed question


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Because the manufacturers of the backerboard say so. (and I figure they are a lot smarter than I am)
From the Hardibacker instructions: Apply a supporting bed of mortar or modified thinset to subfloor using a 1/4" square-notched trowel.
I know you must use unmodified thinset between Ditra and tile, but not for Hardibacker or Durock.
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Old 01-01-2008, 01:56 PM   #6
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Tile underlayment - mortar bed question


Wow I stand corrected, THANKS Jerry, I have been using wonderboard for many years, and was told by mfg rep to use unmodified thinset. Just got off of websights and you are right, I have many hundreds of installations out there without any problems so far But I will change to modified immediately
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Old 01-01-2008, 01:56 PM   #7
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Tile underlayment - mortar bed question


What exactly is "unmodified" thinset? What am I looking for when I go to purchase this?

I looked at the home center and their thinsets say nothing about being unmodified. The premixed one actually says not to use it between a tile underlayment and subfloor.

USG recommends Type I organic adhesive, latex portland cement mortar, or
dry-set mortar for adhering Fiberock to a subfloor.

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Old 01-01-2008, 01:59 PM   #8
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Tile underlayment - mortar bed question


Modified thinset has a latex additive for improved bonding. I happen to like Mapei's Ultraflex 2 or 3 NEVER use premix
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Old 01-01-2008, 04:03 PM   #9
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Tile underlayment - mortar bed question


I am putting ceramic tile down on my bathroom floor (45 sq ft), should i use hardi, wonderboard or dura-rock backerboard?
thank you
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Old 01-01-2008, 06:51 PM   #10
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Tile underlayment - mortar bed question


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I am putting ceramic tile down on my bathroom floor (45 sq ft), should i use hardi, wonderboard or dura-rock backerboard?
thank you
steve
I use Hardi. Easy to cut. Built in screw patterns. Edges are usually sharp and square.
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Old 01-01-2008, 06:54 PM   #11
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Modified thinset has a latex additive for improved bonding. I happen to like Mapei's Ultraflex 2 or 3 NEVER use premix
Second that. Mapei is good stuff. I used kerabond and Keralastic to put down the kitchen floor in my last house. I feel sorry if they ever need to take it up.

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