Underlayment Questions

rrweatherMarch 14, 2013

I am in the middle of a kitchen remodel and need some guidance. Our original kitchen was linoleum when built in 1968, which the original owners covered in carpet. After demo, the original linoleum delaminated and came off in layers. We scraped as best we could but there is some remaining paper/felt layers permanently bonded to the subfloor.

We are planning on installing slate tiles so that is what we were preparing the subfloor for. Our subfloor is tongue and groove 1 1/2 inch planks with a 3/8 or 1/2 inch particle board top.

A friend of mine is a contractor that works for investors that flip homes. He recommended using hardibacker on top of the subfloor. I watched the hardibacker installation video and read the directions. I questioned the use of thinset under the hardibacker to my buddy. He said they never use it and not to worry. He's never led me wrong before so I proceeded to lay down the hardibacker using the recommended screws in the recommended spots. I then proceeded to install our base cabinets on top of the hardibacker.

As our slate should be delivered tomorrow, I began anticipating a weekend of installation. While browsing for some slate sealer, I came across a post describing the use of hardibacker without thinset. There were then repeated posts about how your tile will crack if you do not add the thinset under the hardibacker.

While further looking into the use of hardibacker without thinset, I came across Ditra, which I wish I had seen before.

So, I am fairly certain I will have to remove my base cabinets today, and at a minimum, pull up the hardibacker to at least add thinset under it before putting in back down. I am now wondering if I am going back to square one, maybe I should just opt for Ditra instead and write the cost of the hardibacker off as a learning experience. We are doing the laundry room and a bathroom in the same slate after the kitchen is done so if I switch to Ditra, the labor savings down the road would be significant. My one question is whether thinset will adhere to the leftover paper/felt from the original linoleum that is permanently bonded to my subfloor. Whether I go with re-using the hardi or Ditra, should I cover the leftover linoleum felt with ply to ensure a good bond or will the thinset stick to anything?

Any advice would be appreciated. I hate going backwards but I want the floor to last.

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Perhaps a tile and stone expert will chime in here. I have concerns about your particle board underlayment, as it adds almost nothing to the structural integrity of the floor system. A quick call to the technical folks over at Schluter might tell you if you can use Ditra in your application. Have a look at the Schluter site for good information. There are specific recommendations for what type of thinset to use below the Ditra and between the Ditra and the tile, as well as specific trowel teeth requirements.

I would suggest that unless someone comes in here with expert advice, that you go over to the John Bridge Tile forum with your questions.

However, for now, consider taking up the particle board and installing the right structural underlayment. I don't think particle board is acceptable under ANY circumstances, but I could be wrong.

Find the Tile Council of America handbook on the internet or search for "tile or stone over wood substrates".

Finally, ever consider installing your cabinets AFTER the slate is in?

Here is a link that might be useful: Schluter Ditra

    Bookmark   March 14, 2013 at 6:24PM
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While I appreciate that your contractor has never steered you wrong before, he is clearly out of his area of expertise when it comes to tile floor installation. First of all, the particle board needs to go. Plywood is required. Second, never, never, never install hardibacker or other cement board without thinset under it. The thinset is not there to stick the board down. It is there to fill any and all voids so that you have a solid, non-flexing, dimensionally stable base for your tile. Failure to use thinset = voids = flexing = cracked tile or grout.

If you are concerned about the height of the floor, Ditra is a great choice as it only ads 1/8" of thickness. You still need to remove the particle board and install plywood.

The forum mentioned above is populated with tile experts, lots of them. They are in invaluable resource.

    Bookmark   March 14, 2013 at 6:47PM
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I echo the above post, but I didn't see anything about particleboard in the OP. If it IS, then it has to go. PB will swell up big time if any moisture gets to it...and yes, you absolutely MUST have thinset beneath any CBU. Mix it "loose" and screw it down immediately.

    Bookmark   March 21, 2013 at 12:47AM
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