Best option over diagonal subfloor???

Discussion in 'Ceramic and Stone Q&A' started by motiger2, Nov 1, 2010.

  1. motiger2

    motiger2 New Member

    First, thanks in advance to those who choose to help me out.

    Here is my current situation involving a bathroom remodel.

    I have a bathroom that is about 7'x10' at the widest points. The sub-floor is diagonal lumber (3/4" x 11 1/2") on 2x10 joists, spaced anywhere from 10" to 18" (it varies for some reason, probably due to some HVAC ductwork in the floor.

    My intention is to install porcelain tile over _____?

    The old tile that I ripped out was a small 1" and 2" square pattern on a mortar bed on steel mesh. I never noticed any cracking whatsoever and the floor seems very secure (firm).

    The flooring at the hallway transition is 3/4" hardwood w/ carpet on top, so I have some height to play with.

    I've read up on the Ditra and I'm uncertain whether to go w/ Ditra or cementboard. I was planning on laying 3/8 or 1/2 exterior grade plywood over the diagonal planks, then Ditra. Or, should I just lay down cementboard on top of the diagonal planks. I'm not loaded, but given the relatively small size of my bathroom (75 square feet), I'd rather go with the quality choice over the most inexpensive choice. I'd rather do it right and have it last. I'm not interested in just meeting minimum standards. Any assistance would be greatly appreciated.

  2. Kman

    Kman Tile Expert I Support TFP Senior Member Published

    Hello, Dave. Welcome to TFP.

    Provided your joists in the area of the bathroom don't span more than about 14' without support from underneath, then I would start with a layer of 1/2" exterior grade plywood. You'll want to make sure that the face grade is A, B, or C. In this case, a D is a failing grade.

    Make sure the diagonal subfloor is in good shape because you'll want to screw the plywood directly into the subfloor, but not into the joists. Do not use glue between these two layers, and use this article Position of Underlayment to Prevent Cracked Tile and Grout - Schluter-Systems as a guide to installing the plywood. You'll need only three sheets and a few pounds of 1" or 1 1/4" deck screws, so it's not that great of an expense. Don't cheap out and use drywall screws, they won't hold.

    You can use either Ditra or CBU. I like Ditra for its ease of installation. While the cost is higher (maybe even twice as much as CBU), the fact that you don't have to drive a couple hundred screws helps to make up for that. You also don't have the dust that you get from cutting CBU, and you have maybe 10lbs of Ditra to carry as opposed to probably 150 lbs. of CBU. CBU also requires that you tape and mud all the joints which is not that great of an expense, just an added step you don't need with Ditra. If you don't mind the extra work and want to save around $50-60, use CBU.

    Hope I answered all your questions. Feel free to follow up with any more concerns or questions as your job progresses. :)
  3. motiger2

    motiger2 New Member

    Wow, thanks for the quick response. You already helped a bunch because I would have sunk the screws into the joists under the assumption that "more is better".

    Assuming the use of Ditra, which side of the plywood faces up? I'm assuming the roughest side in order to create the best bond w/ the thinset.

  4. Kman

    Kman Tile Expert I Support TFP Senior Member Published

    I've always put the smooth side up. Most of the plywood I've bought has a stamp on it that denotes "This side up", so I never really had to think about it.

    Important thing is to slightly dampen the surface of the plywood prior to putting down thinset so the plywood doesn't draw the moisture out of the thinet prematurely.

    There are some installation videos on Schluter Systems - Homepage - Schluter-Systems.

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