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Question - new tile hearth pad - skip plywood underlayment?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Bunsen, Nov 20, 2012.

  1. Bunsen

    Bunsen New Member

    Joined:
    Nov 3, 2012
    Messages:
    16
    Hi guys,

    Well I've decided that instead of adding an extension to my current hearth pad for my soon to be installed Lopi Republic 1750, I'm going to tear out the current brick hearth pad and install a new tile one. This stove only requires ember protection, so I don't need very much under the stove.

    I have a very specific question I'm hoping someone can answer. Normally under tile One would have the subfloor, 3/8" plywood, cement board, then the tile. I'd like to skip the plywood and put the 1/4" cement board directly on the subfloor in order to keep the height of the tile roughly even with the height of the finished floor it will meet with.

    The floor joists are true 2x6s, to which I've sistered up modern 2x6s in order to stiffen the area under the stove. This was mostly just for piece of mind, not because I detected any sort of problem. The stove is in the corner of the house, so there shouldn't be much deflection in the floor at the point anyway.

    So my question is this; Will I be OK skipping the plywood? No one will be walking on this surface, but it will have a 430lbs stove on it. I don't want to install all the tile and then find that it all cracks and breaks when I sit a stove on it.

    I'll use mortar under the cement board to give it a 100% footprint, and screw every 6". I do not detect any movement in the subfloor when I walk on them. No creaks or sounds.

    Any thoughts?

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  2. begreen

    begreen Mooderator Staff Member

    Joined:
    Nov 18, 2005
    Messages:
    47,420
    Loc:
    South Puget Sound, WA
    I would want more rigidity there. Tile is not forgiving of movement. But I'm not an experience tile layer. You might ask this question over at the JohnBridge tiling forum where there are some experts.

    http://www.johnbridge.com/vbulletin/
  3. Augie

    Augie Feeling the Heat

    Joined:
    Nov 8, 2012
    Messages:
    468
    Loc:
    North Of Canada
    Plywood/osb under layment FTW. 4x8 OSB 5/8th thick is only another $20.00. Why not add it and use a construction adhesive to bond the backer board to it.
  4. David Tackett

    David Tackett Member

    Joined:
    Oct 17, 2012
    Messages:
    178
    Loc:
    Waynesburg, Kentucky
    Every time I lay tile, I always use cement backer board directly on the sub-floor. I have never added a piece of plywood and have never had an issue. .
  5. begreen

    begreen Mooderator Staff Member

    Joined:
    Nov 18, 2005
    Messages:
    47,420
    Loc:
    South Puget Sound, WA
    Construction adhesive is not heat safe. It's better to use screws at the interval specified by the mfg.
  6. bag of hammers

    bag of hammers Minister of Fire

    Joined:
    Jan 7, 2010
    Messages:
    1,168
    Loc:
    Northern ON
    Tile setters can speak best to this. I'm ok with structural overkill when I can do it for $30. I have existing t&g subfloor on 2x12 floor joists 12" o/c, Already very solid but I added 3/4 ply for the hearth over that, then durock, then tile. I trimmed all around and stained to finish. I like the extra ridigity so i can put a very heavy stove there if i wish and i think it helps distribiute the weight better / evenly across 5 joists. But I like the bit of a raised hearth so this works for me. YMMV....

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