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Planning on laying tile in our house. We're putting down cement backer board, and then ceramic tile. The tiles are 7in x 20in.

My question is, should the tiles be placed so they are perpendicular with the floor joists, or can they be laid parallel to the joists?

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  • Is there something about the floor that makes the direction of the joists apparent?
    – JPhi1618
    Commented Mar 12, 2018 at 14:47
  • I can go down in the basement and see what direction they are going. The reason I ask is a friend was telling me that with putting down hardwood you want to go against the direction of the joists to minimize sagging. He didn't know if the same was true of tile, especially if using backer board.
    – AndyD273
    Commented Mar 12, 2018 at 14:49
  • Well, what I was really asking was if there are visible ripples or warping in the floor that are so bad you can see where the joists are. Like is the subfloor in bad shape or already sagging.
    – JPhi1618
    Commented Mar 12, 2018 at 14:51
  • @JPhi1618 The subfloor does not appear to be in bad shape, though I do need to pull up the existing flooring. I have a few weeks until the tile arrives, so I haven't torn up the carpet/laminate yet.
    – AndyD273
    Commented Mar 12, 2018 at 15:05

2 Answers 2

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Your subfloor should be rigid enough that joist orientation isn't a factor in your flooring design, be it hardwood or tile.

Follow the specifications of your mortar manufacturer, or go with 3/4" solid boards, plywood, or OSB plus 1/2" plywood or cement board as a minimum.

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  • Ok. The house is only around 15 years old, and while I haven't had a chance to double check the thickness of the plywood I believe it is 3/4". I was planning on going with 1/2" cement board. I wasn't sure if I could get away with 1/4" as a way to avoid raising the floor level too much or not...
    – AndyD273
    Commented Mar 12, 2018 at 15:10
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    Most mortar manufacturers want 1-1/4" minimum. If you happen to have 16" joist centers (unlikely), you could maybe go with 1" subfloor.
    – isherwood
    Commented Mar 12, 2018 at 15:11
  • I agree with 1-1/4" especially for large tiles if your base is not solid the tiles may crack. When I have limited thickness I will usually trowel some thinset under the backer to provide a little better base and screw it down well.+
    – Ed Beal
    Commented Mar 12, 2018 at 16:38
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    The thickness of the cement board is supposed to be irrelevant. Meaning, it does not add any stiffness to the floor; this is what I have read in manufacturer's literature. It is only the wooded framing & decking system which provides required stiffness.
    – StayOnTarget
    Commented Mar 13, 2018 at 0:24
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    Then why make it in half inch at all?
    – isherwood
    Commented Mar 13, 2018 at 1:37
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They make backer 1/2" for wall applications where you want a smooth transition from backer to drywall so your bullnose tile can lap and hide the transition seam. About the subfloor and framing. If you have an 1/6" of an inch or greater of deflection in the floor then your floor tile will fail. Guarantee it. No matter the orientation of the subfloor, backerboard or the floor joist.

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    Commented Sep 8, 2020 at 23:32

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