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My home is a single story, about 1200 square feet, and is built on a crawl space foundation with a plywood sub-floor. The people we bought the house from had flipped the home after a foreclosure, and just put in some cheap carpet for most of the house and cheap vinyl in the kitchen and bathrooms. We'd like to put in new flooring, and aren't too picky on what it is exactly. I've installed several DIY vinyl and laminate floors, as well as some hardwood a while back. I've also tiled several showers/bathrooms.

My main question is about porcelain tile in a crawlspace home though - I've heard mixed answers from different people who were experienced at flooring. Some people have told me porcelain tile planks cannot be placed in a home with a floor like this because it will flex too much and crack the tiles. But then others have explained that as long as I install cement board before laying the tile, it will be fine.

TL;DR: Is there any method of installing porcelain tile planks onto a plywood sub-floor that will prevent the tiles from flexing?

EDIT: I’ve included an image of under my house, showing the cinder block supports and the wood supports. I think they’re 2x12s at the point where they intersect the blocks, I’m not sure what the size of the perpendicular beams are, but underneath the insulation I think they may also be 2x12s. I think they span a good 12' or 15'.

Crawlspace Supports

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  • That advice as you describe it makes absolutely no sense. A crawl space is not different whatsoever than a full basement with respect to structure, other than foundation wall height. Are you sure there wasn't more to the recommendation than it being a crawl space?
    – isherwood
    Commented Jun 24, 2019 at 14:32
  • @isherwood Maybe I'm not using the terminology correctly. The structure of my floor is such that it is just a plywood floor several feet above the ground. The way it is currently braced and supported gives it a bit of flex, so if you were to walk heavily or stomp through the halls, you could feel it a room over or shake the walls potentially. I'm not sure if that clarifies your question.
    – ConcernedHobbit
    Commented Jun 24, 2019 at 14:35
  • You haven't told us anything about your subfloor or framing, so....
    – isherwood
    Commented Jun 24, 2019 at 14:40
  • @isherwood Gotcha. I'll get under the house later and take a closer look at it. Is there anything particular you'd want to know that would be helpful?
    – ConcernedHobbit
    Commented Jun 24, 2019 at 14:41
  • Joist type, size, span, spacing and condition. Subfloor composition, thickness, and (if solid wood boards) orientation with respect to the joists.
    – isherwood
    Commented Jun 24, 2019 at 14:42

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The height of the air below the floor on which the tile rests is completely irrelevant. The only real concern is the integrity of the subfloor. Have at least 1-1/4" thickness of appropriate subfloor material (or whatever the particular tile product and mortar calls for) over suitable joists (so the tiles and grout are supported well and won't crack) and be happy.

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    Which is to say - preventing the tiles from flexing and cracking is a matter of preventing the subfloor they are on from flexing, which comes from adequate construction of that sub-floor for the purpose.
    – Ecnerwal
    Commented Jun 24, 2019 at 14:35
  • So would it be better to place a fair amount of the cement backer board onto the floor, or try and additionally brace up the floor from underneath with 2x6s etc.? Or both?
    – ConcernedHobbit
    Commented Jun 24, 2019 at 14:37
  • Downvoter, care to comment?
    – isherwood
    Commented Jun 24, 2019 at 17:42
  • I'm not a downvoter, but 1-1/4" thick subfloor seems excessively thick. I thought 3/4" T&G plywood on good joisting would be sufficient.
    – Jim Stewart
    Commented Jun 24, 2019 at 20:02
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    No. Ceramic and porcelain always require more, and 1-1/4" is considered a minimum. Even polymer-modified mortars tend to want that much. 3/4" plywood on even old-school 16" centers leaves too much flex. Get out to 19.2" or 24" and no way.
    – isherwood
    Commented Jun 24, 2019 at 20:03
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I fully agree with Isherwood that it depends on the stability of the floor. I have attached tile directly to T&G flooring but it was a bit tighter or more solid looking than yours looks like. I would put down backer board in your case I would trowel a layer of thin set with a large notch trowel put the backer board down and screw the backer to the flooring doing the entire floor with backer. This will make your new floor +1/2” thicker but it will last a lifetime. If your floor is really solid you may be able to use larger tiles without backer and not have problems but I would caution not to use less than 2x the width of the existing boards to prevent problems with slightly uneven boards larger tiles are better in this case. After many years of putting tile down I almost always use at least a thin backer with the above method.

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