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Community and Q&A

Basement subfloor

Dan Baker | Posted in General Questions on

Hi, I would like to put a porcelain tile floor in my walk out basement. I live in RI which is zone 5 I believe  and there is no insulation under my slab. My plan right now is to…

1. Put down a dimple mat and tape seams

2. put down 1″ foamular xps 250, tape seams and foam edges

3. put put down DryPly 23/32 CAT PS1-09 Tongue and Groove Pine Plywood Subfloor, Application as 4 x 8 using adhesive in t and g seams.

On top of that I’ll most likely do an uncoupling membrane and then the tiles.

What I’m wondering is do you think I need to tapcon the plywood down or can I let it “float”? Will it be ridged enough? There is a half bath as well… will it be ridged enough around the toilet flange area? Should I cut the foam into 2′ sections and do 1×4 strips of pt lumber for added rigidity? I wanted to avoid fasteners into the slab but I also want my tiles to last. Also there are a couple of non load bering walls that I would install on top of the ply wood that I would tapcon into the slab. Like I said I would like not to tapcon all the plywood or and wooden strips between the ridged foam but I will if I have to. Any input would be appreciated thanks!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Dan,

    You'll probably hear from more experienced tile setters than me, but here's what comes to mind. Before uncoupling membranes were widely available, I was taught to install tile over subflooring, which meant "screwing the heck" out of the the subfloor before installing the tile. I'm not sure if that was actually helpful, and now it is best practice to use an uncoupling membrane, particularly on larger floors. It seems like you would want to do both in this situation: screw the plywood down to prevent unnecessary movement beneath the tile, and use an uncoupling membrane so the tile floor can move freely. I look forward to seeing what advice you get from others.

  2. Dan Baker | | #2

    Hey Brian, I appreciate that. Yea most def going to do the Schluter System membrane I was just hoping to get away with not have to tapcon every sheet of plywood to the slab.

  3. Expert Member
    Akos | | #3

    Next to impossible to have flat floor with plywood without screwing it down, you can't skip this step. Get/rent a nice rotary hammer drill, it doesn't take all that long. Officially you are supposed to use 2 layers of plywood with the seams staggered, 1st layer screwed to the concrete, 2nd layer glued/screwed to the first layer.

    For smaller bathrooms, I've had no problems with tile directly over plywood (concrete -> foam-> plywood (quicktrack) -> tile).

    I haven't use Sureseal, don't know how strong it is. The spec sheet puts it at 50psi, so it should be fine for interior non load bearing walls.

    1. Dan Baker | | #4

      Akos, thanks looks like I'll be getting a rotary drill.

  4. Walter Ahlgrim | | #5

    Thin set tile and concrete should be a very good combination that worked for me.
    If the floor slab is old and was not cracked it seems likely it is not going to at this point. An uncoupling membrane could be insurance against hair line cracks.

    All that wood on a basement floor sooner or later is bound to get wet and it will not be flat if it ever dries.


    1. Dan Baker | | #6

      Thanks Walter... I wanted to get some r value out of the ridged foam but that is an option.

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