1 3/4' subfloor under tile??? Is this necessary?

deedlesJuly 9, 2012


Hoping I can get a quick coupla answers to this. We're being told conflicting info.

We want to install a pebble tile floor with a heat tape mat underneath in our bathroom. We have 3/4 inch plywood subfloor thus far.

One installer said we should prime the subfloor, lay the tape mat and he'll pour a subfloor to encapsulate the heat mat. Then the tile can go on that poured subfloor.

Another installer said we need cement board, THEN pour the stuff to encapsulate the heat mat, then the tile goes on that for a thickness of 2 inches to 2 1/4" total. This installer says you need 1 3/4" of sub-flooring under tile, esp. in our part of the country (wisconsin) or we'll have movement and cracking of the tile or grout.

This amount of flooring seems extreme to me but the guy said anything less is substandard and we'll have problems down the road?

A 2 + inch flooring difference? WTHeck?

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Add a 1/2" BC sanded or plugged ply or 3/8" for that matter. Get the heat mat that's a waterproof membrane as well set in thinset bed and tile over the mat.. Make shore your 2nd ply layer is staggered correctly on joists.

    Bookmark   July 9, 2012 at 1:51PM
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Joist span?

Tile and stone floors do not tolerate the normal movement allowed in a wood floor.

Look around the John Bridge forum for guidance on floor substrate thickness.

Cement board does NOT increase floor stiffness.

    Bookmark   July 13, 2012 at 10:23AM
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It's a bathroom so most likly is 16oc the question is are the joists 2x8, 2x12? Ect... Need at least 1" of wood for any tile install. 1/2" & 3/4" of ply is plenty. Can actually install heat lines over the backer board here is how.


    Bookmark   July 13, 2012 at 3:37PM
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The actual joist span and height are more important than the joist spacing.

the joist spacing does drive the required subfloor stiffness, with 24 inch spacing needing even more thickness than 16 inch spacing.

If the joists are at the limit of span, no amount of thickness from the subfloor is likely to eliminate joist flex adequately for tile.

    Bookmark   July 13, 2012 at 4:37PM
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