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pex retrofit over slab under tile

GradyyGradyy Posts: 4Member
I have a 2000 square foot insulated [under and sides] slab with a 6 mil vapor barrier. I want to add pex and tile and keep the total thickness to a safe minimum. What are my options? I don't really want to go with gypcrete. If I use an additional 6 mil separation barrier between the existing slab and the new mud bed, will I have any issues with the double vapor barrier? The existing slab is well cured - 1980's.


  • hot_rodhot_rod Posts: 11,269Member
    Fasten the tube to the slab with adhesive and anchors, then screed the mudset over the tube, thin-set tile to that. A cleavage membrane between the slab to prevent cracking from telegraphing.

    The tile council website has plenty of installation tips, like this. The RPA Flooring Guide also.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GordyGordy Posts: 9,264Member
    Another good web site.

    Tile mud beds are done all the time on concrete slabs. The second vapor barrier is not necessary. There are plenty of different options for setting tile on existing concrete slabs.

    What condition is the existing slab? Any cracks?
  • GradyyGradyy Posts: 4Member
    Thanks, The slab is good - no cracks except hairlines where screed supports were removed and filled.

    What kind of anchors into concrete - PAF's? That's quite a few.
    I like the idea of adhesive.

    Some say the tile mud bed may or will crack because it will expand at a different rate than the existing slab - so the 6 mil separation membrane - others say not necessary.
  • GradyyGradyy Posts: 4Member
    hot rod, the screen shot shows electric radiant - can you do the same with pex? On your link, RH 110 for hydronic says minimum of 3/4" concrete above pex where it's in a poured slab. What if I were to overpour a thin slab with pex tied to rewire? What's the minimum safe thickness and what are the potential problems?

    As an alternative, F111 says,"where radiant heating pipes are laid over the slab, screed fill flush to top of pipes before placing a cleavage membrane and reinforced mortar bed."

    Does that mean a cleavage membrane would not be needed below the pex and mortar?

    - and then I'd need a reinforced mortar bed over the top of the pex and mortar bed?
  • GordyGordy Posts: 9,264Member
    If you use the link I provided there are many tilers who have done this. They will direct you as the best method for the tile detail. Which doesn’t always coincide with the best method for hydronics.
  • hot_rodhot_rod Posts: 11,269Member
    yes you can embed pex in the motor bed. A separation membrane is mainly to prevent cracking of the slab below from coming thru to the tile set.

    If you use a concrete mix, which included aggregates, stones, then you need a minimum coverage so the rocks can consolidate and have some cement between them :)

    The concrete council suggests 1-1/2 times the aggregate size above the tube. So with a pea gravel mix of 3/8" stone 1-1/8".

    Mortar beds are not really considered structural hence no need for reinforcing or aggregate, it is mainly to level the surface and have a good base for tile to set to. No harm in adding mesh for a way to keep the pex spacing and keep it down tightly, I've seen chicken wire used for that purpose, very thin and flexible in a thin set.

    Some tile setters prefer cement board for tile setting, I still prefer scratch coats of dry pack motor. If you can find tilesetters with the skill to put them in anymore, especially walls :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GradyyGradyy Posts: 4Member
    Gordy, Thanks for the link. I posted my question over there, too.
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