Rebuilding an outdoor deck in upstate NY with 6" X 24" porcelain tile. The foundation will be a 6" concrete bed with rebar. Confused about the need for and the order in which the following should be used:

🔹Uncoupling Membrane (e.g.: Schluter Ditra) 🔹Redgard 🔹Cement Backer Board 🔹Thin set mortar

Have no issue with the expense of using all of the above in order to maximize the life and stability of the deck.

Thanks in advance for any advice you might offer.

  • Is this deck on the ground? Commented Jul 12, 2020 at 14:17
  • Upstate gets rather cold and snowy in the winter. I'm sure you won't be using the deck for sitting around and enjoying the beautiful January weather, but do bear in mind that if you have to walk across the deck (for example, to leave the house to find the dog) and it's wet or snow covered, porcelain tile can be very slippery. I'm sure the tile you've picked has some texture to reduce the risk of slipping, right?
    – FreeMan
    Commented Jul 13, 2020 at 15:22

2 Answers 2


I have tiled a few concrete slabs and have never used anything but thinset, a membrane for a shower stall is great but for outside I believe it could have more problems than straight thinset, if you want to make it stronger add add mix or purchase thinset that is already enriched. Absolutely no need for a backer board on a concrete pad.


If your 6" slab is in good shape then you'll just need to apply the ditra membrane.

Clean the slab with a good pressure washing. Then paint on a primer coat. The primer will help the thinset bond to the slab.

Apply ditra to the slab. Schluter has a great video here: https://youtu.be/qa_l7U7zh0w

Then the tile gets installed on top of the ditra.

  • If this is a concrete slab in the ground, why would you need a waterproofing membrane...? Commented Jul 12, 2020 at 19:17
  • You want ditra for decoupling, waterproofing is just along for the ride
    – Matthew
    Commented Jul 12, 2020 at 20:02
  • It is all cementitious material, at least the thinset and grout, all will move at the same rate, why decouple? The freeze thaw cycle, with the extra moisture in the tile freezing and expanding, and the concrete, because of the Ditra, not having the same amount of moisture, therefore not freezing with the tile at the same rate, would that not possibly fail, since all Ditra that I seen as in the video, is used indoors.
    – Jack
    Commented Jul 12, 2020 at 22:52

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