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The tiles are becoming loose on the steps to my home and need to be replaced. As I pull some of them off I discovered this black sheet under the tiles. It smells tarry and has a texture between fabric and chewing gum. It's somewhat sticky and in places it's very stretchy. Some parts are really easy to pull off of the concrete base, and in other places it sticks incredibly well. This material seems to be only present in that right-hand low plateau, not anywhere else.

  • What is this sheet made of?
  • What is its purpose? Why is it here?
  • Should I use a similar sheet again?

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To add some details, the base is made of solid concrete and it was built only 5 years ago. The tiles are coming loose not because the mortar is poor but apparently because the concrete was surface treated somehow and that treatment is coming off.

I can very easily remove most of the vertical tiles with just two fingers, and the back side of the tile has its glue firmly attached and also a thin layer of this other, darker color which comes off completely from the concrete. You can see it best in the vertical part on the top left of the third picture above. The horizontal tiles are stuck much better but not everywhere.

  • While it might be roof membrane - I think it is shower membrane. That area gets wet this is a moisture barrier - now why it is there to begin with is a mystery. What is under that area could be one reason? However sometimes people float a surface like that so the expansion of the substrate vs. tiles does not cause the tiles to crack. I suspect this is what it is really for; that large area would tend to expand and those tiles would be pulled across and crack. – Ken Jul 18 '18 at 4:20
  • Thank you @Ken for your theory, it makes sense. For the record, there is nothing under the stairs, no basement or anything. – Torben Gundtofte-Bruun Jul 18 '18 at 7:59
  • I'm wondering if that sub-color is supposed to be some kind of waterproofing layer like you might use in a bathroom before putting tile in a splash area... anyway that's just weird. Do all you can to remove it and get to clean bare concrete before retiling – The Evil Greebo Jul 18 '18 at 12:06
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It looks to me like roof membrane. enter image description here

And I have to say I can't imagine why anyone would do the install like they did.

With a solid concrete base, the only water proofing necessary is against the foundation. The tile could have (and should have) been applied directly to the base with mortar.

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    I would agree with roofing membrane oops now even more work and $. + – Ed Beal Jul 17 '18 at 20:35
  • Thank you, I'm glad to hear that I shouldn't put something like this again. – Torben Gundtofte-Bruun Jul 18 '18 at 8:00

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