Tom Hanrahan

07:22AM | 04/21/06
Member Since: 12/28/02
24 lifetime posts
Enclosing our front porch, which is a concrete slab, our plans are to install quarry tile.

My question is do we need to install a moisture barrier of some sort over this concrete slab?

Does cement board need to be installed?

Or, can I install the quarry tile over this slab with no prep other than thoroughly cleaning the slab?

In case it matters, the area will be heated and cooled.


01:42PM | 04/21/06
Member Since: 07/03/05
283 lifetime posts
If the concrete has no cracks, is not sealed or painted, then you could tile right over it with a premium grade modified thinset. The tile will still follow the floor which I assume is pitched to allow for water runoff. If there are cracks then a membrane like Ditra from or one from would be put down first. Cement board is not used over a cement slab, only over wood frame subflooring.

If the pitch is to be removed the floor needs to be refloated with a mud base of a minimum 1 & 1/4" at the high end which means it could be a couple of inches deep at the door, or the existing slab is chopped up and replaced with a new flat one.

Tom Hanrahan

08:04AM | 04/22/06
Member Since: 12/28/02
24 lifetime posts
Thanks for the reply!

The concrete had been painted at one point by previous owners, so it has the remains of paint helter-skelter.

What should I do about that?


02:43PM | 04/22/06
Member Since: 07/03/05
283 lifetime posts
If you can hit it with a sander or grinder that would help. If the paint covers the slab, it, in and of itself, offers a reduced bond ability of the thinset, it also becomes the surface to which the thinset bonds to, rather than directly to the cement so if the paint breaks its bond the tile comes loose by default. Solvents are not a good idea because they tend to leave behind a residue that is also problematic for the thinset to bond to. The floor would need to be flushed and scrubbed with clean water several times to get it clean of that residue.


09:13AM | 06/30/06
Member Since: 06/29/06
1 lifetime posts


07:34PM | 06/30/06
Member Since: 07/03/05
283 lifetime posts
Not sure what you are describing.

If you mean is it necessary to have a cement board under the tile, that depends on what would be under the cement board. If the floor is a concrete slab then cement board is not used at all. If the floor is wood framing and has existing plywood of at least 3/4" in thickness and the floor joists are 16" on center then a cement board would be highly recommended. Vinyl flooring and the luan underlayment do not count and in fact need to be removed first.

Tile crack if the floor under them has too much flex (called deflection in the trade). Adding cement board to a deficient floor wont stop the cracking from occuring because the cement board offers no structural support as plywood does.
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