Replies

1. 
   gfretwell | Nov 18, 2013 11:06pm | #1

   How old is the floor? Is it cracked yet?

   If this is new construction I would definately use the membrane.

   OTOH my house was built in 1963 and the floor never cracked so I had no hesitation in putting tile down directly.

   I suspect they took more care in compaction and used a good bed of gravel in the olden days. These days a builder will tilt his head like a puppy and say "concrete always cracks" then slam it down over whatever is there. "Compaction" seems to be running over it with a tracked bobcat a few times.

2. 
   Mongo | Nov 19, 2013 01:20am | #2

   crack versus uncoupling

   It's always best practice to use a membrane.

   In small areas like a bathroom floor, you can get away without one if the slab is in excellent shape. On a large slab, it's best to use a membrane. While it may not be needed for crack isolation, you'd at least want one for uncoupling purposes, to essentially isolate the tile from the slab. That way they can move independently from one another with the membrane itself absorbing any shear stress from any differential movement.

   FWIW, Ditra is technically not a crack isolation membrane, it's an uncoupling membrane. There are technical differences between the two.

   If you want a true crack isolation membrane use something ike Nobel CIS or an equivalent product. If cracks or future slab movement is not a concern and you simply want to isolate the tile from the slab, then use a crack isolation membrane (like Nobel CIS or equivalent) or an  uncoupling membrane (like Ditra or equivalent).