We are installing a 'mud' pan over an existing concrete slab. The old 'pan' was just mortar type mix. We'll be doing the membrane technique.

In reading the forums and Ontario's steps, they talk about putting a slip sheet of tar paper or plastic between the base and the first layer of mortar mix. But in all the examples, they seem to be installing over plywood rather than concrete.

Do I really need to put a slip sheet down on the concrete before pouring the first, sloping, layer of mortar mix? If so, does it need to run up the sides or just on the horizontal surface?

Thanks in advance,
BillP

No plastic over the slab. Mix up a little thinset and make it a little runny. Comb it out over the slab immediately before you put the deck mud down so that the thinset doesn't dry. This will bond the deck mud to the slab.

I usually put the thinset down in sections, say, 1/4 of the floor at a time. Just don't spread more than you can cover in a few minutes.