The slate is actually installed on tile concrete board as the pond is totaly insulsted with 1.5" cellular foam, the pond is heated with radiant tubing in the concrete floor and also through the bond beam blocks in the walls, I used a high quality thin set mortor and used a latex addative instead of water that makes the mortor kind of rubbery, and is for extierior use, I can get the names of the product today and post them tonight.