What is the best way to create underlayment for slate tile on raised porch?

Question

I have a porch, about 150 sq. ft., that is raised about 24" off grade. There was an old, cracked, concrete slab (about 3" thick) set on an underlayment of sandy clay soil, contained by a concrete block wall. The soil over the years settled about 4", leaving a 4" gap under the concrete slab (poured without rebar or any reinforcement). Needless to say, the concrete slab broke right in half and sunk in the middle. I've removed the slab and the retaining wall (which is on a poured foundation below the frost line). I'm putting in a new block retaining wall, but rather than a new slab I want to finish the porch with slate tile. The question is, what would be the best thing to fill the porch cavity with to make sure the tile is stable over the years? Should I fill it with pea gravel and sand, compact the heck out of it and add some drainage, and then tile on the sand? Or should I fill it with gravel and pour a new concrete slab then set the tile on that using mortar and grout?

Answer 1

To keep movement down to a minimum, pour a new slab.  let dry and then seal with red guard. it is a paint-on membrane to keep the tile isolated against movement of the slab.  In other words, should your slab crack again, and it probably will because it is the nature of concrete, the tile is less likely to do so with a membrane in place.

Answer 2

the fill with gravel and pour new slab option , then you can thin- set the tile and grout it..  all you need for drainage then is a sleeve through your retaining wall at the exterior grade