Seems to be a lot of debate on this, I need to raise the slate about 2in so i was planning on using tapcons to attach either 5/8" advantec or 1/2in hardibacker through 1 inch xps into the slab with slate on top, also looking to eliminate condensation so using the xps would work for that too.
Anyone with experience doing this?
Seems like it would work the xps is fully supported by the slab could it flex more then a standard installation with a rafter/ply sub floor?
I know the tapcons are a lot of work but its only about 70 square feet.
With 1/2" backer board or 5/8" Advantech to distribute the loads over the foam and a slab to fully support the foam, it's a more rigid structure than wooden subflooring on joists. The slate itself can be pretty structural at 3/8" or thicker too.
You don't need many tapcons to keep it from moving- 16" o.c. along the seams would be plenty. Lap the backer-board seams by at least a foot with the foam seams to avoid any edge-rocker developing over time.