What are the best below grade drainage layers for a slab on grade foundation?
...drainage layer of at least 6 inches of clean crushed stone provides the necessary drainage, it also allows you to do your plumbing in the stone, then re-level it. A vapor barrier may be required by code, or if you're in an area with radon gas and installing a slab for a basement, but if not - a 6 inch base of drainage ...