Photo Source: Roxul (Rockwool)

Waterproofing below grade is used to manage and mitigate water that would enter the foundation due to hydrostatic pressure. A barrier is used (continuous on exterior foundation walls) to prevent water from migrating through the concrete, into the foundation. This is used in conjunction with drainage systems including piping that redirects water away from the foundation, and granular fill under the slab prevents water build up and entry into the foundation.

A waterproof slab involves sandwiching a waterproofing membrane between layers of concrete.