A 4-inch-thick (102 mm) base course consisting of clean graded sand, gravel, crushed stone, crushed concrete or crushed blast-furnace slag passing a 2-inch (51 mm) sieve shall be placed on the prepared subgrade where the slab is below grade.
Exception: A base course is not required where the concrete slab is installed on well-drained or sand-gravel mixture soils classified as Group I according to the United Soil Classification System in accordance with Table R405.1.
A 6-mil (0.006 inch; 152 μm) polyethylene or approved vapor retarder with joints lapped not less than 6 inches (152 mm) shall be placed between the concrete floor slab and the base course or the prepared subgrade where no base course exists.
Exception: The vapor retarder is not required for the following:
- Garages, utility buildings and other unheated accessory structures.
- For unheated storage rooms having an area of less than 70 square feet (6.5 m2) and carports.
- Driveways, walks, patios and other flatwork not likely to be enclosed and heated at a later date.
- Where approved by the building official, based on local site conditions.
When a vapor retarder is required, a capillary break shall be installed in accordance with the California Green Building Standards Code, Chapter 4, Division 4.5.