What is a stem wall, and how is it different from a footing or a slab?

The 2021 International Residential Code (IRC) says exterior walls must be supported on continuous footings or other approved systems that carry the loads to the soil.

Concrete footings must be at least 12 inches wide and 6 inches deep, and larger where the tables in the code require it. The width depends on the load-bearing value of the soil.

Exterior footings must be placed at least 12 inches below the undisturbed ground surface, and deeper where frost protection applies.

The IRC's footing tables cover concrete footings with masonry and concrete stem walls. In seismic design categories D0, D1 and D2, the code addresses the joint between a concrete footing and a concrete stem wall.

Foundation walls of concrete or masonry are covered in IRC section R404.

IRC section R506 covers concrete slab-on-ground floors. Floors must be a minimum of 3.5 inches thick. For expansive soils, the code points to section R403.1.8.

The area under the slab must be cleared of vegetation, top soil and foreign material. Fill must be compacted to give uniform support.

Where the slab is below grade, the code calls for a 4-inch base course of clean graded sand, gravel, crushed stone, crushed concrete or crushed blast-furnace slag, with a listed exception for some well-drained soils.