These days, builders are more likely to use poured concrete foundations than foundations built with concrete blocks. Poured concrete has become more popular for a number of reasons: wall forms that can be quickly set up and taken down, concrete trucks that can go almost anywhere, and modern admixtures and pumping equipment that make “impossible” pours nearly routine.

Despite all the advantages mentioned above, concrete block still has its place as a viable foundation material. When the foundation is small, for example for a home addition, a builder can save money by laying a block foundation rather than outsourcing the work to a foundation contractor. Likewise, basement foundations can be built quickly and cheaply from concrete blocks. And for a DIYer with masonry skills, this material provides an opportunity to complete a foundation block by block.

Before delving into this “heavy” topic, it’s worth clarifying some terminology. In the construction industry, concrete blocks are called “CMU”, short for concrete masonry units. Beyond the standard 16 x 8 x 8 inches. cinder block stocked by most housing centers, CMU’s vast universe of sizes, shapes and surface finishes is truly astounding. Most of this selection is intended for commercial construction.

Concrete blocks are made from the same ingredients used in poured concrete walls: gravel (called aggregate), sand, Portland cement, and water. Lightweight blocks are sometimes called cinder blocks because fly ash (ash), a waste product of industrial combustion, is used as a partial substitute for heavier ingredients such as sand and aggregate. The hollow cores in most CMUs make the block cheaper and easier to drive. Cores can be filled with concrete, sand, or foam insulation to improve strength, thermal mass, or insulation value.

Contrary to popular opinion, a concrete block foundation is not inherently inferior to a poured concrete foundation, provided it has been constructed correctly. Both types of masonry require steel reinforcement and must be laid on a solid, stable foundation. Good drainage and waterproofing details are essential. When expansive soils or other external forces damage concrete block foundations, cracks usually appear along the mortar joints that separate the individual blocks.