Building a Block Foundation
How to pour the footing and lay the concrete block, and what to do about waterproofing, drainage, and insulation.
Synopsis: Concrete-block foundations are widely used in some parts of the country. This article, written by a mason, is a guide to building one. He covers all the essential details and includes a well-illustrated sidebar on how to get the right amount of mortar in the right place with a trowel.
Foundation walls are often the most neglected part of a structure. But they are actually the most structurally important element of a house. They support the weight of the building by distributing its entire load over a large area. Apart from structural requirements, foundations have to be waterproofed, insulated, and properly drained.
Although the depth of a foundation wall may vary according to the specific needs of the site or building, the footings must always be below the frost line. If they’re not, the foundation will heave in cold weather as the frozen earth swells, and then settle in warm weather when the ground softens. This shifting can crack foundations, rack framing, and make for wavy floors and sagging roofs.
Concrete blocks are composed of portland cement, a fine aggregate, and water. They have been a popular choice for foundations because they’re not too expensive, they go up in a straightforward way, and they’re available everywhere. Block foundations provide adequate compressive strength and resistance to fire and moisture. They don’t require formwork, and they’re not expensive to maintain.
All standard blocks are 8 in. high and 16 in. long—including the usual 3/8-in.-thick mortar head and bed joints. But they come in different widths. The size given for a block always refers to its width. The size you need depends on the vertical loads and lateral stresses that the wall will have to withstand, but as a rule, most concrete-block foundations are built of 10-in.…