Concrete block is one of the most efficient and economical materials for constructing residential basements. It's strong, fireproof, thermally efficient, termite-resistant, requires no formwork, and is available almost everywhere. DESIGNING THE WALLS Most block basement walls are designed according to empirical design standards. The wall thickness, amount of reinforcement, and the spacing between lateral supports are based on lateral earth pressures and vertical loads normally encountered in light construction. A concrete masonry basement wall transfers lateral earth pressures to the floor slabs. For this reason, the first floor framing should be installed before the walls are backfilled. Vertical stiffness of the wall can be increased by constructing solidly grouted pilasters or integral pilasters. Horizontal stiffness can be increased by adding joint reinforcement every other course or by constructing continuous bond beams at or near the top of the wall. REINFORCING BLOCK BASEMENT WALLS In Seismic Zones 2,3, and 4, Uniform Building Code (UBC) requires fully reinforced construction. Reinforcing bars must be a minimum #3 and spaced no more than 4 feet on center. BUILDING THE WALLS Block basement walls should be built on concrete or concrete block footings placed on firm, undisturbed soil below the frost line. Use Type M or Type S mortar (ASTM C 270). And maintain a standard 3/8-inch-thick joint. Tool all the joints concave. KEEPING THE BASEMENT DRY The wall itself should be dampproofed by parging, grout-coating, or asphalt-coating the outside surface. Composite sheet membranes, synthetic rubber sheet membranes (butyl rubber, EPDM, or neoprene), fiber-reinforced cement surface bonding materials, and bentonite clay are waterproofing materials usually used in commercial below-grade construction.