slab

A slab is a monolithic concrete platform, typically 4 to 8 inches thick, poured directly onto prepared ground to serve as the structural base for a building. It replaces a traditional foundation with footings and stem walls, offering a faster and cheaper installation in appropriate conditions, particularly on relatively level terrain with stable soil.

The concrete is poured over a prepared subgrade, which includes compacted fill, a gravel base layer (usually 4 inches of well-draining stone), and a moisture barrier such as polyethylene sheeting. Reinforcement, most commonly a welded wire mesh or rebar grid, is placed within the slab thickness to control cracking and distribute loads. The perimeter may be thickened with a turned-down edge, called a stem or grade beam, that extends below grade to provide additional bearing capacity and resistance to frost heave in colder climates.

Slabs on grade work best on well-drained soils with bearing capacity of at least 2,000 pounds per square foot. In poorly draining or expansive soils (clay, silt), differential settlement or heaving can crack the slab. Moisture intrusion through the slab is a chronic problem in humid or wet climates; crawl spaces or raised foundations perform better where groundwater control is difficult. Slabs are also unsuitable for steeply sloped sites or locations subject to significant soil movement.

Variants and common uses

Floating slabs are unattached to the building frame, allowing some independent movement. Post-tensioned slabs use high-strength cables under tension to reduce crack width and allow thinner pours on marginal soils. Slabs on grade dominate residential construction in warm, dry climates such as the Southwest and Southeast United States, where frost depth is minimal and soil drainage is adequate.

The term slab derives from its appearance: a simple, flat, slab-like pour. In the wider building industry, it occupies a middle ground between expensive deep foundations on poor soil and economical surface bearing on excellent soil. Its popularity stems from its speed and low material cost, but its lack of crawl space makes utilities more difficult to service or modify after construction.