slab on grade

A slab on grade is a concrete floor system cast directly onto compacted earth or fill material, with no structural support beneath it except the ground itself. The concrete slab bears its own weight and all loads above it through direct contact with the prepared subgrade. This method is common in light commercial buildings, warehouses, and residential construction where deep foundations are not required or economical.

The subgrade preparation is critical to performance. The soil is compacted to a specified density, usually 90 to 95 percent of maximum dry density as determined by a standard proctor test. A gravel or sand base course, typically 4 to 6 inches thick, is then placed and compacted above the native soil. This layer provides drainage, distributes loads, and acts as a capillary break. A vapor retarder or moisture barrier, commonly 6-mil polyethylene, is laid over the base course to prevent moisture migration from the ground into the concrete. The concrete itself is typically 4 to 6 inches thick for residential work, though heavier industrial applications may reach 8 inches or more.

Slab on grade can fail when subgrade settlement occurs unevenly, when moisture penetrates and causes frost heave in cold climates, or when the concrete itself cracks from shrinkage during curing. Poor drainage around the perimeter allows water to pool and weaken the soil. In expansive clay regions, differential soil movement can crack the slab severely. Curling, where the edges of the slab rise or dip relative to the center, happens when the top surface of the concrete dries faster than the bottom, creating tensile stress on the exposed face.

Relationship to foundations

Slab on grade differs fundamentally from basement slabs or suspended floor slabs. A basement slab is a floor built at the bottom of an excavated foundation, sitting on prepared subgrade deep below grade, and supported by foundation walls on all sides. A suspended slab is a concrete floor spanning between beams or support walls and can be at any level. By contrast, a slab on grade sits at or near the finished ground surface and must be designed to accommodate minor movement and settlement without fracturing. It cannot be relieved of load by structural members beneath it.

The term reflects the trade vocabulary: the slab is poured on whatever grade (ground level) is present at the building site. In regions with significant freeze-thaw cycles, unheated slabs on grade are vulnerable to frost heave, so heated slabs or stem walls with perimeter insulation are often specified. In warm or arid climates, a simple slab on grade with adequate drainage is durable and economical. Where expansive soils are present, special design measures such as post-tensioning or thickened edges may be necessary to control cracking.