What is a slab foundation?

Does a slab foundation need footings?

A slab foundation still needs a way to carry wall and column loads into the soil. The DOE handbook distinguishes perimeter grade beams from foundation walls on footings; it also describes extra support beneath interior bearing walls or columns. A separate strip footing is therefore not the only configuration. The handbook explains: “Concrete spread footings must provide support beneath foundation walls and columns.” Source: U.S. Department of Energy / Oak Ridge National Laboratory, read 2026-10-31. “In some cases additional footings (often a thickened slab) are necessary under bearing walls or columns in the center of the slab.” Source: U.S. Department of Energy / Oak Ridge National Laboratory, read 2026-10-31.

What is a slab foundation made of and supported by?

The DOE handbook describes a concrete floor slab together with perimeter supports. Those supports carry the structure above, while the floor slab carries floor loads. Moisture and insulation layers are separate parts of the assembly: DOE describes a waterproof layer below the concrete and insulation around slab edges. These sources say: “The major structural components of a slab-on-grade foundation are the floor slab itself and either grade beams or foundation walls with footings at the perimeter of the slab (see Figures 4-2 and 4-3).” Source: U.S. Department of Energy / Oak Ridge National Laboratory, read 2026-10-31. “To prevent moisture from wicking up into the concrete slab and foundation walls, a waterproof layer should be put in under the concrete.” Source: U.S. Department of Energy, Building Science Education, read 2026-10-31. “For slabs that are poured separately from foundations, the insulation is installed before the concrete is poured.” Source: U.S. Department of Energy, Building Science Education, read 2026-10-31.

Does a slab foundation mean no basement?

There is no basement cavity beneath a ground-supported slab area. A house can still combine foundation types in different areas, as the DOE handbook notes. A raised slab also does not create an empty crawlspace: PNNL describes the space below that slab as filled with compacted soil or gravel. PNNL and the handbook explain: “Raised-slab foundations consist of a continuous perimeter foundation wall that is backfilled on the interior up to the level of the above-grade slab floor (Figure 1).” Source: Pacific Northwest National Laboratory, Building America Solution Center, read 2026-10-31. “They are similar in construction to a crawlspace, except that the floor above the foundation is made of concrete slab rather than wood and the space under the floor is completely filled with compacted soil and/or gravel rather than being left as empty air space.” Source: Pacific Northwest National Laboratory, Building America Solution Center, read 2026-10-31. “Of course, actual houses may include combinations of these types.” Source: U.S. Department of Energy / Oak Ridge National Laboratory, read 2026-10-31.

Can a slab foundation crack?

Yes. Shrinkage cracking is specifically discussed in the DOE handbook, which connects crack control to concrete composition and welded wire fabric. This passage explains why cracks can occur; it does not classify an individual crack as harmless or establish that a particular foundation has structural damage. The handbook states: “The proper use of welded wire fabric and concrete with a low water/cement ratio can reduce shrinkage cracking, which is an important concern for appearance and for reducing potential radon infiltration.” Source: U.S. Department of Energy / Oak Ridge National Laboratory, read 2026-10-31.

How thick is a slab foundation?

The 1988 DOE handbook recommends a minimum 4-inch floor slab with concrete rated at least 2500 psi. That is a recommendation for the slab in its illustrated details, not a universal thickness for the whole foundation: perimeter grade beams and footings are separate structural elements. Actual dimensions depend on the design and applicable local requirements. The handbook says: “A minimum slab thickness of 4 inches is recommended using concrete with a minimum compressive strength of 2500 psi.” Source: U.S. Department of Energy / Oak Ridge National Laboratory, read 2026-10-31.

Sources

• Builder’s Foundation Handbook (ORNL/CON-295), Chapters 1 and 4 — U.S. Department of Energy / Oak Ridge National Laboratory; published 1988; read 2026-10-31.
• Flood-Resistant Raised-Slab Foundations — Pacific Northwest National Laboratory, Building America Solution Center; read 2026-10-31.
• Slab Edge Insulation: Code — U.S. Department of Energy, Building Science Education; read 2026-10-31.
• Capillary Break Beneath Slab — U.S. Department of Energy, Building Science Education; read 2026-10-31.