What goes under a concrete slab — gravel sub-base, vapor barrier, reinforcement, and slab thickness
A slab is more than concrete: a compacted gravel sub-base (~4″), a 6-mil vapor barrier, then the slab (4″ standard, 3.5″ IRC minimum) with the steel held at mid-depth on chairs — not on the ground, where it does nothing.
What this diagram shows
A cross-section of a slab-on-grade, layered from the bottom up: a compacted subgrade, a compacted gravel sub-base about 4 inches deep, a 6-mil vapor barrier sheet, and the concrete slab itself — 4 inches standard, 3.5 inches minimum per the IRC — with the steel reinforcement held at mid-depth on chairs rather than lying on the ground. The top of the slab is pitched about 2 percent to drain. The concrete-volume calculation counts only the slab, so the base, vapor barrier, and reinforcement must be planned separately.
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