Foundation slab handbook

A foundation slab is the concrete surface a structure sits on. In residential work in the Charlotte metro the recorded holder pours three sub-types: an ADU (accessory dwelling unit) slab, an addition slab attached to an existing house, and a detached garage slab. Each carries a building above it and each is permitted work.

Structural foundation slabs — for houses over a certain footprint, for commercial buildings, for slabs on unusual soils — are designed by a licensed structural engineer. Where an engineer is on the project, the engineer's spec governs and the recorded holder's crew pours against that spec. Where the slab is small enough to fall under the residential prescriptive code, the build on this page is the working baseline.

Every foundation slab carries the building's load at its bearing points. On a monolithic slab, the thickened edge is the footing. On a slab-on-grade with a separate wall, the wall sits on a footing poured earlier.

• Thickened edge — typical 12″ deep by 12″ wide continuous around the slab perimeter, deeper where a bearing wall lands. #4 rebar (2 bars) run continuously in the thickened edge, with #3 stirrups at 24″ centers where the depth exceeds 16″.
• Frost depth — Mecklenburg County frost line is not deep, but slabs subject to freezing must reach undisturbed soil below frost. A garage slab poured on frozen subgrade heaves.
• Bearing capacity — the thickened edge is sized against the load and the soil bearing per the residential prescriptive code, or per the engineer where one is on the project.
• Interior thickened lines — under interior bearing walls, an interior thickened line follows the wall footprint.

Every foundation slab that will be inside a heated space carries a vapor barrier under it. Skipping the vapor barrier is a decision that shows up years later as efflorescence, wet flooring adhesive failure, or a musty smell in the space.

• 10-mil polyethylene sheet.
• Laid over the aggregate base and against the inside face of the perimeter thickened edge or foundation wall.
• Seams lapped 6 inches minimum and tape-sealed.
• Penetrations — plumbing rough-ins, electrical stubs, radon lines — sealed at the sheet with a manufacturer-approved boot.
• Aggregate base directly under the sheet is finer material so it does not tear the sheet.

A foundation slab for an ADU, addition or garage is permitted work in the City of Charlotte and Mecklenburg County. The recorded holder pulls the permit against the plan reviewer and files inspection requests at the required checkpoints. See NC permit basics for the full rules.

• Subgrade / footing inspection — after excavation, forms, reinforcement and vapor barrier are in place, before pour. The inspector checks depth, width, reinforcement, dowels, vapor barrier.
• Final inspection — after the slab is poured and cured and any building above it is dried in. On many jobs the framing and MEP inspections happen in between.
• Inspection requests are filed the working day before the required window, per the jurisdiction's rules.

The pour runs the pour-day checklist end-to-end. On a monolithic slab, the pour is placed in the thickened edge first, then across the interior, with vibration where the reinforcement density and pour depth call for it. Screed and bull-float. Trowel finish on the interior for interior slabs; broom finish on a garage floor where a car will park.

Cure runs seven days minimum. Framing on the slab typically waits 7 days for a residential build, longer where the engineer's spec calls for it. Hand-off to the client — or to the framer, on a subcontracted job — happens once the cure window opens and the framing inspection is scheduled.