Concrete Subgrade and Base for Residential Projects

The subgrade is the existing soil prepared to support the project. The aggregate base is imported stone or gravel placed above that soil when the design or site conditions call for it. Contractors may also use the terms subbase or base course, so the written scope should describe the actual material and work instead of relying on a label.

Uniform support matters because soft spots, organic material, uncontrolled fill, poor drainage, and inconsistent compaction can allow sections of a slab to settle differently. An appropriate foundation plan can reduce nonuniform support, but aggregate alone cannot guarantee performance or drain water without a compatible path and outlet.

Structural slabs and work tied to a permitted building should follow the approved plan and inspection requirements. A generic flatwork base detail should not override the project design.

• Removal: vegetation, topsoil, old concrete, asphalt, roots, debris, or unsuitable fill included in the price.
• Excavation: planned depth and how excess material will be hauled or reused.
• Aggregate: material type and compacted depth rather than only a loose-delivery quantity.
• Compaction: equipment, lift placement, moisture treatment, and any required verification.
• Drainage: finished slope, runoff direction, edge conditions, and any drain or water-control work.
• Unexpected conditions: unit prices or approval process for soft soil, buried debris, extra excavation, or imported fill.
• Exclusions: utilities, permits, testing, engineering, landscaping repair, and disposal not included.

Put these details beside the slab thickness and reinforcement requirements. The residential slab thickness guide explains why the concrete and the support below it must be scoped together.

No universal testing requirement applies to every patio, walkway, or driveway. Testing may be required by approved plans, a geotechnical recommendation, a permit, local practice, or the contract. Small nonstructural projects are often verified through the contractor's preparation process rather than formal field-density testing.

If a quote promises a specific compaction percentage, ask what standard, reference density, testing method, and responsible testing party apply. A percentage without that context is not independently verifiable.

• The quote says “prep as needed” without excavation or aggregate details.
• New concrete will be placed over topsoil, organic material, mud, or visibly unstable fill.
• The contractor cannot explain where water will drain after elevations change.
• A replacement quote ignores why the existing slab settled or cracked.
• Extra excavation and imported material have no approval process or unit price.
• A structural slab is priced without reference to approved plans or required inspections.