Concrete Slab and Foundation Planning

Start with the soil

Piedmont clay holds water, drains slowly and can move with moisture changes. Soft spots, fill and organic material must be removed or compacted before building. On some sites, a soil evaluation or engineering is worth doing before design.

Grading and drainage

Water should flow away from the foundation, not toward it. The finished grade, gutters and downspouts, and swales all play a part. See yard drainage on Carolina clay and slab foundations on sloped lots.

Slab, crawl space or basement

Flat lots suit slab-on-grade foundations; sloped lots often suit crawl spaces or walk-out basements. Garages, shops and outbuildings are commonly built on slabs. The choice affects cost, access to plumbing, moisture control and how the building sits on the land.

Footings

Footings spread the building's load into the soil. They must bear on undisturbed or properly compacted soil and extend below the frost line set by code. Footing excavations are usually inspected before concrete is placed. When adding onto an existing building, new footings must tie in correctly — see connecting new footings to existing foundations.

Base, vapor retarder and reinforcement

A compacted stone base gives a slab uniform support. Under interior slabs, a vapor retarder limits moisture coming up through the concrete. Reinforcement and thickness depend on how the slab will be used — a living space, a garage, a shop with equipment or a driveway all differ. AGCS's standard is 4,000 PSI reinforced concrete. See slab and driveway thickness.

Placing and curing

Weather affects concrete. Hot, dry or windy days speed drying and raise the risk of cracking, so curing practices matter. Control joints guide where shrinkage cracks form. See hot-weather curing and control joints.

Permits and inspections

Foundations for homes, additions and many accessory buildings need building permits and inspections from the county or city building department. AGCS handles permits on contracted projects.

Plan for what goes under and through the slab

Once concrete is placed, changes are hard. Before the pour, plan for plumbing lines, floor drains, electrical conduit and any sleeves for future utilities. In shops and garages, think about where equipment, lifts or workbenches will go and whether thicker areas are needed. For homes, under-slab plumbing is typically inspected before concrete is placed. Marking everything on a plan and walking the site before the pour avoids cutting concrete later.

• Plumbing and drain lines
• Electrical conduit and sleeves
• Thicker areas for heavy loads
• Future utility needs

Common problems to avoid

Many slab problems trace back to the ground rather than the concrete. Pouring over soft, wet or uncompacted soil, skipping the stone base, ignoring drainage, or placing concrete in very hot or freezing weather without precautions all raise the risk of cracking and settling. Cutting control joints too late or too shallow leads to random cracks. Good planning, proper base preparation and attention to curing prevent most of these issues. See our concrete pouring services.

Driveways and walkways

Driveways and walkways follow many of the same rules as slabs: a well-compacted base, the right thickness for the loads, proper slope for drainage, control joints and good curing. Driveways that carry trucks or heavy equipment need more thickness and reinforcement than walkways. Where a driveway meets a road ditch, a properly sized culvert keeps water moving. See driveway thickness and driveway culverts.

Plan it with one contractor

Because AGCS is a licensed general contractor, the site work, foundation and the building above can be planned together. Learn more about our concrete slabs, foundations and concrete and sitework services.

Frequently Asked Questions

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