Foundation types explained: slab, crawlspace, basement and piles

A foundation carries the building load into the ground and keeps water, frost and movement away from the structure. Here are the four common types, what decides between them, and what to ask your engineer.

The four common types

• Slab on grade: a concrete slab, usually thickened at the edges, poured directly on prepared ground. Common on flat, well-drained sites and in warm climates.
• Crawlspace: a short perimeter wall lifts the floor above the soil, leaving a low void for services. It gives access to pipes and keeps the floor off damp ground, but the void needs moisture control.
• Basement: full-height walls below grade. It adds usable space, but it carries the highest waterproofing and drainage demand.
• Piles or piers: columns driven or drilled down to firmer soil or rock, with a beam or slab on top. Used where surface soil is weak, flood-prone or steeply sloped.

What decides the choice

The soil bearing capacity and the depth of the water table do most of the deciding, followed by frost depth, flood and seismic exposure, slope, and the weight of what sits on top. None of these can be judged from the surface. A geotechnical report, or at minimum local engineering knowledge of the soil, sets the starting point.

Light and heavy superstructures

A light steel or timber building puts less load on the ground than masonry or concrete frames, which can allow a simpler foundation. Whatever the system, the foundation has to match the anchor and connection details of the structure above, so confirm the anchor layout before the concrete is poured. See foundation interfaces for modular buildings for what that handover involves.

Questions to put to the engineer

• What soil data is the design based on, and who produced it?
• What is the design frost depth and groundwater assumption?
• How is water kept away from the foundation, and what is the drainage detail?
• What tolerances apply to the slab or anchor positions the structure needs?

Quick answers

• Which foundation is cheapest? It depends on the site. A slab is often simplest on flat, stable ground, but poor soil or a high water table can reverse that. Get quotes against the same engineered design.
• Do I need a soil test? For anything beyond a minor structure, yes. The foundation design depends on soil data, and guessing is how settlement problems start.
• Can the foundation type be changed after design? Only through the engineer, because the structure, anchors and drainage are designed around it.

Keep reading

• Foundation interfaces for modular buildings
• Waterproofing and damp control basics
• Concrete basics: mix, pour, cure and joints

Always check the project drawings and specification, and confirm requirements with the engineer of record and the local authority.

Questions people ask

Which foundation is cheapest?

It depends on the site. A slab is often simplest on flat, stable ground, but poor soil or a high water table can reverse that. Get quotes against the same engineered design.

Do I need a soil test?

For anything beyond a minor structure, yes. The foundation design depends on soil data, and guessing is how settlement problems start.

Can the foundation type be changed after design?

Only through the engineer, because the structure, anchors and drainage are designed around it.

Sources and further reading

• International Code Council
• American Concrete Institute
• FEMA, Building Science

Text Digital Oro

Digital Oro International advertises on The Company Chronicle. This guide was written by our editorial team from public sources and general professional practice; it does not describe a guarantee, quote or offer from Digital Oro International. Confirm current terms, prices and availability with the operator before you decide.