Foundation Basics

What the foundation does

It does two things, and the second one catches people out. It carries the building's weight down into the ground, which is obvious. It also resists the forces the structure pushes outward and upward — arch buildings exert outward thrust at the base, and wind tries to lift any roof off its walls.

That second job is why anchoring is engineered rather than improvised, and why a foundation that would happily hold the weight can still be inadequate.

Foundation types

Concrete slab-on-grade

A single poured slab serving as both foundation and floor. The default for garages, workshops and anything with vehicles or equipment inside. Most expensive of the common options, and the one you want for occupied or finished space.

Continuous perimeter footing

A footing around the perimeter carrying the walls, with the interior floor handled separately — gravel, a separate slab, or left as dirt. Sensible where the building encloses space that does not need a finished floor, such as equipment or hay storage.

Piers and grade beam

Individual piers at load points tied by a grade beam. Useful on sloping sites, poor soils, or where a full slab is not justified. Requires more engineering attention than the others.

Gravel or compacted base

Possible for some open or lightweight structures where anchoring is engineered for it. Generally not acceptable for enclosed, permanent or occupied buildings, and your building department will usually say so.

Frost depth

In any climate that freezes, footings must extend below the frost line. If they do not, the ground lifts them as it freezes and drops them as it thaws — repeatedly, every year, unevenly. Over a few seasons that racks the structure, and the damage shows up as doors that stop closing and panels that stop lining up.

Frost depth is published locally and varies from a foot or so in mild regions to several feet in the northern tier. Your building department has the number for your address.

Slab thickness

Determined by what sits and drives on it, not by the building above. A slab for passenger vehicles is a different specification than one carrying a loaded forklift, a combine, or racking with concentrated point loads.

Tell your concrete contractor the heaviest thing that will ever be on that floor, including equipment that only visits. Reinforcement, thickness and control joint layout all follow from that number, and it is not something to revisit later.

Anchors

Anchor placement is not approximate. Your stamped drawings specify the type, size and position of every base connection, and the building is engineered on the assumption that they are where the drawings say.

• Pour from the drawings, not from the nominal building dimensions.
• Give your concrete contractor the anchor plan before the pour, not on the day.
• Verify positions and squareness before the concrete sets — this is the last cheap moment to fix anything.
• Check the diagonals. A rectangle that measures correctly on all four sides can still be a parallelogram.