The foundation is the part of your house you will never see, and the part you most want to get right. The type your home needs depends on your soil, your site, and your local code. Here is a plain guide to the most common house foundation types and when each one makes sense.

Slab on grade

A slab on grade is a single layer of concrete poured directly on the ground across the whole footprint of the house. It works by bearing on the soil, and by using the friction between the concrete and the soil to resist sliding during wind or an earthquake.

A slab on grade works best when the soil under the whole house is even and competent. If part of the ground is weaker, that area can settle while the rest stays put, and that differential settlement causes cracks.

Post tensioned slab

A post tensioned slab is a variation of the slab on grade. Steel tendons are tensioned inside the concrete, which lets the slab span over softer spots in the soil and spread the load more evenly across the whole footprint. This type is common in states like Arizona, Nevada, and Texas, partly because builders there have moved toward higher standards to reduce liability. In California it is less common for homes, but it is a real option.

Shallow footing and T footing

A shallow footing does not cover the whole footprint. Instead it runs under the perimeter walls and sometimes under interior shear walls, usually in a T shape in cross section. Like a slab, it works in bearing and sliding. It is a common, economical choice when the soil is good.

Pier and grade beam

When the soil is bad, a pier and grade beam foundation is the go to. Expansive clay soils swell and shrink as they take on or lose water, which can heave a shallow foundation around. To avoid that, engineers drill piers down to deeper, more stable soil. The piers hold through skin friction with that deeper soil, so they do not move when the surface soil swells. A concrete grade beam then spans from pier to pier, usually with piers spaced about 8 to 12 feet apart, and carries the house. This takes more work than a shallow foundation, so it costs more.

Helical screw piles

Helical screw piles are essentially large screws driven into the earth. The helices keep the pile from moving up or down, and installers turn them in until they hit the right torque. They are not a common residential foundation, but they show up on certain projects, and some builders are finding creative ways to use them for sideways resistance as well as vertical support.

How your engineer decides

Here is something useful. An engineer can learn a lot about your foundation from your address alone. The address tells them which local code and municipality govern the project, and each has its own minimums. As an example, one city may require a footing at least 12 inches deep while a neighboring one requires 18. Hazard maps tied to your latitude and longitude then flag whether you are in a high wind, liquefaction, or landslide zone. A soils report fills in the rest. All of that shapes which foundation is right before a single line is drawn.

Spacial provides structural engineering for homeowners, architects, and builders across California.