how are slab foundations made

What is a Slab-on-Grade Foundation and How Are They Made?

You’ve heard or read the term “slab” or “slab-on-grade” foundation and just need a little more *background* on what that means. Maybe you just moved to the area and are new to this type of home foundation and want to know more about it.

Anchor Foundation Repair

At Anchor Foundation Repair, we have repaired thousands of foundations in the Brazos Valley and the vast majority of them are slab-on-grade foundations. So what we’re saying here is that we know a lot about slab foundations and can tell you about them.

This article will briefly explain the residential slab-on-grade foundation by defining the words used in the term, describing what these foundations look like, and how they are made.

What is a Slab-on-Grade Foundation? Definition

According to our Wiki-good friends, a slab-on-grade foundation is just one of five basic types of shallow foundations used in construction. It is considered a shallow foundation because it essentially “floats” on the soil surface without a great deal of deep underground support. 

slab foundation
Regular ol‘ slab

Not that it doesn’t have any underground support, but we will get to that in a minute.

Slab-on-grade foundations are very popular in warmer climates like we have here in Texas. They are not used in colder climates where the ground freezes regularly and for long periods like in the northern United States. More northern locations will tend to use basement-type foundations.

Generally speaking, slab-on-grade foundations are easier and cheaper to install and are preferred by home builders because of this.

Defining the Words in the Term “Slab-on-Grade”

Let’s also make sure everyone knows what defines a slab-on-grade foundation by exploring the words. There’s the word “slab,” it’s not a slab of meat, it’s not a slab of marble, it’s a slab of concrete that is poured all at once.

What does “on-grade” mean? Good question. In the construction industry, “grade” is another word for ground. So the slab is poured on a prepared ground lot and there is no space between the ground and the slab. Not super sure why they didn’t just call it a “slab-on-ground” foundation (that would have been easier) but that might just be a mystery for the ages.

Knowing what these words mean only gets you halfway there to understanding this type of foundation. The rest of it comes from understanding how it’s made, which is coming up real soon.

What Does a Slab-on-Grade Foundation Look Like?

what does a slab on grade foundation look like?

Regular slab foundations can be most easily identified by the flat solid concrete piece that your home rests upon. You will see concrete near the ground on the outside perimeter of your home, you will feel concrete under the floors of your home. 

If you were seeing a new slab-on-grade foundation before a house is built on top of it, you would see a large continuous piece of concrete shaped like the footprint of the home. There would likely be various pipes sticking up from it that will later become under-slab plumbing drain or supply lines, gas lines, or a few conduits for other utilities.

Some Pier and Beam Homes Look Like Slab Foundations

Some pier and beam crawl space type foundations can trick you into thinking they are slab foundations at first sight. These homes might have a concrete beamed perimeter near the ground but there is no slab running under the whole home. 

pier and beam house that looks like a slab foundation
Looks like a slab foundation, but the vents tell you it’s pier and beam

The biggest giveaway that a slab-looking foundation is actually a pier and beam is the air circulation vents. A slab-on-grade foundation will never have these kinds of air vents because there is no “air space” between the slab and the ground that would need to be vented.

Another way that you can tell if a home has a pier and beam foundation is on the inside. The floors will feel and sound more hollow. They might give or creak slightly if you were to jump up and down.

How Are Slab-on-Grade Foundations Made?

slab on grade foundation before it is poured

Conventional slab foundations are the most prevalent of all home foundation types in Texas today. Slab foundations have been around and widely used since the 1950s. So if your home was built in the last 60 years or so, it’s probably a conventional slab-on-grade foundation, or a slight variation called post-tension slab foundation is gaining in current building popularity.

This type of foundation is made by pouring concrete all at one time directly onto specially prepared ground. The slabs are about 4 to 6” thick. A perimeter and grid of trenches are dug (looks a bit like an irregular waffle) to make the foundation. The concrete is reinforced with steel rebar throughout. A post-tension foundation uses a woven grid of cable instead of rebar.

The edges aka “beams” of the foundation are thicker (from 24 to 30” or more). The beams go deeper into the ground around the perimeter of the home, while interior support beams tend to be less deep than the perimeter beams.

Basic Steps in Constructing a Slab-on-Grade Foundation

Here are the general steps a home builder would go through in order to create a new slab-on-grade foundation. This is just for everyday folks and all the mega-detailed construction stuff might not be included. Here we are just focussing on the slab itself.

  1. Prep the Lot with New Dirt: Fill, Slope, and Level as Needed
  2. Place Wooden Form Boards to Frame in the Edges of the Slab
  3. Dig Grid of Trenches to Form the Footings for Interior and Exterior Beams
  4. Line Trenches and Ground with Plastic Moisture Barrier
  5. Insert Steel Rebar Reinforcements Into the Footings and Perimeter
  6. Add More Rebar for the Rest of the Slab Surface
  7. Pour Concrete into the Vertical Footings and Fill in the Rest to Create the Flat Slab
  8. Allow to Cure for Optimal Strength
form boards for a slab on grade foundation

Constructing a slab-on-grade foundation is definitely a multi-phase and careful process. Even though we says it’s *quick and easy* compared to some other types of home foundations, it’s still step-by-step, and layer-by-layer to create the thing that your whole home rests upon.

What Can Go Wrong with Slab-on-Grade Foundations?

expansive clay soil in BCS

These slab foundation things sound really strong, don’t they? There’s all this prep work, and steel reinforcements and whatnot. Concrete is really strong too. But things can still go wrong with slab-on-grade foundations, they are not impervious to natural forces and other flaws.

At Anchor Foundation Repair, we know what tends to damage slab foundations the most. After 35+ years of working in the expansive clay soils in Bryan, College Station, and other communities like Madisonville and Brenham, we can tell you that it’s usually the dirt’s fault.

Expansive soils are to blame for more foundation problems in the United States than any type of sudden natural disaster. But nothing about damage from expansive soils is sudden, it all happens very slowly over time. Read all about it here: “What is Expansive Clay Soil? How Does it Affect My Home’s Foundation?