12 Things to Know About Concrete Slab Foundations


In this series of blog posts about building a custom home, we’ve talked about planning, finding an architect or designer, getting your finances in order, and more.

Now let’s turn the corner and talk about different elements of your custom home. Let's start with the foundation. Specifically, let's talk about the poured concrete slab foundation. It’s the most common kind of foundation used in central Texas today.

Here are 12 things to know about a poured slab foundation:

Engineered plans. If you want to get an accurate cost estimate from your builder, want to get your project permitted by most local authorities, and want a bonded warranty on your home, you’ll need to hire a structural engineer to design the foundation. The engineer calculates load capacity and designs the foundation accordingly.

Geotechnical soil testing. Soil conditions vary widely across the Austin and central Texas area. A soil test by a geotechnical engineer provides information about the soil’s composition and its ability to support a structure, plus its absorption and drainage rate. This information is critical for the structural engineer.

Keep it simple. The more complex the foundation layout, the more opportunities to create the potential for water leaks sometime in the future. That’s why it’s better to keep the layout of the foundation – and the layout of the house and its roofline – as simple as possible. The more like a rectangle, the better. Doing this can save you money on the cost of the foundation, too. That’s money you can spend elsewhere on your custom home.

Stepping the foundation. No, we’re not talking about the Texas two-step. On certain sloped lots, you can economize in excavation and foundation walling by stepping down the foundation from one area of the house to another.

Basements. Basements are not common in central Texas for good reasons. Just don’t bother. It’s not worth the headache you’ll be dealing with, not to mention the added cost.

Form boards, trenches, and plumbing. The foundation crew follows the engineer’s detailed plans to set up form boards around the perimeter. They dig trenches that will be filled with concrete. These trenches can be three feet deep or more. Between the deep trenches are mounds of fill material, which will also be covered in concrete 6 to 10 inches deep. The plumber lays his water and sewer lines.

Fill material. We prefer to use road base instead of sandy loam. When the road base gets wet, it hardens like concrete, whereas sandy loan can compact unevenly and create voids under your foundation.

Vapor barrier. To prevent ground water from seeping up into your foundation, the crew lays sheets of thick polyethylene vapor barrier over the mounds of fill material between the trenches.   

Steel. Concrete needs steel for strength. The foundation crew ties steel rebar everywhere per the engineer’s instructions. The engineer’s plans tell how much and what size.

Pre-pour inspection. When everything is ready, the engineer inspects the work to make sure it’s put together according to his specifications. It’s time to pour concrete.

Early-morning pour. In the heat of a central Texas, trucks line up early. Concrete can be coming down the shoot as early as 5 am. The crew has already been there an hour or more getting ready. Try to be there for the show if you can. It’s fun to watch.

The foundation as your floor. Most Solluna homeowners choose to stain and seal the concrete foundation and use it as the finished floor.

We've barely scratched the surface when it comes to foundations. If you want to geek out and read more, you could look up information about the post-tensioned slab, sometimes used where there are active soils as in the area east of Austin. Also pier and beam foundation, a type of foundation you'll find in many older homes. 

-- Wayne and Mary