Why You Should Consider Pier and Beam Foundations for Your House Build Project
Foreword by Ian Thompson, Editor
Many geotechnical, civil, and structural engineers will specify a particular foundation type based on the weight and type of the structure, as well as the soils that the building sits on, or in. However, there are reasons why you might want to consider other foundation types to get more functionality and future-proofing for your building.
For instance, in Australasia, we predominantly use slab-on-grade (flat slab) foundations, they’re easy to construct and relatively cost effective. But that doesn’t mean they are the only choice worth considering. Pier and beam foundations, for example, could also be used on many of the same house building projects. This type of foundation would add the ability to include a short basement that allows you to provision all your services underneath the house. This allows for easy maintenance and significant plumbing and electrical changes in the future without having to rip up the slab.
Pier and beam foundations also offer extra height protection from flooding and the potential to create a better thermal envelope. Yes, it might cost more, but in the grand scheme of things, the additional cost is not considerable when you factor in the benefits gained.
One characteristic I particularly appreciate is that it allows for a suspended wooden ground floor, which is better on your joints than walking on hard concrete floors, and a sustainability consideration.
Join Matt Risinger as he discusses why he chose a pier and beam foundation for the SIPS house in the deep south, USA.
If you’re building a new home in the south, one of the most important decisions you’re going to make is the type of foundation to use. If you get your foundation right, everything is going to go well for your house long term. On the other hand, with a bad foundation, no matter what you do in the build, you’re going to have problems with your house. It’s not often you see full in-ground foundations in the south like you do in the northern zones. That’s mainly because we don’t have a frost line and code does not dictate that we need to dig down several feet.
So, in the south, the two most common foundation types are either a slab on grade or a pier and beam foundation. For this new home we’re building behind us, which is a SIPS construction house, we’ve decided to go with a pier and beam foundation. I want to tell you a couple reasons why we chose that and why I think pier and beam is a great choice.
Firstly, we need to talk about the soil type here. We did some soil testing and had a geotechnical engineer out. We found very early on, before we got into any of the engineering, that we had a very high clay content in the soil underneath us. We have about a foot or two of regular dirt and then underneath that is very high clay content soil all the way down until we hit rock at about 20 feet.
This house, in particular, if we were to build a slab on grade, we would experience a lot of movement over the years of that slab. That means as the soil contracted and got dry, it would move and as it got wet after it had been dry for a long time, it would expand and have a tendency to move this house around. So, what we did here was we sunk really deep piers all the way through that clay so that we could hit the bedrock. In effect, we’re building kind of like a beach house up on stilts here where that clay is in between us, but we’ve got these concrete and rebar piers all the way down to the bedrock.
Now, that clay soil could still affect our foundation, so what we’ve done is we’ve isolated, or are about to isolate, this foundation and the concrete on top from that clay movement. The way we do that is what’s called a curtain form. This is in effect a cardboard box that will crush over time and now the concrete on top, the perimeter beam, and also the slab that’s going inside our conditioned crawl space, is going to be able to stay constant and the soil below that could have some movement. It’ll crush that carton form, expand and contract, and not lift or move the house.
We’ve also chosen pier and beam for a couple other reasons that I like. In particular, because we have great access to all our utilities underneath the house later. So all we have to do for this foundation here is sleeve through that perimeter beam for our sewer, our electric, and our water, and then everything can be done underneath the house.
Let’s pick up this video in about two weeks as we move on in this construction phase and get to the point where we’re getting close and I’ll give you a tour then.
As you can see, the foundation is complete and we’re looking really good over here. We’re ready for the structural framing to come in, including the floor framing, but we’ve got one last step on this foundation. We’re going to be pouring a concrete slab on top of that vapor barrier down inside this conditioned crawl space. I’m not going to get into those specifics now but stay tuned for a future video on all the details of
how we got the conditioned crawl space.
Let’s take a second to review what this foundation is now that we’re complete over here. Remember, we sunk the piers all the way down to rock. We added the void forms inside our framing for this foundation and now all these perimeter beams that you’re seeing here, as well as the front porch, the back porch slab, and the garage slab, are on top of an air space.
This is akin to that parking garage that you drive on where you’ve got a concrete slab that has air underneath it, and the concrete slab, along with the rebar in there, is transferring the load to the piers which will go all the way down to the rock. That’s exactly what’s happening here. This is an excellent foundation for clay soil. This can also be used in other types of soils as well, but particularly where we’ve got clay and we’re worried about those upload and download forces on the house, this is really the best practice.
Had we not had clay here, we would not have had to do a void form underneath this pure beam foundation. For more information on foundations in general, visit my blog at mattressinger.com. If you want to follow this house in particular, we’re going to be posting a bunch more videos from here, including some SIPS videos. And if you’re following us on Instagram or Twitter, we’re always going to post from this job site as Rice and Grocer SIPS.
We’ll see you next time on the build show.