Why Slab Foundations Work for Both Heated and Unheated Structures In Cold Areas

No matter what type of structure you’re looking to build, you’ll need to have a foundation beneath it. In some ways, one could argue that it’s the most important construction element. It provides safety and stability to your home while carrying the load of the whole building. However, depending on numerous factors, the cost of such an element can be pretty high.

People who’re on a more moderate budget can instead opt for a slab foundation. It is cheaper to build and does its job without any fuss. The only real downside of a slab foundation is that your home won’t have a basement or crawl space. However, some homeowners don’t need lower-grade levels at all, and that’s what makes slabs a perfect choice for them.

Their only concern is how will this substructure fare with the local weather conditions. Will it work in their cold climate, and what difference does it make if you do or do not heat the structure above the slab? Here’s what you need to know.

What Is a Slab Foundation?

The slab is a construction element that combines concrete and steel rods that provide additional reinforcement. In most cases, the slab is four to eight inches thick, with even bulkier edges to further increase its durability. When building it, the construction crew pours the concrete all-in-one, making the slab a monolithic structure. This means it’s “of one stone”.

It has both pros and cons like everything else. The main reason slabs are so popular in America is that they’re budget-friendly and simple to build. They’re also pest and mold-free, easy to extend, and most importantly, long-lasting. On the other hand, slabs can interfere when you have plumbing issues, and its concrete can crack.

Either way, just like any other concrete composition, the slab requires time to cure. This process can take up to 28 days, depending on the weather conditions. Yet, after a day or two, the slab will become firm enough for the builders to remove the surrounding forms. And in a week, it will be ready to carry the weight of the crew walking over it while preparing further work.

What Does Climate Have to Do with Slab Foundations?

During the winter, the soil beneath a structure will freeze when the temperatures drop. This can cause serious structural damage as it will affect the foundations by moving them. Therefore, builders need to take into consideration the frost depth. In other words, they will need to build footings beneath the line at which the ground freezes.

The team will excavate the perimeter trenches below the frost depth, pour the concrete footing, and build concrete or block walls. They will then backfill the walls and compact the soil inside the perimeter. Afterward, the builders will place the gravel, a plastic vapor barrier, and finally, pour the concrete slab within the foundation borders.

However, there’s an easier way to get slabs to function properly in cold areas. With the use of frost-protected shallow foundations (FPSF), there won’t be a need for deep footings below the frost depth. This technique requires the builders to simply insulate the ground under the building, keeping it safe from freezing.

Slabs in Cold Climates

The main reason frost-protected shallow foundations are a popular choice in St. Louis, Missouri, and the rest of the Midwest is the fact that they can raise the frost depth. In some cases where it was logical to dig 40 inches for footings, you can now do it at 12 inches without any risks. By combining the heat from the earth’s core and the one that leaks from the structure above, the insulation of FPSF will keep the ground warm enough for it to not freeze during the winter.

Depending on the climate zone, the builders working on the slab will need to figure out how far the insulation foam will need to extend. The American Society of Civil Engineers suggests that the zone of insulation should correlate with the purpose of the structure. In other words, it will depend on whether you’ll heat the space above the slab or not.

Heated buildings will expose the warmth into the ground and keep it from freezing. But if you’re looking to have a garage atop the slab foundation, you will probably heat it from time to time, which means that the warmth will only occasionally go into the freezing ground. Hence, the purpose of the structure will dictate how you’ll go about soil insulation.

If you have an unheated structure, you’ll need to insulate the soil beneath the slab as well as extend it outward. This way, you’ll use the heat that’s coming from the earth’s core to keep the ground from freezing and endangering your foundation. You can do this by extending the two-inch insulation layer for about four feet beyond the footing area.

Should You Opt for a Slab Foundation?

If you’re living in St. Louis, Missouri, and have a moderate construction budget, a slab foundation will work wonders for you. Besides being cost-friendly and easy to build, it’s also mold-free and long-lasting. You will no longer have to worry about frost depth if you insulate the slab and the surrounding ground properly. With frost protection, your foundation will be safe from numerous structural damages that can occur due to cold weather.

Luckily, we can help with that. Contact our professionals in Missouri at Foundation Recovery Systems to solve this and all other foundation problems. Our crew will come down to your home and inspect it for free. After that, they’ll discuss with you what measures you can take to make your substructure safe and stable for years to come.

Leah Leitow

Content Writer

Leah is a Content Writer for Groundworks with nearly ten years of experience working in the foundation repair industry. Her experience ranges from working with homeowners to find the right solution to training inspectors and staff. In her background as a Michigan journalist, she gained invaluable insight into people's lives throughout our state. Leah lives in metro Detroit with her husband and two sons.