Frost Heave and the Dawn of Basements

The majority of homes we inspect in the Triad area of North Carolina are built on a crawlspace or a slab.  Comparatively, very few have basements.  Why is that? The simple answer lies in one word: frost.

heaving.png

Frost expands soil and exerts tremendous pressure which can lift up houses or push foundation walls in. Builders in the northern part of the country have to dig down far enough to get below the frost line (the depth to which frost penetrates the soil) to place the foundation footings.  These foundations have to be tall enough to extend up through the soil above the grade so that builders can put the house on top of the foundation.  Since the builder has to dig a trench for the footings and foundations, they may as well create a hole and use the below-grade space. That is how basements were invented.

If the building is not likely to see frost to any great depth, there is little risk of the building heaving.  As a result, the weight of the building can be spread out on footings near the surface. Adding a basement becomes quite expensive, which is a major reason most homes in our area have the majority of the living space above grade.  Incidentally, when homes are built in southern climates at grade level, the organic topsoil is removed from the surface since it is not very stable.  While builders might scrape off the surface, that is different from digging holes to get below the frost line.

In the mountain regions of NC, frost can be an issue and many of those homes do not have basements.  Remember our blog about soil type? If you build on bedrock in frost areas, frost won’t be an issue and footings do not have to extend down below the frost line. Similarly, if you build on gravel or coarse sand that is free-draining, and the water table is far enough down, frost isn’t likely to be a problem (like at the beach).  Free-draining soils allow the water from rain and melting snow to fall through very quickly, and as long as the water doesn’t stick around, it doesn’t matter how cold the sand or gravel gets; it won’t expand if there is no water in it and heaving isn’t likely. Many of these homes are built on piles. frost-heave

In northern areas, where basements are more necessary, it is important to understand that the home must be kept heated all winter long.  The purpose is to stop frost from getting under the building, so as long as we keep the inside of the building heated, frost can’t get down under the basement floor. As long as the foundations are deep enough to expend below the frost line outside, the frost can’t get under the building from the outside either. A problem arises when there is a house with a basement that is left unheated over the winter. If the frost depth in an area penetrates 3-4 feet into the soil, the footings have to be at or below that depth.  An unheated house allows frost to penetrate the soil 3-4 feet under the basement floor. If there is adequate moisture in the soil, it will heave, picking the whole house up with it, or more commonly, parts of the house. Very serious structural damage can result.

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