Floating slab foundations are the most basic type of concrete foundation. They consist of a solid slab of concrete with no surrounding foundation to support it and are designed to float with the soil as it shifts during weather and moisture changes. Floating slab foundations are often used for garages because they’re the simplest and least expensive type of concrete foundation to build.
The Basic Design
The structural design of a floating slab will vary depending on your local building codes, but some characteristics are shared by all floating slab foundations. For example, all floating slab foundations are designed as a solid slab of concrete with thickened edges that serve as integral footings, and all have rebar around the perimeter. Rebar or fiber mesh is run throughout the slab to provide reinforcement as well, and the slab is supported by the soil or a layer of gravel.
Floating slab foundations are typically used when building a structure that does not have a basement, such as detached garages, sheds or barns. They're most ideal for detached garages in regions where frost and freezing aren’t an issue. However, the addition of an insulating layer makes it possible to use this type of foundation in cold climates as well.
Homeowners have a few options when building a floating slab foundation. Your climate will influence which type is more ideal for your garage. Desert slab foundations are ideal for dry, warm regions where soil moisture and freezing are uncommon. This type of floating slab is poured directly into forms on the soil so the slab rests on the ground. In warm, wet areas where freezing isn’t a problem, the same type of slab is used, but a layer of gravel is placed between the soil and the concrete as a moisture barrier. Floating slab foundations in regions that experience temperatures below freezing require insulation. These slabs typically extend below the frost line and rest on a bed of gravel; they are insulated with rigid board insulation placed beneath the slab or within the concrete layers.
While they're simple in design and construction, floating slab foundations do have some drawbacks. For example, if you’re building a garage that will be attached to your home, a floating slab foundation can't be used, because this type of garage requires frost protected footings and a block foundation. If the site is poorly prepared or the foundation is placed directly on wet soil, moisture and major shifting of the soil can lead to cracks that affect the foundation's structural integrity. This can be costly and time consuming to repair.
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