What Is a Floating Slab Foundation, and Why Is It Used?

Slab foundations are some of the most commonly used in the U.S., but many homeowners don’t know about the different kinds of slab foundations, each with a different purpose and use case. In this guide, we’ll explain what a floating slab foundation is, how it differs from traditional slab foundations and other styles, when it’s useful, and more.

What Is a Floating Slab Foundation?

A floating slab foundation is a concrete foundation that sits directly on flat, level soil, hence the name “floating” slab. Since it only gets support from the ground directly underneath, it’s not ideal for use under larger structures, like entire homes. Unlike a slab-on-grade foundation, which is more common in home construction, a floating slab doesn’t sit on footings that sink further into the ground for more reliable support.

Types of Floating Slab Foundations

There’s really only one type of floating slab foundation, which is a concrete slab that a contractor pours into a concrete mold directly on the soil. There are two other types of slab foundations which some people refer to as floating slabs, even though this is technically a misnomer.

• Slab-on-Grade Foundation: A slab-on-grade foundation usually has concrete footings that add support to the slab. The footings sit a few feet under the ground to reach below any soil that might freeze or expand in the presence of moisture.

• Monolithic Slab Foundation: A monolithic slab foundation also has footings that reach down into the ground, but the home builder will pour a single concrete slab that includes built-in footings rather than pouring them separately.

How Much Does a Floating Slab Foundation Cost to Install?

The average cost to have a concrete foundation company near you install a floating concrete slab is between $4.00 and $5.00 per square foot. A floating slab for a shed will average around $400, while a floating slab for a detached garage will likely cost around $1,700. Of course, the size of the foundation you need will dictate the total cost.

Floating slabs are the most affordable foundation to install because they require no excavation for footings. The contractor can simply level the ground, construct a mold for the foundation, and pour the concrete.

Pros and Cons of Floating Slab Foundations

Like all other types of foundations, floating slab foundations have their place in construction. There are some upsides and downsides to this foundation type to consider before having one installed on your property.

Pros

• Most affordable foundation style

• Suitable for all soil types

• No exposure to high water table

• Can be suitable for small home extensions

• Possible to DIY for small structures

• Minimal moisture issues

• Minimal risk of pest infestation

Cons

• Subject to frost heave

• Not ideal for colder climates

• Not suitable for large structures

• Can be expensive to repair

Common Problems With Floating Slab Foundations

Since floating slabs sit entirely on top of the soil, they’re unaffected by high water tables and aren’t prone to flooding like basements and crawl spaces are. However, there are a few issues that stem from not having footings sink below the surface of the earth.

Frost Heave

Frost heave is the upward pressure exerted on the underside of a foundation when the water in the soil below it freezes and expands. The upward pressure can crack the concrete and cause structural issues. A clear sign of this foundation issue is a circular crack on the surface of the slab.

Since floating slabs don’t get additional support from footings that sit below the frost line, any expansion of frozen ground will put upward pressure on the foundation and could cause issues. As such, floating slab foundations are sometimes not viable in colder areas where the frost line is deep.

Differential Settlement

Another issue comes from soil movement in response to changes in moisture, specifically expansive clay soil. Clay soil absorbs water readily, and as it gets wet and dries, it expands and contracts. Since a floating slab sits directly on the soil, it is exposed directly to that expansion and contraction.

If the soil movement occurs unevenly under a floating slab, the difference in support the slab gets from the soil can cause the concrete to crack and lose strength. This could cause structural instability for the shed, detached garage, or home addition that sits above it.

Floating Slab Vs. Other Foundations

Most other types of foundations are common in home construction, and that’s the biggest difference between them and a floating slab: floating slabs aren’t suitable bases for standard homes. Instead, they’re more common under sheds, detached garages, and other small structures.

There are several other differences between block foundations vs. poured foundations, and other differences even among the different types of slab foundations.

Floating Slab Foundation

• Ideal for small structures, like sheds

• Minimal problems with moisture and pests

• Suitable on all types of soil

• Affordable

• Not suitable for entire homes

• Not great in areas with cold weather or expansive soil

Standard Slab/Monolithic Slab Foundations

• Suitable for home construction

• Minimal issues with moisture and pest infestation

• Suitable for all soil types

• More affordable than most other foundations

• No underhome area

• Expensive to repair

Pier and Beam Foundations

• Great for areas prone to flooding

• Easy to access for repairs

• Space to run utility lines

• Prone to moisture build-up and pests

• Require more excavation than slabs

• More expensive than slabs

Crawl Space Foundations

• Great for colder climates

• Easy access for repairs

• Room for utilities and storage

• Prone to issues with moisture and pests

• Second-most expensive style due to excavation

• Overkill for small structures, like sheds

Basement Foundations

• Ideal for cold climates

• Provide tons of space for storage or living area

• Easy to access for repairs

• Adds the most value to your structure

• Overkill for small structures

• Prone to problems with moisture and pests

• Most expensive foundation type

When Are Floating Slab Foundations Used?

Floating slab foundations are most common under sheds and detached garages. They’re affordable and fast to install, and they usually provide enough support for these smaller structures while keeping installation costs to a minimum.

Many homeowners also choose floating slab foundations for driveways, patios, sidewalks, and walkways since they’re affordable and should provide plenty of support for foot traffic and light vehicle traffic.

Floating slab foundations are sometimes suitable under small home additions as well. The contractor installing a floating slab can connect the new foundation to the existing one with ease, making it a fast and affordable option, provided your soil type and climate permit it.