Floating slabs sit directly on soil and are best for lightweight structures
Floating slabs are the fastest foundations to construct and require no excavation.
They’re typically only used for lighter structures, like detached garages and sheds.
They’re not ideal in colder climates where there’s a deep frost line.
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.
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.
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.
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.
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
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 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.
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.
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
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.
A floating foundation can be a good option under small structures like sheds and detached garages, under carports, or as sidewalks and concrete driveways because they’re the most affordable to install and usually provide enough support under lightweight structures and activity. However, they aren’t suitable for whole-home construction, as they don’t provide sufficient support for larger or heavier structures. They also come with their own issues, like being prone to frost heave and differential settlement, especially in areas that see a lot of rain or have clay soil.
A floating slab is a single chunk of concrete that sits directly on flat soil, while a monolithic slab is a single piece of concrete that includes built-in footings that reach several feet below where the majority of the slab sits. Monolithic slabs provide more support, so they’re suitable for home construction, whereas floating slabs get minimal support from the soil directly below and are more common under sheds and detached garages.
Yes, slab foundations can sink just like any other foundation type. Floating slabs are the most prone to differential settlement and foundation sinking because they sit directly on the surface of the earth. Slab foundations that have footings—including slab-on-grade foundations and monolithic slab foundations—get additional support from soil further down and are less likely to sink over time, especially under larger, heavier structures.
