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A structure’s foundation is its support system. Project owners must choose the right type or risk the structural integrity of the entire building.
But with so many to choose from, determining the best foundation for your commercial project can be challenging.
This article will examine one of the most common foundation types: the slab-on-grade.
We will discuss:
Table of Contents
1. What Is a Slab-on-Grade Foundation?
2. Types of Slab-on-Grade Foundations
3. The Advantages of Slab-on-Grade
4. Some Downsides to Slab-on-Grade Foundations
5. Slab-on-Grade vs. Mat Foundations
6. Slab-on-Grade vs. Slab-on-Ground Foundations
7. Slab-on-Grade vs. Crawl Space Foundations
“Slab” means a flat, continuous pour, and “grade” means ground. So, a slab-on-grade foundation is a concrete foundation placed directly onto the ground.
It is a simpler foundation type often used for home and even DIY shed foundations.
Slab-on-grade usually has thickened footings in the corners and edges of the slab and below load-bearing walls. It can also have foundation walls along the perimeter of the slab itself.
Several different types of foundations are grouped in the slab-on-grade definition, but each has unique attributes.
A monolithic slab foundation is poured continuously, creating a single, unified slab that includes both the footing and the slab itself.
The edges of a monolithic slab are thicker (typically 18-24 inches) to serve as the footing, while the center portion is thinner. The concrete is reinforced with steel rebar or wire mesh to improve strength.
Learn more: What’s the Best Concrete Strength Testing Method for Your Project?
Monolithic slabs work well in areas with stable soil and mild climates. They’re often used by homebuilders and for small buildings. Open floor plans without beams or columns could also use a monolithic slab foundation.
The T-shaped slab foundation consists of a separate footing poured first, followed by the slab on top. The footing is wider than the slab and resembles an inverted “T” shape when viewed in cross-section.
The footing is poured first and allowed to cure, providing a stable base for the slab, which is poured afterward.
T-shaped slab foundations suit colder climates where frost heave is a concern. It’s also used in areas with weaker or expansive soils.
An FPSF is explicitly designed for cold climates, where the slab is insulated to protect against frost heave without the need for deep footings.
Insulating materials, such as rigid foam, are placed around the slab’s perimeter to prevent the ground below from freezing. The slab itself is poured over the insulated ground.
FPSF foundations are ideal for cold climates with significant frost depths. It’s commonly used in residential and light commercial construction in northern regions.
Slab-on-grade foundations are a popular choice when possible because they have many benefits, which are listed below.
Slab-on-grade foundations have lower construction costs because less excavation and materials are needed.
They are much cheaper than crawl space or basement foundations. They also require less labor since pouring directly on the ground reduces the labor needed for digging, constructing foundation walls, and elevating slabs.
These foundations also provide faster installation since the process demands fewer steps than other types of foundations. You also don’t need quite as much effort for site preparation.
One of the most significant benefits of a slab-on-grade foundation is appreciated by the future users of the building.
These are the thermal mass advantages of this foundation. The slab will absorb heat during the day and release it at night, which reduces heating and cooling needs.
Slab-on-grade foundations also have reduced air leaks because they don’t include a crawl space or basement.
These foundations also require less maintenance with fewer moisture or mold issues.
As well, they provide pest resistance for the building. The slab acts as a barrier to pests reaching the structure, especially termites, which can cause extensive damage to wood.
Since the slab is on the ground, the entrance to the structure is also lower, allowing those with minimal mobility to enter more easily.
Related: Overview of Slab Construction
On the flip side, the simplicity of these foundations can also be their downfall. Here’s why.
Accessing under-slab plumbing, electrical, and HVAC can be difficult and expensive since they are embedded into the concrete slab. If any of them need repair, the cost and difficulty are significant.
Since the slab covers the pipes, plumbing leaks are highly likely to go unnoticed. Undetected leaks can cause significant damage before they are finally fixed.
Slab-on-grade foundations have a higher risk of cracking. Poorly compacted soils can shift under the slab, creating a cracked foundation. Clay soil is especially bad. This limited flexibility cannot accommodate soil shifts as raised foundations can.
Since the concrete slabs are in direct contact with the structure’s flooring, the floors are often cold, especially in the winter. Humid climates can cause condensation to rise to the slab’s surface, creating moisture issues and mildew.
A vapor barrier can be installed between the subgrade and slab to provide some sort of waterproofing, but this adds more time and money to the process.
Since these foundations bring the structure closer to the ground, heavy rainfall or poor drainage will make it more susceptible to flooding.
They also have no drainage crawl space, meaning there is no buffer zone to manage water overflow.
The limited foundation depth limits the types of buildings that can be set on it. This poses the most problems in areas with frost heave or deep foundations.
They also come with aesthetic constraints. Split-level designs aren’t an option when using slab-on-grade.
Finally, they are also not a good choice for a sloped property since extensive grading or filling would be needed to make them possible, increasing the structure’s cost and complexity.
When slab-on-grade foundations are not an option, many structural engineers design the building with mat foundations.
Although these two techniques are similar and often mistaken for each other, they have subtle differences that affect their structure, purpose, and usage.
A raft or mat slab foundation is a thick, reinforced concrete slab that covers the entire building footprint, distributing the load over a large area.
The slab is typically thicker than standard slabs and heavily reinforced with steel rebar or post-tensioning cables. It’s poured in one continuous process over a prepared base.
Mat foundations are typically used for heavy structures or buildings on weak, compressible soils where a standard slab would not provide adequate support.
When compared to slab-on-grade foundations, here are the key differences:
Mat foundations are designed for much heavier loads and larger structures than slab-on-grade foundations.
Mat foundations are preferred in poor soil conditions, while slab-on-grade foundations are best suited for stable, well-drained soils.
Slab-on-grade construction is more cost-effective and straightforward, while mat foundations are expensive, complex, and suited for larger, more demanding projects.
Slab-on-grade is typically used for residential and light commercial buildings, while mat foundations are more appropriate for heavy, multi-story buildings or challenging soil conditions.
Another commonly mistaken foundation for slab-on-grade is called slab-on-ground, or floating slab. Again, they are similar but have slight differences.
A floating slab is a slab-on-grade foundation designed to “float” without footings or deep foundations.
It’s commonly used in garages, sheds, and other outbuildings.
The slab is poured directly onto a prepared gravel or sand base without a separate footing. The slab edges may be thickened slightly, but they essentially sit on the ground surface.
Floating slab foundations are best for non-heated structures or small buildings where frost heave is not a significant concern.
The differences between slab-on-grade and slab-on-ground foundations are explained below.
Slab-on-grade foundations often have thickened edges that provide more structural support than some slab-on-ground foundations, which may not have this feature.
Slab-on-grade is typically used for buildings where the foundation must support substantial loads, while slab-on-ground can be used for lighter structures or non-load-bearing applications.
Slab-on-grade may involve more complicated construction, especially when integrated with footings or insulation, whereas slab-on-ground can be simpler, especially in applications like patios or walkways.
The terms are sometimes used interchangeably, but “slab-on-grade” generally refers to a more specific type of foundation with particular design features. At the same time, “slab-on-ground” can be a broader term.
Besides slab-on-grade foundations, one of the other more common foundation types is a crawl space foundation.
A crawl space foundation elevates the building slightly above ground level, creating a small space (usually 18 inches to 4 feet high) between the ground and the first floor. The crawl space is supported by perimeter foundation walls and possibly piers or posts.
Crawl space foundations are often used in areas with moisture concerns, sloped terrain, or where additional height is needed for flood protection.
Crawl space foundations are typical for areas with poor drainage, high humidity, or flood risks. They can also be used on sloped or uneven terrain or buildings requiring elevated floors or easy access to utilities.
There is a significant difference between crawl space foundations and slab-on-grade foundations. These are explained below.
Slab-on-grade foundations are generally more affordable and quicker to construct, while crawl spaces are more expensive due to the additional labor and materials.
Slab-on-grade foundations are good in mild climates with stable soil, while crawl space foundations are better suited for regions with moisture issues, poor drainage, or uneven terrain.
Crawl spaces can provide better insulation and comfort but require more maintenance and careful moisture management. Slab-on-grade can be energy-efficient but might lead to cold floors unless additional insulation is used.
Slab-on-grade foundations require minimal maintenance, whereas crawl spaces need regular inspections to address moisture, pests, and structural issues.
Crawl spaces allow easy access for repairs and upgrades, while slab-on-grade foundations make this more difficult and costly.
Learn More About Slab Foundations
Slab-on-grade foundations are perfect for specific projects due to soil conditions, climate, cost, and time constraints.
However, as the information shows, they have drawbacks and limitations. That’s why there are multiple ways to construct a foundation.
Understanding the difference between foundations and the possible benefits and drawbacks of each can help you choose the right type for your site and project.
Contact FMP Construction for insight on which foundation would be best for your needs.
