The FMP Blog

Mastering Commercial Slab-On-Foundations: A Comprehensive Guide to Unlocking Essential Knowledge

  • May 30, 2024

Project owners have a variety of choices regarding the foundation of their commercial buildings, one of which is slab-on-grade.

How does this type of foundation compare to the others, and what would motivate a project owner to select this one as the best option?

This article will answer those questions and many more. 

Contents:

1. What is a Slab-On-Grade Foundation?

2. Two Types of Slab-On-Grade Foundations

3. Advantages of Slab-On-Grade Foundations

4. Pitfalls of Slab-On-Grade Foundations

5. Slab-On-Grade vs. Mat Foundations

6. Slab-On-Grade vs. T-Shaped Foundations

7. Best Applications for a Slab-On-Grade Foundation

8. How to Build a Slab-On-Grade Foundation

9. Recap

What is a Slab-On-Grade Foundation?

Simply put, this type of slab foundation is a slab of concrete placed directly onto the soil. “Slab” means one continuous pour, and “grade” means ground.

Slab-on-grade is considered a shallow foundation because there isn’t much support under the soil. It usually has thickened footings in the corners, edges, and below load-bearing walls.

Two Types of Slab-On-Grade Foundations

There are two different versions of slab-on-grade foundations:

Supported Slab-On-Grade

A supported slab-on-grade foundation has standard footings called control joints, such as isolation joints, that support the columns above. The concrete slab rests on a vapor barrier, crushed stone, and reinforcing steel.

This option is stronger and more durable.

Monolithic Slab-On-Grade

This type has no footings. The slab itself bears the entire load of the structure above. Constructing a monolithic concrete slab-on-grade is cheaper, easier, and faster, but it’s not as strong as supported slab-on-grade. 

Advantages of Slab-On-Grade Foundations

Granted, slab-on-grade is often chosen for home foundations, since these builders are looking for the most economical choice for the homeowner — especially for basement foundations. 

Slab-on-grade works well in these situations, where the living space above is generally light and easily supported by a floating slab. Unless the plans call for a crawlspace foundation, this is a very popular residential strategy.

But what about commercial structures? Can a slab-on-grade work in the commercial sector?

First, the pros:

Affordable During Construction

Since the construction process of slab-on-grade is more straightforward than most, it is much cheaper when it comes to labor expenses. Less material is needed to construct a slab-on-grade foundation, so you also save money there. 

For example, no wood framing is necessary for the subfloor, since the concrete slab doubles as a ground-level concrete floor. There are also no extensive concrete footings around the foundation wall, saving even more on materials.

Longer Lasting

Even though this is a simple foundation, it is still stronger than many other types because it’s poured directly onto the soil, minimizing the chances of cracking. 

This reduces the need for foundation repair in the future. 

Cost-Efficient After Completion

There is no crawlspace under a slab-on-grade foundation, making it better insulated. 

In cold climates, where heating costs are hefty, slab-on-grade foundations are a great choice because they reduce the amount of radiant heat escaping through the concrete. 

Lower Environmental Impact

Concrete is considered one of the most sustainable materials in structural engineering. This is because less water, energy, and machinery are used to make concrete than many other substances. 

Faster Construction

Weather permitting, pouring slab foundations takes less than a day. Of course, this is after prepping the site and placing the forms. It also takes less time to dry.

The process only lasts about 4-7 days, depending on the project size. 

When considering commercial projects, minimizing the foundation’s construction time is preferable and can ultimately ensure its success.

Since the foundation is the first step of construction, every other stage depends on its completion. 

Less Susceptible to Pests

Since there is no space under the foundation for pests, such as termites or rats, to live, slab-on-grade foundations make structures less susceptible to creatures that can cause damage and headaches. 

Pitfalls of Slab-On-Grade Foundations

Now that we’ve examined the positive aspects of a slab-on-grade foundation, let’s look at the flip side. 

Cracking

Pitfalls of Slab-On-Grade Foundations

If improperly constructed, a slab-on-grade foundation can easily begin to crack. This type of foundation must be placed at the proper depth with the correct cement mix. It is also crucial that the concrete slab is adequately cured and given enough time to dry. 

Another problem is frost heave. When wet soil freezes, it expands. 

When this happens to the soil under a slab-on-grade foundation, which is placed directly on the ground, it pushes the slab up and can cause the foundation to crack. Spreading crushed stone before pouring the slab can minimize this effect, but it is still a possibility. 

Flooding

Because this foundation is thinner, the structure is closer to the ground. This makes the building more susceptible to flood waters. Even a few inches of water can cause great damage. 

Soil Settlement

After the slab is poured directly on the ground, the soil can shift or settle and cause foundational movement. This can result in uneven floors, sticking doors, and crooked windows. 

The thinner the slab, the more susceptible it is to soil settlement, so you can mitigate this by increasing its thickness. 

Site preparation, such as earthwork, is also important to ensure the foundation is laid on compact, level ground. 

Moisture and Drainage Issues

Concrete is a porous material that will absorb any moisture from the ground. 

Without proper moisture barrier application, such as polyethylene, the foundation can absorb moisture and cause damage, such as mildew, to accumulate in the structure above.

Heating Loss

Heat loss can occur with thin slabs or without the correct concrete mix or foundational depth, allowing heat to escape through the foundation itself. 

When the foundation is placed deeper into the ground, the soil acts as insulation, stopping the heat from escaping. 

Slab-On-Grade vs. Mat Foundations

Footings support most of the load for a slab-on-grade, and the edge of the slabs are thicker.

With mat foundations, the load is distributed across the ground’s surface, and the slab has a continued thickness throughout.

Mat foundations are exponentially stronger than slab-on-grade ones, making them better for larger, heavier buildings. 

Slab-On-Grade vs. T-Shaped Foundations

Slab-on-grade foundations are just one big chunk of concrete poured directly onto the soil. 

To support the load, T-shaped foundations have concrete structures placed under the frost line. These foundations are most often used in areas where freezing is an issue.

Since they are directly on the soil, slab-on-grade foundations are susceptible to frost heave. 

Extra steps need to be taken to make slab-on-grade foundations frost protected, such as skirt insulating.

Best Applications for a Slab-On-Grade Foundation

Since several types of concrete foundations exist, what circumstances would call for a slab-on-grade?

Areas With Warmer Climates

Concrete is damaged over time when it goes through constant freezing and thawing cycles. We’ve already discussed the problem of frost heave.

Most other types of foundations will have something added to mitigate this problem, but slab-on-grade does not. You could add frost-protecting insulation around the exterior of the concrete slab, but this is often not ideal for commercial projects. 

That is why areas with warmer temperatures year-round are the best place to install a slab-on-grade foundation. 

Areas With Clay Soil

Slab-on-grade does an excellent job of countering expansion and shifting, a common problem in clay soil. 

Projects With Low Load-Carrying Needs 

Slab-on-grade foundations aren’t considered the strongest ones available. Their load-carrying capacity is low, but they are economical.

This makes them a good choice for projects with low budgets and a light structure, such as garages, storage sheds, and the like. 

How to Build a Slab-On-Grade Foundation

The construction of a slab-on-grade foundation follows the basics overall, but with some key differences. 

These are the steps taken to build a slab-on-grade foundation:

1. Site Preparation

Before a foundation of any kind can be built, some soil-related tasks need to be completed. With slab-on-grade, the ground is leveled and compacted to about 90%.

Then gravel or crushed stone is added as a base under the concrete and for drainage purposes, as the water can escape through the cracks between the rocks. 

A French drain can be installed with a slope at the bottom of the crushed stone to help with any excess stormwater. 

2. Form Construction

Batter boards are used to keep the concrete pour in place. The boundaries can be drawn out by placing stakes at the corners and string lines between each stake. 

If the batter boards are higher than the depth of the pour, mark these boards with chalk to delineate the pour line. 

3. Waterproofing

After the plumbing and mechanical systems have been installed, insulation panels can be cut and placed around the piping and equipment. The building codes for insulation levels vary by region, so careful attention must be paid to ensure these are met for your specific area. 

Next, add a vapor barrier (usually polyethylene lining) for waterproofing. Spray foam can be used for additional insulation if necessary. 

4. Installing Reinforcements

Rebar and/or wire mesh are placed across the entirety of the foundation floor to add strength to the concrete slab. 

These reinforcements must be installed carefully so as not to puncture the vapor barrier below. 

5. Pouring Footings 

The footings are poured before the rest of the concrete. The engineer must calculate their proper depth, width, and placement. The size and type of structure will determine these needs. 

Reinforcement rods are installed in each footing as a connector to the rest of the structure. 

6. Main Pour

concreate pouring

The remaining slab is then poured, with thicker concrete at the edges. The concrete is cured for three days and needs to be kept moist for the entire time by spraying it with water and covering it with a tarp.

After curing is finished, the concrete must be allowed to dry before any further weight is placed on it and the forms removed.  

Read more: Concrete Forms and When to Remove Them

7. Finishing

There are several finishes to choose from for your slab-on-grade foundation. Since it doubles as the floor, the finish is actually an important decision that needs to be made. 

Of course, you need to ensure the proper trowel work is done to flatten and level the pour. What used to be done by hand is now completed with a ride-on power trowel, making the job much easier and faster.

After the trowel work is finished, the simplest and cheapest option is just to buff the slab to your desired sheen. However, there are many concrete flooring options available for those looking for a more aesthetic result. 

To mitigate hairline cracks, expansion joints can be installed and hidden under division walls. 


Recap

Slab-on-grade foundations are common choices for many residential structures as well as some commercial. They are most popular in areas with warmer climates and clay soil. 

Remember, this is only one of several decisions you’ll need to make for a commercial project’s foundation, and it might benefit more from a different type of foundation. 

To help decipher which foundation type would be most suitable for your unique circumstances, contact FMP Construction today.

Latest Posts

© 2023 FMP Construction. All rights reserved.