For builders and homeowners, slab-on-grade foundations are popular, thanks to its many advantages. Also known as a floating slab foundation, it consists of a thick concrete slab, poured directly on the ground, and serves as the base of your structure.
In this structural engineering practice, the concrete slab is formed from a mold set into the ground.
This concrete slab usually serves as the foundation for a structure by transferring its weight directly to the soil or rock beneath it. Without leaving any space between the structure and the ground, the concrete is poured directly into the mold.
This foundation construction method is mostly used and idea in warmer climates. This is because the heat that’s ducting beneath the building’s floor is not necessarily needed.
Additionally, this type of construction is also utilized in places where seasonal thawing and freezing of the ground is not much of a concern.
To add, reinforcements are not necessarily needed in this type of foundation method. While reinforcements may be added, they’re not required, just like how footings and scaffoldings are not needed.
This process helps in saving a lot of time and money, while still providing the same structural integrity as any other type of foundation construction method.
Formwork is used minimally, batter boards are provided at the corners of the slab, and expansion joints are used at the slab’s edges. Since the slab on grade rests on earth, it does not require beams to support it.
How Is Concrete Slab on Grade Foundation Constructed?
The entire construction process for a slab-on-grade foundation is typically straightforward.
The first thing done during the construction of the concrete slab on grade is the excavation of the earth up to the required depth. The earth is also compacted at a minimum of 90 percent compaction in most cases to remove air voids.
Once the earth is compacted, gravel or crushed stone is added to serve as a base for the concrete slab. It also helps in providing drainage, so water does not accumulate beneath the concrete slab on grade and get absorbed by it.
Before pouring the concrete, batter boards are made use of and are marked on the ground to help in forming the required shape and size of the slab. These batter boards act as a concrete mold by helping identify slab corners.
Once batter boards are in place, proper investigation is done to ensure that all of the required components, like reinforcing steel and material for expansion joints, are in place. The soil also requires thorough investigation to determine the thickness of the slab.
Depending on the results of the soil investigation, layers of moisture barriers and gravel are then poured on the ground. These layers prevent the penetration of moisture into the slab.
The concrete is then poured thicker at the edges, forming an integral footing. From there, reinforcement rods are used to strengthen the edges of the slab.
The concrete is then allowed to cure and dry for a few days. Additionally, expansion joints are also put in place to minimize the effects of expansion and contraction due to temperature changes.
Once done, the concrete slab on grade can then be used for building construction. It is important that proper inspection is done before any kind of construction begins and the concrete has completely cured.
2 Types of Slab on Grade Foundations:
Slab-on-grade foundations come in two distinct varieties: supported and monolithic:
Supported Slab on Grade
A supported grade slab foundation is used when the common footings are already framed on-site. This type of slab-on-grade is supported directly on the framing and can accommodate the water and soil pressure.
When it comes to the integrity of the slab and its long-term performance, the supported slab-on-grade is superior to the monolithic slab-on-grade. The grade slab rests on a layer of moisture and barrier, crushed stone, and reinforcing steel slab to provide the required level of support.
Monolithic Slab on grade
In a monolithic grade slab, the concrete slab acts as a footing for the structure because the grade slab itself does not have any footings. The walls and columns are raised from the grade slab, which directly bears the entire load of the structure.
To frame this type of slab, batter boards are provided around the concrete slab, and the concrete is poured inside the batter boards. Acting as a mold, these batter boards identify the slab corners.
Monolithic slab on grade foundation is poured all in one go. This makes the process quicker, which significantly lowers labor costs. Instead of footers, the thick concrete slab is then poured on the outside to help with the load-bearing of walls.
Monolithic slab foundations are best used for flat ground, unlike supported slab foundations, which can be used on both flat and sloped ground. Because a huge amount of fill dirt needs to be used to level the ground, this may cause problems in the future since the concrete slab will crack if the floor is not properly compacted.
Another advantage of using monolithic slabs is that it’s easy and faster to build. Simply pour the concrete for a day, given that the soil has been compacted, add the perimeter trench, followed by the gravel spread.
The concrete dries faster than they do in other foundation construction types since they just need to be 4 inches thick at its thinnest part. If properly constructed, this slab foundation will last a very long time. Once you have installed your anchor bolts and reinforcements, you will have a strong foundation that can endure a lot of weight.
This type of foundation construction does not require regular maintenance either. Month-by-month inspections are no longer necessary since the concrete slab is already reinforced and thoroughly compacted. All you have to do is make sure there are no cracks or holes in the foundation that could cause a problem later on.
Advantages of Using Slab on Grade Foundation
Slab on grade foundations aren’t popular for no reason. There are quite a few advantages to this foundation construction type:
Lasts Longer and has Higher Quality
Because the concrete slab is poured directly onto the grade, it is more resistant to cracking and other structural issues. The concrete itself is also stronger and of higher quality than most traditional foundations.
Improved Quality of Life
When you build your foundation above ground, the structure gives off a lighter hue, allowing more natural light to enter your home. This can do wonders for the overall quality of life inside the home and can even lead to increased property values in some cases.
Additionally, this type of construction decreases the probability of mod buildup, while boosting clean air quality within the building.
With that being said, this type of construction simply makes your building more comfortable to live in. The interior temperatures are regulated by the thermal mass within the structure. This thermal mass absorbs and stores heat which is how they control the temperatures in the building.
It is easier to keep your living space cool in summer if your home has significant thermal mass inside the building. Good airflow and well-regulated temperatures equal a good quality of life.
Compared to basement foundations, slab-on-grade slabs are significantly cheaper since it requires minimal labor costs and time. The labor only needs to spread the gravel, pour the concrete slab, and adjust drainage lines as needed.
You don’t need to build a wooden subfloor on top, or an 8-foot concrete wall or footing to support the entire structure either. This makes it a budget-friendly option for those looking to save money during their construction process.
Without needing wooden materials, you are also reducing the amount of wood being used, which is beneficial for nature. Additionally, there’s no crawlspace beneath the foundation, which means that your utility costs will significantly decrease.
Having a concrete slab on grade increases the overall insulation of your home, creating an energy-efficient structure. The temperature isn’t allowed to escape as easily and the air inside won’t suffer from extreme temperatures.
Overall, slab-on-grade foundations are easy to construct and maintain, which is why they are so popular today. They provide high-quality results that are cost-effective, making them one of the best foundation construction options available.
It gets cooler the further you get from radiant floor heat. Instead of heat being concentrated in the highest point of the building where people don’t usually stay, it centralizes at the ground level where people are.
Slab-on-grade foundations store and release heat during the winter and reject heat in the summer. If you want a passive building then this is a good starting point for you.
Reduces Environmental Impact and Carbon Footprint
Slab-on-grade foundations can significantly reduce the environmental impact of your building, as well as its carbon footprint. The material used for these types of foundations is often more sustainable and less energy-intensive to manufacture than other foundation materials such as wood or steel.
It also eliminates the need for deep excavations which decreases deforestation as well as the use of heavy machinery. This type of construction can also reduce the amount of water used in the building process, as well as electricity and fuel used in construction activities.
Cost-Efficient in the Long Run
Most people argue that a slab-on-grade foundation will only produce homes that have less value than an equivalent basement foundation home. This may be true in the beginning, but not for long.
Though buildings with a basement foundation will sell for more than an equivalent building with a slab-on-grade foundation, buildings with a basement foundation will cost more to build and live in.
Factoring in the savings on cooling and heating and the savings you will likely have in the building stage of a slab-on-grade home, you will find that you are in an equal or better financial position than someone selling an equivalent basement foundation home.
Shorter Construction Time
Creating a concrete slab foundation does not take long. The entire process requires only one day, on top of the initial preparations that will be done. As long as the weather permits and the workers are well-organized, the work can be completed quite quickly.
On average, the entire construction can be completed in as fast as four days – this is significantly shorter than the time needed to build a basement foundation, which could take up to several weeks.
More Flooring Selections
Another advantage of this foundation type is that it allows for a broader selection of flooring options such as scored or stained concrete.
Significantly Takes Less Time to Dry
A slab-on-grade foundation takes less time to dry. This means that construction can continue without much delay. You don’t have to wait the number of days it would take for concrete to cure and dry in a basement foundation.
Decreases Chances of Getting Damaged from Gas Leaks or Flooding
Slab-on-grade foundations reduce the risk of damage from gas leaks and flooding from a crawl space or a basement into your house.
Helps Keep Pests Away
This type of construction can protect your house from insects like termites since your building does not have any open space under it that would have provided access to these insects. Apart from insects, rodents, mold, and mildew is not an issue with this type of construction. Your house simply does not have space under its slab for rodents and insects to nest and for mold to form.
Disadvantages of Slab-On-Grade Foundations
Since slab-on-grade foundations are cheap, quick, and easy to install, they are commonly used for tract housing. This type of construction is a great building practice when designed and installed properly, but if the slab-on-grade foundation is not installed correctly it can cause serious damage to your home, such as:
Cracking of the Foundation
A slab-on-grade foundation that is not designed or installed appropriately will crack. If the concrete is not dried or cured properly, it can develop cracks that will allow water and moisture to enter your home.
Although cheaper to build, it requires a delicate technique to properly construct a slab-on-grade foundation. The concrete slab has to be poured to the proper depth, and you have to get the correct concrete mix to avoid any problems.
Homes that have foundations with cracks have structural issues that only damage the building.
A slab-on-grade foundation is placed directly on the soil, so it can be affected by organic material and other particles that can cause the soil to settle. This type of settling will force the foundation up or down, creating problems for your house such as sticking doors and windows due to an uneven floor.
The solution for this problem is to choose the location for your foundation carefully and make sure it is placed on a solid surface that won’t shift.
Additionally, if the slab is not poured to the proper depth, slab-on-grade becomes more susceptible to soil settlement. Most times contractors/subcontractors do not make the form for the slab deep enough (either from the desire to save money, from oversight, o laziness), this makes the slab too thin and therefore susceptible to soil movement and cracking.
It is hard to repair soil settlement damage to a slab-on-grade foundation. You cannot place jacks under the slab to raise and steady the foundation since the slab lays directly on the soil below.
Boring holes and installing piers through the slab-on-grade foundations is the commonly used method in repairing soil settlement damages to slab-on-grade foundations.
Pumping a mixture of water and cement (slurry) under the slab to lift it is another way to fix soil settlement. The problem with these repairs is that they can cause extensive damage to your home if they are not done well.
Moisture and Drainage Issues
Since cement is a porous material/substance, moisture, and drainage problems can result from this type of foundation. Moisture can move from the slab into the living area of the structure above if no proper barriers are separating the slab and the soil below it.
Water stains, mold, and mildew are common signs of moisture and drainage problems. If left unchecked, moisture can make the frames of the structure rot.
If the slab-on-grade foundation is not thick enough, then heat loss occurs. This usually happens if the contractors did not dig out a form that is deep enough for the foundation.
The heat from your home can escape through the slab, causing your heating bill to increase significantly.
To ensure that this does not happen, you need to make sure that the contractors have dug out a deep enough form for your foundation and it is poured with a mixture of concrete that will provide insulation against heat loss. Additionally, installing an insulation membrane between the slab and the soil can prevent heat loss.
With slab-on-grade foundations, homes rest very low to the ground. This makes them susceptible to flooding. If the surrounding properties or yards have not been well-graded, a structure on a slab-on-grade foundation can flood. Unlike basement foundations, the positive thing about this is that you will have minimum damage from the flood.
Remember, you can only experience these disadvantages if your slab-on-grade foundation is not properly designed and constructed. Otherwise, this type of construction has many advantages as can be seen above.
Slab-on-grade foundations are an excellent choice for a sturdy and economical flooring option. When handled properly and installed correctly, they provide a great foundation that can support many types of buildings.
With modern insulation and moisture-control systems, you can count on a slab-on-grade foundation to last for many years.
However, if you are facing problems such as soil settlement or flooding due to your current slab-on-grade foundation, it is best to get help from a professional building contractor who will be able to diagnose and fix the issue.
Investing in proper repairs now can save you from a lot of headaches in the future.