SLAB ON GRADE | Explained

A slab-on-grade foundation is also known as a floating slab foundation. 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. 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 in warmer climates where the installation of heat ducting beneath the floor of a building is not needed. This type of construction is also used in places where seasonal thawing and freezing of the ground is not much of a concern.

In this method of foundation construction, reinforcements are not needed. You may wish to have or not have reinforcements. This method also does not require footings to support and scaffolding to construct.

Formwork is used minimally, batter boards are provided at 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.  

Construction of Concrete Slab on grade

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 minimum of 90 percent compaction for most cases to remove air voids, Before the concrete pour, batter boards are marked and placed in the position the plan suggests.

These batter boards act as a concrete mold by helping to identify slab corners. Investigation of the soul is done to create the thickness of the slab. Depending on the results of the soil investigation, layers of moisture barriers and gravel are poured on the ground.

These layers prevent the penetration of moisture into the slab. The concrete is then poured thicker at the edges, form integral footing. Reinforcement rods are used to strengthen the edges of the slab.

The concrete is then allowed to cure and dry for a few days. Also, control joints on the slab are marked using chalk lines. This is to prevent or minimize cracking on top of the concrete.

Types of Slab on Grade

The two different types of Slab on Grade are

  1.  Supported Slab on Grade.
  1. Monolithic Slab on Grade.

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. The wall stands on a footing.  

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.

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 quick and helps lower labor cost. To help with the load-bearing of walls, there are no footers, and a thicker concrete slab is poured on the outside.

Monolithic slab foundations are best used for flat ground. A lot of fill dirt is used to level the ground If it is not flat. This may cause problems in the future since the concrete slab will crack if the floor is not compacted properly.  

Monolithic slabs are ideal because they are quick to build. You can pour the concrete for a day if the soil has been compacted, the perimeter trench added, and 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 their 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. You don’t need a month-by-month inspection. You just have to inspect it at intervals to make sure there are no cracks in the foundation.  

You don’t waste energy heating the air below the house since it does not have any space between the home and the ground. This makes this foundation cheaper in both the short and long term.

Advantage of Slab on Grade Foundation

1. Quality of life

When you build above ground, the structure tends to have more natural light, unlike basement structures. Slab on grade foundations decreases the probability of mold and helps boost clean air quality in the building.

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.

2. Affordable

This type of construction is more affordable than a basement. Unlike with a basement, you don’t have to build a wooden subfloor on top or an 8-foot concrete wall for the building project to be brought to the point where it is ready for the main floor to be framed.

This makes slab on grade foundation more affordable since you don’t need to build any extra thing. This type of foundation is generally less expensive to construct than other types of foundations. Slab-on-grade construction does not need wooden members such as floor joists.

This means that the price of these wooden members is eliminated. Utility bills that you have to pay concerning heating and air conditioning are lowered since there is no crawlspace under this foundation.

3. Comfortable and balanced

A slab-on-grade foundation in a cold climate zone usually requires additional measures such as insulation to avoid frost heave. This is because there are no 5 feet of dirt in the slab-on-grade foundation, unlike in basements.

These additional measures do not add to the overall cost of slab-on-grade foundations. This is because you can just pay for these measures with the money you would have used to buy concrete for a foundation wall.

This type of foundation offers a comfortable and balanced environment since the floors accommodate radiant floor heating easily. This situation     transforms the concrete slab into a radiator.

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 are not, 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.  

 4. Reducing your environmental impact/carbon footprint

The slab-on-grade construction method reduces your carbon footprint by providing a wall that has better insulation. It also does this by reducing the amount of carbon dioxide produced when the materials are being manufactured and transported.

Slab-on-grade buildings use foundation walls in place of above-ground walls. This means that an energy-efficient wall assembly will be used in place of the more expensive concrete walls.

5. You can save money 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.  

6. Short creation time

Creating a concrete slab foundation does not take long. You have to make some preparations first, but you can pour the concrete slab in just a day. As long as the weather will not cause any issues, the entire foundation process can be finished in a maximum of four days.

7. Wider Flooring options

Another advantage of this foundation type is that it allows for a broader selection of flooring options such as scored or stained concrete.

8. 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.  

9. Less Risk of Damage from Flooding or Gas Leaks

Slab-on-grade foundations reduce the risk of damage from gas leaks and flooding from crawl space or a basement into your house.

10. Protection from Pests

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.  

Read more about when to call for Slab on grade inspection.

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.

Some of these damages includes:

  1. Cracking foundation
  2. Soil settlement
  3. Heating loss
  4. Moisture and drainage issues
  5.  Flooding

Cracking foundation

A slab-on-grade foundation that is not designed or installed appropriately will crack. If the concrete is not dried or cured properly, the foundation will crack.

The slab-on-grade foundation is a delicate technique that needs attention to detail. 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.

Read more about how to know if you have a slab leak ?

Soil settlement

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 the 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 the common signs of moisture and drainage problems. If left unchecked, moisture can make the frames of the structure rot.

Heating loss

If the slab-on-grade foundation is not thick enough, then heat loss occurs. If the contractors did not dig out a form that is deep enough for the foundation, the concrete slab laying on the soil can cause significant heat loss from the structure above. This can lead to financial loss and discomfort to homeowners.

 Flooding

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. The positive thing about this is that you will have minimum damage from the flood, unlike basement foundations.

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 from above.

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