Shipping container homes need to rest upon a firm foundation. Although there are many types of foundations that are used under buildings, three are the most commonly used for shipping containers.
The three most common types of foundations used under shipping container homes are made of reinforced concrete with either piers at the four corners when the soil is firm enough to support the load of the building, stem with spread footings when the load needs to be spread out over less supportive soil, and turned down slabs over expansive soils like soft clay.
Although not common because of their expense, piles are used in weaker soils.
A professional who understands your specific project should help you design the perfect foundation for your shipping container. But, to get started, it is helpful to know what the three common types of foundations are.
Also, let’s look at the basics key factors that will affect what the best type of foundation should be used.
Three Most Common Foundation Types Under Shipping Container Homes (& One Uncommon Type)
1. Pier Foundations
Pier foundations are round and usually made of reinforced concrete.
This type of foundation is one of the quickest, easiest, and cheapest type of foundations.
A cylinder form is placed into the dugout soil. Quik-Tube and Sonotube fabricate these cylinder commonly used for constructing steel reinforced concrete pier foundations.
Here is a link to Home Depot’s product that will form piers called Quik-Tube.
Since concrete is only good to support compression forces and will fail if bent or twisted, steel is added to concrete structural members to withstand the tensile forces that the concrete might be having to withstand.
The steel reinforcing bars (“rebar”) are placed inside of the form and then concrete is poured all around the rebar.
Then, threaded rods are placed into the concrete before it sets and hardens.
After the concrete cures, then 1” of grout is placed on top of the pier so a metal plate can be leveled on top of it before the grout sets.
Then, a metal plate that is welded to the bottom of the shipping container is placed over the grout with the threaded rods penetrating through holes in the plate. The plate is held down with two bolts at each threaded rod.
Another way the shipping container is attached to the reinforced concrete pier is with rods welded to the bottom of the shipping container connector.
Then the shipping container is twist-locked to the connector. Some municipalities may require you to weld this twist lock so it will be considered a permanent foundation.
If the soil is not strong enough to support the load that is imposed upon the pier, then the structural engineer may design a spread footing under it so the load is distributed to a larger surface area.
The spread footing is also part of the next type of foundation discussed below.
2. Stem with Spread Foundations
The stem part of this foundation system is built with either steel reinforced concrete or masonry concrete units (CMU). The stem is continuous around the perimeter of and below the building.
The spread foundation is the steel reinforced concrete that is poured under the frost line (see discussion of the frost line below). It is also continuous around the perimeter of the and below the building.
The stem is built on top of the spread footing.
It is important to keep water away from the foundation. Often, waterproofing is placed on the outside of the stem and spread footing.
The top of the stem wall of the foundation should be several inches above grade (grade is the top of the ground). This will help prevent water from running into the building.
A continuous pipe is place near the bottom of the spread footing at the outside perimeter to pick up any water that seeps into the soil. This pipe has holes on the bottom of it. There is gravel placed all around the pipe so the water can percolate to the bottom of the pipe and be picked up through the holes in the bottom of the pipe. The pipe takes the water away from the foundation and often to a dry well that is a safe distance from the building.
A permeable fabric is wrapped around the gravel to keep any silt from mixing with the gravel. If silt was allowed to mix the grave, the gravel would not be able to conduct the water through it very efficiently.
The shipping container is attached to the top of the stem wall foundation similar to the way it is attached to the pier foundation mentioned above.
The illustration above shows the base plate welded to the bottom of the corner casting of the shipping container.
If you are building a basement or cellar under your shipping container home, then this type of foundation maybe your best choice.
The walls of the basement or cellar would be just a deeper stem wall.
Just be sure to use proper waterproofing along the foundation to keep the inside of your basement or cellar dry and mold free.
3. Turned Down Slab Foundation
This type of foundation is also referred to as a “raft slab” because is it flat and virtually “floats” on top of the soil like a raft.
If the soil is wet and expansive, like the clay soil in much as Fort Worth and Dallas, Texas, then a popular type of foundation is the turned down slab foundation. This type of foundation allows the building to virtually float on top of the soil. So, if the soil moves, the foundation moves with it.
The edges of the foundation are thicker and acts as an “anchor” and helps keep the slab from moving around very much. The movement of this foundation is so slight that it is not noticeable. But, without it allowed to move, the foundation could crack.
One disadvantage of using this type of foundation is that all utilities are buried under the reinforced concrete slab. If they there is a leak, for example, in a pipe that is under the slab, the slab will need to be cut up and opened to give access to the pipe needing repair.
The slab is usually poured over a vapor barrier membrane and gravel to help keep any moisture from seeping up through the slab and into your new home.
4. Piles Foundation
Piles foundations are not very commonly used when building foundations for shipping containers. So, it was not listed above as one of the three most common types of foundations used.
But, it may be an option if the soil, such as sand, does not support much weight.
Instead of bearing upon a sturdy subsoil layer (unless it exist deep under the top of the ground level), the structural engineer may design the pile footing to support the load imposed by the weight of the shipping container building by considering the friction along the outer surface of the piles that pass through the weak soil (like sand).
Piles are one of the most expensive types of foundations and may be cost and time prohibitive for most cases.
13 Key Factors that Will Determine Which Type of Foundation to Use
Consider these 13 items before you and your professional chooses the best foundation for your shipping container.
1. Bearing Capacity of the Soil
Probably the most important factor to consider is knowing how much load the soil under your shipping container home will support without letting the building sink. Soil that allows the building to sink are illustrating a “shear failure.”
To find out the soil bearing capacity, you can have it field tested by experts. Even the experts who have the experience in testing soils still add a safety factor on top of their findings because they know that the testing results can vary on the same site.
If it is found that the soil bearing capacity is high, meaning it will support a lot of weight, then the footings usually do not need to be very deep. Piers or stem walls with spread footings are commonly used.
If the foundation needs to be deep, then you might need to hire a contractor to drive piles () deep into the ground. Piles are either long round metal caissons filled at the top with concrete or pressure treated wooded poles that are driven into the ground.
Be sure to check with the local officials to see if your foundation needs to withstand seismic forces (like earthquakes) or other lateral loads (like tornados).
2. Frost Line
During cold seasons, the soil will freeze. If the bottom of the foundation is designed to rest onto a firm soil, you do not want that soil to change its bearing capacity during different times of the year.
Like anything else, soil will contract when it freezes and expand when it is thawed. This shrinking and expansion can break up the foundation.
To avoid this, design the foundation to bear on soil several inches below the frost line.
The frost line is at a depth of the soil when it no longer freezes when the above soil air temperature is below freezing. This depth will be generally consistent in the region that you are building your home.
For example, the frost line is 36” below the top of the soil in New Jersey where I live.
3. Climate
The climate will affect the weather. The weather will affect how much rain or any other type of moisture will fall around the foundation.
It will also affect the relative humidity of the air that will affect the curing of the concrete foundations.
The sun plays a factor too. Too much heat can cure the concrete too fast and diminish the strength of the concrete when it is curing.
Depending on the climate, you might need to put admixtures in the concrete to accelerate or slow down its curing time.
4. Ease of Construction
If you are going to do it yourself, choose the correct foundation best for the support of your new shipping container home. Also pick the type that you feel most capable of constructing.
Do it yourselfers tend to choose pier foundations because they are the easiest type of foundations to construct.
You might not have the option to use an easier foundation type and will need to pay a professional contractor to construct the more expensive and complicated foundation with their equipment and man-power.
5. Permanence of the Building
How long will the shipping container be at the location it is built?
Some municipalities require the shipping container to be permanently part of the foundation. If so, you will want to place the home over one of the foundation types discussed in this article.
If the shipping container building is to be movable. Then, the foundation of that building may be on temporary skids or on a chassis with wheels.
6. Ground Water Level
Find out where the water level is location under the top of the ground. There are underground streams and rivers that will not serve your building well as a strong foundation for obvious reasons.
This is not uncommon. I was hired to examine an existing school’s cellar walls in the Bronx, NY. It had water seeping through it because there was an underground river that ran up against it. The river was discovered on very old maps that were drawn before America won its independence from Great Britain.
Be sure if the ground water level is not so high that it could erode and wash the soil away from underneath your building’s foundation.
Also, slope the dirt downward and away from the foundation all around the building’s foundation. This will cause the water at the top surface of the ground to run off and away from the building’s foundation.
7. Thermal Conditions
The temperature of the air above the foundation and the soil around the foundation will affect the success or failure of the building’s foundation as mentioned in the above discussion about the frost line and the climate.
8. Radon and Other Contaminates
Radon is a radioactive gas that seeps out of the soil and can cause lung cancer.
It is colorless, tasteless, and cannot be seen or smelled.
It is found throughout America and is particularly evident in North Dakota and Iowa. According to epa.gov, nearly one out of 15 homes in United States is estimated to have an elevated radon level.
Be sure that soil you want to build your foundation is tested and found to be safe from radon.
If the soil was once a site of a factory, dry cleaners or a farm, there may be toxic chemicals mixed in it.
This type of soil should be replaced with clean soil before building a home over it and can be especially hazardous to children.
9. Termites and Other Disruptive Insects
There are some home built upon timber foundations that could be susceptible to termites.
Reinforced concrete stem with spread footings is continuous all around the perimeter of the building.
It acts as a barrier against unwanted insects like termites and can be a desirable type of foundation if there is a threat of termites in some locales.
10. Local Market Preferences
The availability of certain construction materials over others and the skill level of the available local work force can affect the type of foundation that is chosen for certain parts of the world.
11. Construction Costs
The “bottom line” is an important factor when building your shipping container home.
If one type of foundation is as good as another, then cost can play a big part at which one to use.
For example, piers are usually less expensive than other foundation types.
12. Construction Schedule
With some types of foundation requiring more time to build than others, you might need to choose the one that can be built faster if there is a time restraint to the construction schedule.
For example, if the season of freezing temperatures are fast approaching, you may want to choose the easiest and fastest type of the foundation to build, like piers, if the other factors also support this choice.
13. Topography
If the site that you are building your shipping container is hilly or with sharp land features, such as cliffs, then you will need to be very selective as to where you will construct your building foundation.
Some of my home designs have been on edges of cliffs and near a waterfront for spectacular panoramic views.
Sometimes you will need to let the site dictate how it wants your home to rest upon it.
Removing Uncertainties One Step at a Time
Designing and building your shipping container dream home has many issues you will need to address. When launching your shipping container home journey, it can be overwhelming while thinking of all the obstacles that are in front of you. It doesn’t have to be that way. Watch the free 90 minute seminar title, “6 Step Formula to Design and Build Your Own Shipping Container” and take the uncertainties away one step at a time.