Along with the appropriate insulation, making sure you use the correct foundation for your shipping container home is crucial for a successful build.
We discussed How to Build Your Shipping Container Home the Right Way previously. Since this new article is a continuation of that discussion, if you have not done so already, you may want to take the time to read the other article before you continue here.
In short, you will always need a foundation for your shipping container home. This is because the ground moves a considerable amount. The ground can rise, sink or slide. This movement can be sporadic and is usually very slow. Even though it is often barely noticeable, this slight movement can affect how level your home is.
A foundation provides a solid, stable platform for your building. Without this solid platform, the ground’s natural movement can cause the containers to split and separate.
The ground underneath your building can also be comprised of different materials. For example, part of the ground could be hard rock and the other part soft clay. This creates an unevenness which can allow your home to shift since the load isn’t equally distributed. One result could be having doors that are incredibly difficult to open and close.
A solid, well built, foundation will ensure the weight is appropriately distributed. It will also help prevent moisture and the corrosion that occurs as a result of this moisture.
Note that if the shipping container home will be relocated within a few months, it is sufficient to use railroad ties for this short time frame.
The four main foundation types which can be used with container homes are pier, pile, slab and strip. There are other types of foundations but these are the most commonly used with container homes.
We will outline when you should use each one and discuss the strengths and weaknesses for each.
Pier foundations are the most popular choice for shipping container homes for numerous reasons. They are relatively inexpensive, DIY friendly, and quick to construct.
As seen in the photo above, a pier foundation is comprised of concrete blocks. Each concrete block, or pier, is generally 50 cm X 50 cm X 50 cm and containers reinforcing steel inside to improve the concretes strength in tension.
With shipping container homes, the concrete piers are generally laid at each corner of the container. And, with the with larger 40-foot containers, an additional two piers can be placed midway down each side of the container.
You save a lot of time and money with pier foundations because you don’t need to excavate a lot of earth at all. You only need to excavate the ground for the piers, which are generally 50 cm X 50 cm X 50 cm.
Compare this to a slab foundation where you will need to excavate basically the entire area under the container.
Another great reason to use a pier foundation is that other foundations, such as pile foundations, require expensive specialized equipment which can obviously be difficult for DIY builders.
This is by far the most popular shipping container foundation and is the one we recommend for most people.
Pile foundations are used when the soil type is too weak to support a concrete base. This type is the most expensive type of foundation covered here.
If you remember, pile foundations were used in the Graceville Container Home Case Study.
Piles (cylindrical solid steel tubes) are hammered into the ground through the soft soil until the piles reach more suitable load-bearing ground.
Example of Pile Foundations
Once the piles are secured in place they are traditionally capped with a block of concrete. So once you have secured all your piles you end up with a grid system of concrete caps which above ground look visually similar to concrete piers.
Pile foundations are not recommended for a DIY builder. A contractor would be needed to install pile foundations due to the specialized equipment needed, such as the pile driver.
A slab foundation is a popular choice when the ground is soft and requires an equal weight distribution. However, it is more time-consuming and more expensive to build than a pier foundation. If you are going to use a slab foundation, be prepared to dig a lot!
As shown in the photo above, a slab foundation is a concrete slab onto which your containers are placed. The slab foundation is generally slightly larger than the footprint of your home.
If you are building with two 40-foot shipping containers, your slab foundation would typically be 18 feet wide by 42 feet long. This would provide an overhanging foot of foundation around the perimeter of your shipping containers.
A huge benefit of slab foundations is that because it provides a solid base, there is no hollow space in the foundation. This prevents future problems such as termite infestations.
Unfortunately though, because of the additional concrete used and the vast amount of space which needs to be excavated, slab foundations are significantly more expensive than pier foundations.
We often see slab foundations used in warmer climates where freezing isn’t a concern. However, they do increase the potential for heat loss when ground temperatures drop below the interior temperature because the container can conduct heat into the ground, which transfers more heat than via convection into the air.
Note that with slab foundations, once the concrete has set, there is a lack of access to utility lines. If you have a leak in your water pipe, the concrete will have to be cut to access the pipe. With a pier foundation, you will always have access to your utility lines.
A strip foundation (also known as a trench foundation) is somewhat of a combination of the previously mentioned pier and slab foundation.
The strip foundation, shown below, is simply a strip of concrete which is laid to support the containers. The concrete strip is normally 1-2 feet wide and 4 feet deep.
The strip can either go around the perimeter of the containers or it can be laid at the top and bottom of the containers instead.
It is ideal to use when you’re looking for a cheaper alternative to the slab foundation but have slightly less firm ground to lay the foundation on.
For locations where the ground stays damp much of the time because of large amounts of rain, a rubble strip foundation using loose stone below the concrete strip can be used. This stone allows the water to run through and drain away.
Like all the foundation types mentioned, strip foundations also have their weaknesses. For instance, strip foundations have low resistance to earthquakes. Also, due to their shallow form, strip foundations are best suited for small and medium-sized builds.
The most popular way to attach containers to the foundation pad is through a steel plate. While the concrete is still curing, a steel plate with vertical bars is set that sinks into the wet concrete.
Paul Chamber’s video shows an excellent example of this.
Once the concrete has cured, the shipping containers can be placed on the steel plate and welded together.
Optionally, if you don’t want to use the steel plate technique, you can bolt the containers down into the concrete using concrete anchors. This is a simpler technique, but the hold isn’t as strong as welding the containers.
Finally, if you are opposed to permanently attaching the containers to the foundation, you can just place the containers onto the foundations. You sacrifice some additional strength when you choose this route.
The only exception to the suggestion of permanently attaching the container to the foundation would be if the container is being built to be portable.
If you chose to use either a concrete pier or slab foundation, this section is extremely relevant for you.
Once people have decided to use a concrete foundation, their next question is normally about what strength of concrete to use.
The strength of concrete you need to use for your foundation will be primarily decided by the geo-technical engineer’s report.
The concrete strength will be referred to as a C value. C15 concrete, a general all-purpose concrete, is made by using 1 part cement, 2 parts sand, and 5 parts gravel. The higher proportion of cement used, the stronger the concrete. For instance, C30 is very strong concrete made up of 1 part cement, 2 parts sand and 3 parts gravel.
Example of Concrete Pier Foundation
If you are mixing small quantities, then you can either do this by hand or by using a cement mixer. For anything more than 1 cubic meter, consider having the concrete delivered directly to your site, ready to use.
Note that, if you are mixing the concrete yourself, make sure you thoroughly mix all the elements together properly, otherwise the strength of the concrete is greatly reduced.
To determine how much concrete you need, just calculate the cubic meters of your foundation. Multiply the width by the height by the depth.
For example, to calculate how much concrete is needed for a 10-foot wide, 22-foot long, 2-foot deep slab foundation, multiply 10 x 22 x 2. The amount of concrete to be ordered would be 440 cubic feet.
Once the cement is mixed with water, it will start to cure. Ensure the concrete cures properly, since this improves its strength and durability. The concrete only cures properly if the concrete’s temperature is kept within a suitable range (refer to the manufacturer’s packaging).
It typically takes from 5-7 days for the concrete to cure. During this time it needs to be kept moist.
If you are laying concrete in hot weather, it’s crucial you prepare your site correctly before pouring the concrete. Place temporary sun shades up to block any direct sunlight onto the concrete. Also, before you lay the concrete you should spray the ground with cold water. While you are mixing the concrete, make sure you use cold water.
Another good idea is to pour the concrete either later in the evening or first thing in the morning to avoid peak temperatures.
Like pouring concrete in hot weather, special measures need to be taken when pouring concrete during cold weather.
Cold weather is classed as the average temperature being below freezing for more than three consecutive days.
Before you pour the concrete, make sure any snow or ice has been cleaned from the base and forms. Remove any standing water. Once you have laid your concrete, cover it with insulating blankets right away. Use the blankets for 3-7 days while the concrete is curing. After the concrete has cured, remove the blankets gradually, so the concrete doesn’t crack due to quick temperature change.
Now you’re clear on why you need a foundation in the first place, and how to select the proper one for your build. We made our selection of foundation types based on those that are most commonly used and the most DIY friendly, since many of you reading this article are self-builders!
You should also understand how to mix the cement for this type of foundation. Make sure you pay special attention to the advice regarding how to lay in extremely hot and cold climates as this can make or break your container’s foundation.
The next step now is getting your containers delivered and installed on your foundation.
Let us know below what foundation choices you’ve made.