When building a shipping container home, you need to lay a foundation which the containers can be placed upon.
The foundation ‘type’ you can use will vary depending on: your budget, structural requirements, local soil type and local building conventions. You need to make sure you choose the right foundation type for both your budget and the local environment.
Getting the decision about the type of foundation to use wrong can be devastating and it can bring your entire project to an abrupt holt- in fact this is one of the biggest mistakes people make when building a container home.
Today we are going to explore the various types of foundations you can use for your shipping container and address the pros and cons of each foundation type.
What Type of Foundation Should I Use?
The two major factors that need to be considered when designing your foundation are cost and what’s required structurally. Let’s address them each point at a time, firstly: what’s required structurally.
To calculate what’s structurally required it is best to consult with a qualified builder or engineer, however both your soil type and the amount of containers you are planning on using have a large impact here.
If you are building on a soft soil type then you will need a deep foundation type such as pile foundations- more on that later.
Whereas, if you’re building on a hard soil type, you can use minimal foundations because the majority of your containers’ weight will be distributed onto the existing hard ground.
Sometimes people prefer to ‘over-spec’ their foundation, which is where you make your foundation stronger than necessary, either for peace of mind or because you prefer that particular style of foundation.
Ultimately though, the decision will revolve around cost, design and what’s structurally required. Now let’s take a look at the types of foundation you can use.
Types of Foundation
Typically any foundation which is used in ‘traditional’ home construction can be used for shipping container home foundations.
These can be classified into either shallow or deep foundations.
Shallow foundations are laid very near to the ground level, as opposed to deep foundations which can be laid at depths of up to 10 meters.
We are going to focus on the three most commonly used foundations: concrete piers, raft foundation, and piles.
Concrete piers are a type of shallow foundation and are one of the simplest and cheapest routes to go.
In their basic form, they are concrete cubes which have reinforced steel bars within them. A reinforced steel bar is either a steel bar or a mesh of steel wires and is used to strengthen the concrete.
Image From Larry
This method of foundation is definitely the most DIY-friendly, and is the cheapest form of foundation. A great advantage of using this type of foundation is that because the containers are up off the ground it allows for ventilation and prevents condensation forming underneath the container.
Six concrete pier are laid for each container. One pier is laid for each corner of the container and two piers are used in the centre to support the middle of the container.
Image From Sea Cabin
This is by far the most popular shipping container home foundation and it’s the one which I would recommend in 8 out of 10 cases.
Slab on Grade
A raft (also known as Slab-on-Grade) foundation is more time-consuming and more expensive than a pier foundation, but it is an exceptionally good foundation to use on softer soil types. If you are going to use a raft foundation be prepared to dig… a lot!
Slab on Grade Foundation
We often see the raft foundation used in warmer climates where freezing isn’t a concern. Its advantages are that it’s quick and easy to build; also because there are no hollow spaces in the foundation they are less vulnerable to termite infestation.
The disadvantages of a raft foundation are the lack of access to utility lines once the concrete has hardened and there is potential for heat losses where ground temperatures drop below the interior temperature.
Pile foundations are used when the soil type is too weak to support a concrete base- they are the most expensive type of foundation covered here.
If you remember, they were used in the Graceville Container Home Case Study.
Piles (which are 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 concreate piers.
Typically a DIY piles installation isn’t recommended. A contractor would be needed to be used to install pile foundations due to the specialist equipment needed- such as the pile driver.
Strength of Concrete to Use for Your Foundation
If you chose to use either a concrete pier or slab on grade foundation this section is extremely relevant for you.
Once people have decided to use a concrete foundation their next question is normally: “what strength concrete should I use?”
The strength of concrete you need to use for your foundations will be primarily decided by the geotechnical engineer’s report.
It will be referred to as a C value, for instance C15 concrete which is a general all-purpose concrete is made by using 1 part cement, 2 part sand and 5 part gravel. The higher proportion cement used the stronger the concrete. For instance C30 is very strong concrete made up of 1 part cement, 2 part sand and 3 part of 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 however for anything more than 1 cubic meter consider having the concrete delivery to your site ready to use.
Note: 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 calculate how much concrete you need, just calculate the cubic meters of your foundation. To do this you need to multiply the width by the height by the depth.
For example if we were calculating how much concrete is needed for our 10 foot wide, 22 foot long, 2 foot deep raft foundation, we would do 10x22x2. So we need 440 foot cubed of concrete ordering.
Once the cement is mixed with water it will start to cure. You need to ensure the concrete cures properly as this improves its strength and durability. The concrete only cures properly if the concrete’s temperature it kept within a suitable range (see the manufactures’ packaging).
It typically takes from 5-7 days for the concrete to cure and during this time it needs to be kept moist.
Pouring Concrete in Hot Weather
If you are laying concrete in hot weather it’s crucial you prepare your site properly before pouring the concrete. You should 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. Whilst 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 air temperatures.
Pouring Concrete in Cold Weather
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 0 Celsius for more than three consecutive days.
Before you pour the concrete make sure any snow or ice has been cleaned from the base and formwork; also extract any standing water. Once you have laid your concrete, cover it with insulating blankets straight away. You should use the blankets for 3-7 days whilst the concrete is curing. After the concrete has cured make sure to remove the blankets gradually so the concrete doesn’t crack due to quick temperature change.
Make Your Choice
So hopefully by now you will be able to select the type of foundation you require and also know exactly 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/cold climates as this can make or break your container’s foundation.
The next step now is getting your containers delivered and fitted to your foundations.
Let us know below how you got on laying your foundations!