04 Dec Start Your Container Project With A Good Foundation
For a Level Playing Field, Start With Your Foundation
Today, shipping containers are being modified into everything from homes to computer server storage units and everything in between, along the way containers are getting heavier and heavier. A dry 40 foot container weights about 6,800 pounds or about 21 pounds per square foot. Once it is modified into a small commercial store, a cabin or simply becomes a storage container or tool container the additional weight of framing materials, drywall, insulation, lighting, HVAC, plumbing, plus furnishings and fixtures and the pounds per square foot can easily increase by multiples of eight to ten times!
While this is still not an astronomical number in terms of construction loads it is significant enough to be concerned about the proper design and of a pad or foundation to set your building on. If the foundation or pad that you wish to set your building on is not suitable, even an empty or lightly loaded container will settle, causing the floors to be un-level and worse, doors that are very difficult to open, close and lock. Once a container (or any structure for that matter) is out of level, the condition will continue to worsen as additional dead loads (building weight) will be directed to the low point of the structure. If you are planning to construct a shipping container house in an area where building permits are required than a fully engineered foundation design will need to be a part of your submittal package. However if you are setting a storage container or a cabin or in a rural area where permitting is not required then make sure you do some prep work before taking delivery of your container.
The simplest type of base or pad for your container would be a level spot, ideally excavated approximately 4 inches down to a flat, level surface, spread crushed concrete and compact it to about two inches, then spread some on 21A crushed limestone on top and compact it to about two inches as well. Both of these products have a binder in them that will help the pad stay together yet still allow the moisture to drain through. This method will allow the container to move around with the frost but it will work well for storage containers and other uses where you will not have utility connections or the container is attached to other structures.
If you are going to live in the container, have utilities connected to it or connect it to another building then you should consider installing a pier foundation. This is still a fairly straight forward project and if all the proper tools are available, you should be done in a couple hours. Don’t pinch pennies here if you don’t have a post hole auger, rent one. A 12 inch bit will work but I would recommend a 16 inch or 18 inch bit if available. The less stable your soil is the larger diameter your piers should be, if you have sandy soil, wet soil, or it has been filled in the past then you should go to 18” piers.
Let’s get started:
First, lay out and mark the four corners of your container, don’t forget the 3, 4, 5, rule
to make sure your layout is square.
You’ll want to install a pier every ten feet down both sides of the container. That means you’ll have ten piers for a 40 foot shipping container and six piers for a 20 foot container. Mark each pier location.
You will want the container to rest in the middle of the pier so theoretically half of the circumference of the pier will be under the container and the other half outside of the container.
Auger out your holes down to the frost level in your area, you should end up with a nice smooth cylinder with a relatively flat bottom. You should try to have the concrete delivered soon after your done drilling holes. If you try to wait overnight and it rains you’ll have a mess on your hands.
Fill the holes with concrete making sure you don’t leave any voids. It’s a good idea to press a round concrete form like Sonotube, the same diameter as your hole, over the top 4 to 6 inches of your concrete and level the Sonotube form.
VERY important, use a short two by four to screed the concrete level, the top of your piers must be level otherwise you will not gain the support you need.
Wait at least 24 hours (preferably more), then carefully set your container on top of your piers. You will more than likely need shims, make sure you use hardwood or steel shims, soft wood shims will do you no good.
The steps we’ve outlined here may seem like overkill but it’s a lot less work than emptying out your container, moving it, and then performing the steps outlined here at a later date. A good foundation will give you a solid basis for your container to provide you years of service.