Caveat: before you read the main part of this post, please note that this would be my approach to things. That means what you are about to read is all opinion from personal experience. I get a lot of visitors who are owner-builder types like me, so I figured I would create this post to help some of you out there. You can do things the way you wish and use your own approach; so please don’t contact me with issues or problems or accidents resulting from your own incompetence, lack of knowledge or skill level, or equipment failure. I don’t want you to hurt or kill yourself, so be smart! You can easily create a situation where you can get seriously injured or die when working with things as heavy as shipping containers. Just use common sense and safe building practices… and if you are not sure of yourself, just hire a professional, they are everywhere. Use this information at your own risk and responsibility. OK, with that said…
If you are an owner-builder, you have a few options when it comes to a foundation: concrete slab on grade, crawlspace, full basement, or pier footings. My favorite? Definitely, the pier footings. I am speaking from the standpoint of the do-it-yourself perspective and cost. The pier foundation requires less concrete, less digging and doesn’t take a lot of tools and equipment to get the job done. Having the same benefits as the basement and crawlspace foundations, the pier footings gives you the freedom to get under the container for modifications and repairs.
One thing to keep in mind is that you don’t want any moisture underneath the container home, so you have to seal the bottom and insulate it as well. Placing the home directly on dirt or on a slab could facilitate corrosion. Moisture is the enemy of container homes. Seal, coat and avoid moisture where possible.
So how to make those big, deep holes in the ground? You don’t want to use a shovel–that’s too much work and isn’t as precise. You really want to get your hands on an auger. Preferably a two-man gas-powered auger. If you are capable of getting a hold of heavy machinery, then go for that–but its rather expensive to rent and I hope you don’t screw something up on it or you will be responsible for some costly repairs.
After you get the holes bored out of the earth, you will want to employ some forms (huge preform tubes or made from pieces of plywood) in order to precisely pour the proper amount of concrete. As for rebar (reinforcing bar), you definitely want to get those into the forms before you pour the cement in order to reinforce the concrete and add greater load-bearing strength.
You will need some heavy gauge flat metal plates (at least 1/4″ thick) to set into the cement, letting the concrete cure solidly around the base of the plates; leaving the top part of the plate uncovered and exposed to air. You will be setting the container corner directly on top of the plate, so keep it flush and level. The underside of the plates will need some arms or support bars welded to them. In the past, while working on some pier footings for some outside structures, I had cut some 1/2 inch rebar about a foot long and welded four of them to the underside of some 8×8 plates–this worked beautifully.
Note: with any foundation job, one of the biggest “oops” moments could result from not keeping each footing level with one other because you rushed or just didn’t think about that facet of your build. Talk about a goof that is hard to fix properly! There are methods and instruments to ensure you have a level foundation, just Google them and please use them. You don’t want a crooked platform for your container home. The image below shows one technique with a string run across a series of footings. Another thing, if you are working with an inspector because you are in one of those jurisdictions, you can’t get ahead of yourself. Be sure you let them see every stage of what you are doing so that they can approve it for the next stage of your construction.
After your footings have cured properly, the main load-bearing corners of the container(s) sit on top of the plates and are then welded in place. Don’t weld the container(s) to the plates until you are positive you have things positioned right! Grinding off welds is a bitch if you have never done it–it sucks.
If the footings, even after you took the time to get things right, happen to not be level, you can possibly weld additional plates to the embedded plates to create more elevation BEFORE you set the container(s) on top. You can check for level several times before setting the container(s) to ensure you haven’t goofed up. OK, that’s about it. Be careful and stay alert!