My go-to resource
My primary resource for this project is the book Building Thoreau’s Cabin, for two reasons:
It’s geared toward building a smaller structure, that doesn’t necessarily adhere to residential building codes or need to incorporate extensive utilities.
It explains why you’re doing what you’re doing, rather than giving an Ikea-esque series of follow-these-literally instructions. This is hugely helpful, because by the time you’re in a pickle and need to improvise (which is inevitable), you’ll understand the tolerances, principles, and key factors that will guide your judgment calls in the correct direction.
Choosing a foundation: A case study
The book mentioned above thoroughly outlines how to build one type of foundation, but there’s no guarantee that this will be the correct foundation for everyone. It wasn’t for me.
So as an example of the type of judgment calls that go along with a project like this, here’s a case study on how you’d go about choosing the best foundation for you.
If you are building this project in a residential area, you’re going to need to research what building codes apply to you. Even in rather rural areas, you might be surprised by how extensive some of these restrictions can be, so always check before you start.
With a project like this, your primary concern will most likely be staying under the size that triggers building code requirements or permit applications. Many times what type of foundation you choose will also affect whether you need to file a building permit, so keep that in mind before proceeding. The last thing you want is to complete your project only to discover that it will need to be torn back down.
If you’re lucky enough to be working on land that doesn’t have any restrictions, building codes are relevant only to the extent that they can give you some good suggestions about how to build a structure that’s secure and avoids frost heaving in your area.
As the ground freezes, liquid water in the soil forms ice crystals that expand and cause the ground to “heave.” This is an uneven, lumpy phenomenon, which means that if you just build on the bare ground, you run the risk of having one corner of your cabin pushed up by several inches. This puts a lot of strain on your structure, and is very annoying if you accidentally drop a pencil.
The solution to this problem is to secure your foundation to some type of support that extends below the frost line for your area. Frost depth can vary greatly — from practically zero inches to many feet. There are maps online that chart frost depth across the country, but I found them to vary by as much as a foot. You can call your local zoning office and ask them what the official frost depth is for the building code in your area, but that might (1) be overkill, or (2) draw more attention to your project than you’d prefer.
My method was to:
Look at a few maps to determine at least a ballpark estimate for my frost depth.
Wait until a really hard freeze, and then dig down and see how deep it went.
Pick a number that roughly matched up with these figures without being overkill. For me that was about 10”.
If you live in an extremely cold area, you’ll probably need to rent a big, gas-powered machine to drill down far enough to get below the frost line. That seems like overkill for this kind of project to me; another alternative is to just to build something smaller, toss it on some skids, and tow it elsewhere if the cabin gets tilty.
First thing’s first: drive around your area and eyeball what everyone else did. When you spot a structure similar in size to what you’re planning to build, see what foundation they opted for. How is that structure holding up?
Some go-to options, in ascending order of labor, are:
If you live somewhere like Joshua Tree, you may be able to get away with building your cabin on essentially no foundation at all — maybe just a couple cement pavers to keep it an inch or so off the ground and allow rain to pass under. Tip of the hat to you!
There are different ways of making skids, but the simplest is to get a few pieces of 4”x4” lumber the length of your cabin (rated for ground contact), set them on the ground, and build on top of those. For smallish cabins, you can tow your cabin around, which is pretty neat. I wanted to build something bigger than would make this feasible, but it’s a good option for extremely cold climates, where the frost depth is ridiculous.
Deck blocks w/ gravel
This is what I opted for. Essentially, I:
Measured where the deck blocks (pre-cast concrete blocks that accept 2”x lumber) should sit.
Dug holes below my frost depth figure.
Lined the holes with landscaping cloth.
Filled the holes with sand and gravel.
Set deck blocks on top of the gravel pads.
Used concrete pavers to level out the blocks (the ground slopes slightly).
One drawback to this method is that the final result isn’t particularly pretty, but I intend to cover the exposed deck blocks with sandstone hauled from a nearby wet-weather stream bed.
This process is outlined in detail in Building Thoreau’s Cabin, but in a nutshell, it involves:
Digging holes below the frost depth.
Putting vertical wooden beams into the holes.
Filling the holes with concrete.
Leveling the beams and attaching the deck of your cabin to them.
While this is a solid option (literally), I wanted to do everything in my power to avoid having to pour concrete at my cabin site. Beyond the labor of hauling in all the concrete mix and all the water, it would’ve been a big headache to figure out how to clean everything up with no plumbing and without contaminating the stream. That said, if your cabin site is near a plumbed area, this could be a great option, since it’s very sturdy.