By Catherine Wanek – Concrete foundations are often a big part of the cost of even a small building—besides being high in embodied energy. Since the function of a foundation is to support and distribute the weight of a building’s walls and roof, it stands to reason a small, light building could get by with less than a multi-storied edifice. So if you learning how to build a chicken coop, pump house or even a small self-sustaining home on a budget, you might consider one of these low cost construction techniques.
Your building’s foundation requirements will vary, depending on climate and your site’s soil. Typically, a foundation is dug as deep as the ground can freeze during the winter to prevent “frost heave,” which is caused by moisture in the soil expanding as it freezes. Even thick concrete foundations can be pushed about by this powerful force of nature, taking your house with it. Here in the mountains of New Mexico the frost line is about 18 inches, while in Canada it may be six to eight feet. (This is a big reason nearly every home in cold country has a basement—they have to dig deep anyway.) Generally sandy soil drains well, so will be far less prone to frost heave than clay soil, which tends to hold onto moisture. But sandy soils will shift and erode more easily. So check with local experts and/or code officials to determine what is typically required in your region and your specific site.
Drainage strategies can minimize potential frost heave. The “rubble trench” foundation employs compacted gravel below a grade beam, on which your structure is built. The gravel-filled trench allows moisture to drain away, plus freezing moisture can expand into the spaces between the gravel, reducing the risk of frost heave. Architect Frank Lloyd Wright had success with relatively shallow rubble trench foundations, even in the cold winter climates of the Midwest.
If your site is on a slope, it’s wise to incorporate a “French drain” to divert water around your foundation. A French drain is located uphill from your building, typically using 4” diameter perforated pipe at the bottom of a gravel-filled trench, backfilled with larger stones. Landscape fabric surrounding the perf pipe will prevent it from clogging with soil over time and ensure it will continue to function for decades. Of course, it is unwise to build anything on a flood plain—eventually Mother Nature will show you why.
In supporting the structure above, foundations utilize two basic approaches: point loading and spreading the weight. Post-and-beam structures, which concentrate the building’s weight, are more appropriate for point loads and create less disturbance on your site—but you must dig them deeper. Spreading out the weight allows for a shallower, but wider foundation. This is the principle of earthship homes and buildings, in which tires filled with compressed earth spread the weight of the heavy walls, without excavation. Since straw bales are a wide, relatively light wall system, bale buildings can also utilize this approach. For post-and-beam and bale structures, the point loading technique also works, but you still need to get the bales up above ground level—atop a stem wall or grade beam—a good six to 10 inches, and perhaps even more in some situations. Fortunately for those on a budget, this low cost construction technique can be accomplished with inexpensive materials—plus time and effort.
Dry-stacked stone is probably the oldest foundation method known, and is still a useful low cost construction technique today. We’ve used it on small buildings such as a straw-bale chicken coop in our heavy clay soils without problems. You need a supply of large and medium rocks on site (moving them is heavy work, so use machinery if possible). Dig your foundation trench as deep as you feel necessary for your site and a couple inches wider than your bales—this way, your plaster will have a foundation to rest upon, too. With your foundation trench laid out, basically you use the nicest rock surfaces to face out, and anchor them securely into the ground on both sides of the trench. Wedge smaller and less attractive stones into the center of this foundation wall as tightly as possible, then fill the space left with pebbles. There will still be plenty of cracks between the stones for moisture to expand into.
To create a level surface for bales to rest upon, you can pour a relatively thin concrete grade beam, or use “Earthbags.” These are simply polypropylene sandbags (available for about 50 cents apiece), filled with moistened sub-soil and compacted with a homemade tamper. You can probably use the dirt excavated from your foundation trench. By varying how full the bags are, you can fill in all sorts of irregularities in your stone base, achieving a flat, level foundation with the help of a transit and a little practice. The bags do double duty as a vapor barrier, too. Earthbags alone work as a stable foundation, stacked and tamped in layers in a running bond, and secured together with barbed wire.
Another option for a bale or cob out-building or landscape wall are rot-resistant poles atop a rubble trench. Choose a locally available, rot-resistant tree—in our region this tree is the juniper, which is widely available for the price of a fuel-wood permit. Be sure to peel the bark, which will attract insects. Charring the logs with a torch will also increase their longevity, as it creates a carbon layer that discourages fungal growth. (You can also use old telephone poles or railroad timber, but be aware that they are treated with poisonous chemicals.) Stake or otherwise secure your logs to the ground so that they support the walls at the outside edges, and fill in the space between with stones or gravel. Make sure the site is well drained, and that you create a vapor barrier between your foundation and your wall system. The top of the foundation is also a good place to stop your plaster or stucco, so moisture from the ground cannot wick up into your walls.
Modern natural builders have pioneered the use of recycled chunks of concrete—dubbed “urbanite”—mortared together with cement or mud. Chunks of concrete are commonly found where buildings are being demolished. It is major work to get big chunks to your site, but you may find contractors willing to deliver them for free, especially in areas where they must pay to dispose of the concrete in landfills. Recently, we used this technique for a 16′ x 20′ straw-bale pump house foundation. The concrete chunks came from right on site—a patio slab that was left after a trailer burned down nearby.
Utilizing a local backhoe to break them into chunks and drag them to the pump house site, we placed them into a shallow trench, choosing the largest, smoothest piece for the threshold. With our local clay mud (which hardens like a rock), we mortared the chunks together, overlapping joints. Earthbags were employed to level the foundation, before stacking the bales above. We used an old garden hose to provide a chase for strapping to tie down the roof on this load-bearing structure. It turned out quite solid, and our only foundation cost was fuel for the backhoe.
Foundations are critical to the long-term success of a building, so they should be constructed with thoughtfulness and care. Still, deep and wide concrete foundations can be overkill for a modest sized, relatively light structure. In fact, we recently excavated next to the two-story, two-foot-thick brick wall of our 120-year-old lodge, and were surprised (shocked, really) to discover that what serves as its foundation is about a foot and a half of rubble. After all these years the stucco reveals no shifting of these heavy walls, built on clay soil.
So, consider the use and budget of your small structures, and the amount of risk you are willing to take. Living and building lightly could be a reasonable choice. Good luck employing these low cost construction techniques!
Published in 2002