Building footings and a stem wall that provide sufficient load bearing strength at the lowest cost in labor and materials was our goal as DIY homebuilders. We used three different methods.
In keeping with our goal of creating a pay-as-you-build desert homestead, we used our own labor and, as much as we could, hand tools. This included digging out the trenches for building footings and stem walls to support our first two buildings.
By the time we were ready to build our main house, however, my age (the wrong side of 65) and the magnitude of the task at hand dictated acquiring something easier on the old body than pick and shovel for moving dirt.
Buying a used Terramite TC5 mini-backhoe/frontloader, dubbed the Tonka Toy, solved our problem. It was powerful enough for our tasks, while small enough to maneuver among our desert plants without doing too much damage to the native plants.
When we built our three buildings here on the desert homestead, we had three different sets of conditions for building footings and stem wall. We adapted building footing and stem wall methods that we felt met criteria for both strength and economy and were amenable for DIY home building.
Building #1 – The Adobe Bear Cave
Our first building had the heaviest walls by far, being solid adobe. But the adobe was also the thinnest wall, about half of our two-foot straw bale walls.
We believed that a poured concrete stem wall over a wider concrete footing, both with steel rebar in the pour, would give us the most economical stem wall while preventing extensive settling and cracking in our adobe wall. Above the footings, we formed in and poured a single pour stem wall about 19 inches deep and 14 inches wide. The extra width on the stem wall gave us a plaster shelf for the adobe.
Because we were putting on a moisture barrier of felt paper between the stem wall and the first course of adobe block, we couldn’t use plaster or mortar to bond the first course to the stem wall. To keep the wall from shifting on the stem wall, we put rebar pegs into the stem wall and mounted the first course of block with holes drilled to match.
Our homestead site is on a bajada, a sloping alluvial fan, so we had to consider the slope of the ground as well as the wall weight and width.
The Bear Cave is 320 square feet and the Annex is 192 square feet.
With these smaller buildings, having a slope of about eight inches from high to low points, we simply built with the slope, digging in on the uphill end and building up on the downhill.
The Annex : Our First Straw Bale Building:
The Annex, our straw bale utility building, had thicker and lighter walls than the adobe Bear Cave. In the various books we read, owner-builders building with straw bales have used everything from rocks to railroad ties for a foundation. We had a lot of adobe on hand and not a lot of money, so decided on a treated adobe block foundation for the little straw bale Annex.
We used the same “recipe” of aggregate, straw, and adobe for adobe block as we did in the building of the Bear Cave except that the liquid used to make the “mud” for the blocks was about 10% asphalt emulsion. The asphalt mixture made the blocks a little darker, as expected, but they were hidden. We laid two rows of block and filled the space between the rows with more wet adobe with asphalt. The cost of the foundation, including the 10 gallons of asphalt emulsion, was less than $75 (USD) in 2006. This, of course, does not put a price tag on our labor.
When the wall cured (we gave it two weeks), we applied a heavy coat of uncut asphalt emulsion to the outer surface of the stem wall with a paint roller. We let the coating cure about a week and then backfilled. We built the stem wall high enough to provide clearance of about 10” between the ground and the first course of bales.
A side note on the asphalt emulsion blocks durability: To protect from sheet flooding during monsoons, we had built a 2 foot eyebrow berm around the uphill side of our homestead. We had 8 blocks made with asphalt emulsion left over and put them on the weather side of our berm. They have endured the Arizona sun and rainfalls as heavy as 1 1/2 inches per hour and show modest rounding on the edges of the block. They remain intact and strong.
Our Straw Bale Casa: At Ground Level
The main house was larger and had one more course of bales than the Annex to give us the nine foot interior walls we wanted, so it posed yet another set of problems in building footings and stem wall. Because it was larger, the building site of the main house sloped about 18 inches from high to low.
For economic and environmental reasons, we chose not to have a contractor come in to tear up and tamp down the base for the house. So, we did it ourselves.
After stubbing in the plumbing and waste lines, we used some of the dirt from the septic digging and brought the main house building site nearly to grade.
Our nearby sand and gravel people brought in a load of AB (a mix of sand, clay, and aggregate used for road beds here).
We spread that over the site with the Tonka Toy and then leveled it with a rake and tamped it with a hand tamper. With the nearest equipment rental facility about 75 miles one way, we typically used hand tools for most projects. After wetting the surface with a hose sprayer and tamping a few times, we had a good base for our building footings and stem wall. Again using the Tonka Toy when we could, we dug into our new base for footings and stem wall.
To provide support for the foundation, we built a riprap retainer for the AB base. You can see the beginnings in the picture. It now is four rows and there has been no erosion of the AB.
We laid two courses of block 27” apart over a reinforced concrete footing. For insulating the floor of a straw bale house using block as a stem wall, we suggest filling the space between the courses of block with vermiculite.
It has been more than eight years since we began building the Bear Cave. We have been living in our straw bale Casa for four years.
All three of our explorations in building footings and stem wall have worked well. Out adobe Bear Cave is stable and crackless. We have no erosion on the Annex foundation and our main house is snug, comfortable, stable, and paid for.
Our homestead takes and has taken a lot of work, but every minute of labor has been worthwhile.
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