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The Last Straw -
The International Journal of Straw Bale and Natural Building
- Promoting natural building, and sustainable design and development, through research, training, education and consulting services.
by Owen Geiger
In most cases, even small improvements in a building process are quickly documented and published in books, journals and videos. Perhaps the most astounding fact about tractor cob is that even though it reduces the time and labor of building with cob by up to 90 percent (according to Pete Fust), very little has been published on the matter.
My involvement with cob is limited to brief experiences at natural building workshops, but what I have observed has been a painfully slow process – adding a few inches to a cob wall took a crew all day. This set into motion my quest to find faster, easier alternatives to help spread this building method. After all, building with earth taken from the jobsite is perhaps the most sustainable of all building systems. Cob is such a great material and building method that, hopefully, faster techniques will open it up to a whole new range of people.
Interview with Pete Fust, natural building educator and proponent of tractor cob:
Owen: When did you first come up with the idea of making tractor cob?
Pete: It was the fall of 1995. (Pete knows the exact date because it coincides with the upcoming 10-year anniversary of his front-end loader.)
Owen: What has been the general response?
Pete: Upon first learning of tractor cob, the most common response from workshop participants has been, “Oh, I didn’t know you could do it that way. Gee, I wish I’d known this earlier.”
Owen: Why do you think tractor cob has been slow to catch on?
Pete: Cob has so much going for it. It’s virtually free, nontoxic, and has very low embodied energy. The durability of cob is amazing. Cob structures can last hundreds of years, even in harsh climates. But like rammed earth or earthships, cob suffers from stereotypes. Almost everyone thinks, “This is how it’s supposed to be done.”
From my experience, most cob structures have been done the traditional time-consuming way by people that are building small and have more time than money. And for some people and projects, that may be fine. After seeing how much work was involved, I was determined to find a way to mechanize the process and speed things up. And that’s the beauty of tractor cob. It makes use of this wonderful material, but pretty much eliminates the main drawback of cob – the fact it requires so much labor.
Owen: What are the best uses of cob?
Pete: Cob is perfect for hybrid structures that combine aspects of strawbale and earth. In a home insulated with straw bales, interior walls of cob add thermal mass, soundproofing, and help moderate humidity. Privacy walls of cob are much longer lasting than those built with bales. (Many straw bale privacy walls I’ve seen around the country are failing because, without a roof over them, moisture can get in the inevitable cracks in the plaster.) Cob is also great for stairways, earthen floors, pizza and bread ovens, and built-in furniture such as benches and shelves.
Owen: What points would you like to emphasize?
Pete: I might be cobbing to the choir, but it’s hard to beat cob when it comes to low cost and practicality. As they say, it’s dirt cheap. You can use the natural material right on your land, and minimize the transportation and use of factory made materials.
Tractor cob eliminates up to 90 percent of the mixing work so you have more energy available to be creative as you apply the cob. And it’s far easier to get friends and others to volunteer, because now it’s almost completely a fun, creative process.
Owen: Explain how tractor cob reduces most of the work.
Pete: Mixing is the lion’s share of the work with cob building. Mechanizing the mixing process speeds up production so much that I can keep 30 people busy all day with just 20 minutes of mixing (producing three cubic yards in 20 minutes). A front-end loader (or tractor) speeds the entire cob production process of moving and/or excavating raw materials, mixing and pulling the cob apart, moving the cob to where it’s needed and raising it to the necessary height.
With the cob poised at the perfect height, and the bucket slightly tilted, it’s relatively easy to place it on the wall. Mechanizing the process also eliminates the need to screen the soil and to form it into balls, or loaves. By far the best tool to use is a 4-tined garden fork to move the material from the tractor bucket onto the wall. If you come across any large dry clumps, just throw them to the side. (Caution: Do not over-tilt the bucket. If the cob slides out too fast, it could crush someone. Use common sense precautions whenever using machinery.)
Owen: What are some other advantages?
Pete: There’s little need for a cobber’s thumb or to “stitch” the cob together with this technique. I mix it wetter than traditionally done, and get perfect bonding. The secret to this technique is to add just enough water to get the proper consistency: the clay particles are coated with water, but not fully saturated. The courses of cob bond to each other, but dry very quickly (at least in New Mexico). This allows additional courses to be placed in one day without slumping. One course can go on in the morning, another course in the afternoon, at least here in New Mexico.
Cob landscape walls have lots of advantages. It’s possible to firm the sides and smooth them with a wet trowel before they dry, eliminating the need for plaster. The wall by our chicken coop is about 8 years old and barely weathered, even without a cap. (Note: Pete is fortunate to have near ideal soil for making cob and lives in a fairly dry region. Unprotected cob in many climates will not hold up this well.)
Owen: Explain to us how you make tractor cob.
Pete: There are four main steps:
1. Dig a 2-foot deep pit slightly larger than the width of the tractor bucket and long enough in the other direction to hold the desired amount of cob. (To accommodate Pete’s front-end loader, the size of his pit is about 8’ wide by 5’ long by 2’ deep.)
2. Add about 6 – 12 inches of water in the pit.
3. Add sand and clay in your desired ratio using the front-end loader. This ratio can vary dramatically, so experiment to discover what works best with your materials. (Pete uses the local clay just as he digs it from the ground, including small to medium stones, without additional sand.) Mix the materials by scooping up and dumping the mixture three or four times. I use the heel of the bucket as a depth gauge to guide the bucket into the pit. You have to feel your way between the wet material and the bottom of the pit. Raise the load to full height and dribble the material out of the bucket, observing the process as you go. Dribbling reduces splashing and helps mix the ingredients. Spray the mix with extra water, if necessary.
4. Add straw gradually between drops, approximately 5 – 10 percent by volume. The straw can be wet. You can use almost any length of straw or grass, including wheat, rice, barley, meadow grass or Bermuda grass. Cob just needs some kind of fiber to help hold it together. For my one-cubic-yard front-end loader, I use one 2-string bale for each cubic yard of cob (about 9 heaping wheelbarrow loads). At this point, I drop the mix 8 – 12 times from full 9 ft. height to break up any clumps. (His term for this is “impact mixing.”)
Owen: What are some other ways of mixing cob mechanically?
Pete: Bobcats and rototillers work well. Mortar mixers can work, but they tend to get jammed. You can also drive over the mixture with 4-wheel drive vehicles or tractors. One good method is to locate a low spot with clayey soil, add water and sand and straw as needed, and slowly drive a vehicle back and forth over the mix. The tires do all the mixing. You can also mix earthen plaster in the same way.
The real art of making cob is figuring out how much water to add. This is influenced by climate, type of materials, and the time of year. Too wet, the material will tend to slump; too dry, it will be hard to work with and may not bond properly. My experience is that it is easier to mix thoroughly if the cob is a little too wet, and then add dry material to reach your preferred consistency.
Owen: Do you know anyone else who has used tractor cob?
Pete: Sure, Sun Ray Kelly from Sedro Wooley, Washington developed tractor cob quite independently, with his own backhoe. And Rob Bolman, from Eugene, Oregon, used a bobcat to mix cob for his earthen floor. He rented a bobcat just for the job and mixed the material on the city street, which enabled him to complete the entire floor in one day. The cob was kept moist under tarps until Rob was ready to use it. Poured earthen floors can take weeks or months to fully dry. Cob is a stiffer mix with less water, and therefore faster drying. That’s an important point when you’re trying to get your house finished. For floors, be sure to screen out the larger aggregate.
Owen: Does this technique work with every kind of clay?
Pete: There are many types of clay, some much easier to work with than others. Glaciated clays and caliche, for example, are particularly difficult to work with because they tend to be non-soluble. In northern Europe they used such clay, but would leave it in 1 –2 foot layers all winter to let the freeze/thaw cycles break up the heavy clay.
It might be easier to truck in clayey soil from elsewhere, rather than struggling with difficult clays. Excavators are often eager to dispose of clay soil at little or no cost. Many homesteaders interested in cob look for land with a good source of clay, in addition to weighing all the other considerations that go into selecting land.
Owen: Explain your technique of adding rocks.
Pete: I embed rocks about 1-2 feet apart horizontally. Vertically, I add enough cob to stick them together, but make sure that the rocks touch each other, so the rocks carry the weight of the wall and prevent it from slumping. I can build a 6-foot high, 8-foot long, 2-foot thick wall in 4 hours with one unskilled helper using this stacked rock method. Additional wall strength is achieved by randomly interconnecting the vertical stacks of rocks. The end result is similar to a skeleton of rocks that has been fleshed out with cob. This type of wall is inherently dangerous while being built, and should be 2 – 3 ft. thick minimum to add stability. You can also add interesting rocks for decorative effect or to make steps and shelves.
Sun Ray Kelly was asked why he likes to add rocks to cob. After some thought, he replied “Because they’re already made.”
Owen: How does tractor cob relate to standard cob wall building techniques?
Pete: Standard cob techniques apply.
Owen: Is tractor cob appropriate for developing regions?
Pete: Sure. Even most developing places have tractors, bobcats and 4-wheel-drive vehicles. Historic accounts indicate that pre-industrial cob buildings may have been mixed by hoofed animals.
Owen: Are there any drawbacks?
Pete: A machine of some type is needed, of course, which will require fuel and maintenance. Another drawback, although minor, is mud splattering. Don’t stand nearby if you plan on staying clean! Tractor noise is probably the biggest complaint, so I try to mix the cob before people arrive, while they’re at lunch or busy doing other things.
Owen: So why hasn’t tractor cob caught on?
Pete: Traditionally cob has been taught to be mixed by hand and feet, which is a great way to learn to love the material. But then people aren’t aware that machinery can help them reduce the labor involved.
Owen: How do you justify the environmental cost of using machinery?
Pete: Compared with the cost and embodied energy of manufactured materials shipped to a building site, tractor cob comes out way ahead, especially as you don’t have to run the tractor constantly.
Owen: What is the future for tractor cob?
Pete: Because the process is so expedient, it’s ideally suited to large projects and for contractors who already have the equipment. This opens cob to conventional builders who would never dream of doing it by hand.
Pete Fust is a well-known natural building educator at the Black Range Lodge in Kingston, New Mexico. He can be reached by email at: [email protected] or by phone: 505-895-5652. www.strawbalecentral.com
Dr. Owen Geiger is the Director of the Geiger Research Institute of Sustainable Building, co-author of the Builders Without Borders Straw-Bale Construction Guides and a correspondent for The Last Straw. www.grisb.org
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