Mixing Cob on Tarps for Your Hand-Built Home

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“The Hand-Sculpted House” is theoretical and philosophical, but intensely practical. You will get all the how-to information to undertake an earth and straw building project, and learn why cob cottages are the ultimate expression of ecological design.
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How do you know when you have added enough straw and your mix is done? As you add straw, you should be able to feel the consistency of the mix changing under your feet. Suddenly it will reach a point where it feels like a tough, cohesive substance rather than like loose mud.

Cob cottages are made of the oldest and most natural resources on earth — earth, clay, sand, straw and water. In The Hand-Sculpted House (Chelsea Green Publishing, 2002), authors Ianto Evans, Michael G. Smith and Linda Smiley cover step-by-step how to build one of these whimsical, yet practical structures and explore the deeper meaning of owning a home made of materials from its natural surroundings. Cob houses have a long tradition in Europe and are set to redefine the future of building in America. In this excerpt from chapter 11, “Making the Best Cob,” learn how mixing cob is a “dance” that can be done more efficiently by two people and learn about the types of tarps that work best for this endeavor. Also, pick up some tips for speed and efficiency.

You can purchase this book from the MOTHER EARTH NEWS store: The Hand-Sculpted House.

A great breakthrough in manual cob mixing occurred in 1994, when Becky Bee developed a system for mixing on tarps. Before that, we had been mixing with shovels on a level platform made of tamped earth, concrete, or plywood. The tarp method is quicker, easier on the lower back, and requires fewer tools. It has now diversified into several quite different techniques–“different folks, different strokes!”–and it pays to change technique as circumstances demand. Experiment!

You will need a squarish piece of durable, slick, and water-resistant material, six to eight feet on a side, larger than your armspread by about a foot. Some people prefer a tarp a little longer in one dimension, 7 × 8 feet for instance, or 8 × 10.

Lay the tarp out on clean and level ground, close to your cob ingredients and your building site. We have found it saves work to dig out a shallow dish for the tarp to lie on: about 8 feet in diameter and 6 inches deep in the center works well. Spread the ingredients out on the tarp, alternating buckets of sand and clay to accelerate mixing. We normally use from three to five 5-gallon buckets of ingredients for a single batch of cob. This is the largest amount most people can handle easily and repeatedly.

When mixing by yourself, you can simply grasp one or two corners of the tarp and walk backward over the tarp until the mix is folded back upon itself. Do this repeatedly, rotating to a different corner each time, until the dry materials are mixed.

Though it is quite easy to mix alone, the initial stages of mixing are faster with a partner, so call over a helper if one is nearby. Each person should firmly grasp two adjacent corners of the tarp, then lean back slightly. Keeping their spines straight, both partners slowly rock side to side from one leg to the other, using the greater strength of their legs to roll the material on the tarp back and forth. Part of the tarp and most of the weight should remain solidly on the ground as you roll the dry materials across the tarp. After a few long rolls, in which the mix travels all the way from one side of the tarp to the other, stop and rotate positions 90 degrees. Then rock and roll in the other direction to make the mixing more thorough.

The dry mixing stage should take less than a dozen rocking motions. Once you can no longer see pockets of different colors and textures in the dry mix, it’s time to add water. Using your hands or feet or a shovel, make a crater in the middle of the mound of mixed earth, and pour in water. Always add a little less water than you think you’ll ultimately need. It’s easier to add more than to compensate if you add too much.

With the water added, repeat the mixing process described above, rocking the mix back and forth a few times. Then tread a little, roll the mix, add more water if necessary, tread, roll, and so on. Don’t get stuck on any one motion, and stay alert to what will have the most effect for the least effort.

For treading, take your shoes off. Barefoot mixing keeps you in contact with changes in the mix and is really enjoyable and therapeutic. It is advisable to have toughened your feet in advance by going barefoot for a few days. If you have to wear boots because of cold weather, skin problems, or lots of rocks in the mix, try leather workboots or other flat-soled footwear. But each day, at some point, take off your shoes and dance barefoot. It’s exhilarating, even in freezing weather, and the sensors in your feet, adapted by evolution to fine-tuned alertness, will tell you instantly how well your mixes are developing. There is no substitute for the sensitivity of barefoot mixing.

Learn to dance a twist on the cob mix, using a smearing motion to break up lumps and distribute clay particles. Either raise your knees high and jump on the mix or use a heel-first tread to maximize pressure. Try different methods.

If you are working with a partner, approach the task like ballroom dancing, formally. One treads, the other lifts tarp corners. Treader retreats from turner, turner lifts, rolls, replaces tarp. Turner moves clockwise, to the left. Treader, facing turner, without breaking the rhythm, moves to his or her right, also clockwise, onto the most unmixed part of the mass. With rhythm, you can work very fast, very smoothly. When either tires, she or he shouts “change!” and trades positions. Two people, energetic and rhythmic, can make and build a cubic meter a day (35 cubic feet, about a ton and three-quarters wet).

Once the lumps are all broken up and the water evenly distributed through the mix, begin to add straw. Hold a flake off a bale (2 to 3 inches thick) under your elbow and allow it to sift down onto the mix as you dance. Keep treading, using your heels now, until all the straw is dirty and worked into the mud, then turn the mix by pulling one corner or edge of the tarp toward you until the mix folds over on itself. Do this repeatedly as you add more straw, pulling from a different corner each time, making certain to pull the tarp far enough to turn the center of the mix, so you don’t end up with an unmixed mass there.

How do you know when you have added enough straw and your mix is done? After practicing with the process, you will know intuitively, but here are several tests to get you started. As you add straw, you should be able to feel the consistency of the mix changing under your feet. Suddenly it will reach a point where it feels like a tough, cohesive substance rather than like loose mud. It will get harder and harder to sink your feet in. If the straw is long enough, you will reach a point where as you turn the mix it rolls up like a burrito and holds together in a single mass instead of cracking along the line where it is folded.

When working in a little more straw seems to be taking a lot of effort, stop adding it. Your material should be a good consistency for building. Reach down and seize a double handful of cob. Is it difficult to pull away from the rest of the mass? Does it hold its form and pack readily into a ball that you can throw and catch without breaking? Have your partner hold on to one end of a cob loaf and take the other, your fingertips touching theirs. Try to pull it in two. The straw in the mix should resist your pull, making it difficult to break.

Another way to tarp mix solo is to work on a sloped site, carefully spreading all the ingredients out across the top of an 8- × 10-foot tarp, for instance. Add water carefully. Jump onto the pile and tread, then grasp one top corner and pull back diagonally over the whole pile, flipping the mix toward yourself as you tread. When the mix gets to the side of the tarp, grasp the second top corner (don’t let go of the first). Dance backwards, pulling diagonally until in turn this side can’t go any further. Work your way backward down the tarp, pulling up more tarp with each hand as you go. By the time you get to the bottom of the tarp, the mix may be finished. If not, lay the tarp flat and haul both the bottom corners uphill, rolling the mix up with you (you may need help for this maneuver). Then repeat, until the cob is well mixed.

Yet another technique involves a large tarp; 10 × 14 feet (or up to 30 feet) is ideal, allowing two people to make larger mixes if the materials are dry. Spread the materials across one end of the tarp, then each grasp a corner closest to the pile. Walk back across the length of the tarp, both of you, rolling the material all the way to the far end. Repeat in the opposite direction. Often only three or four rolls will have thoroughly mixed sand, clay soil, and straw. Then add water and tread, rolling the heap as needed.

A basic goal of any heavy manual work is that it be rhythmic and effortless, a smooth and comfortable dance. The joy comes from the dance itself, not from anticipation of finishing the job.

The key to making mixing easy and enjoyable is a consistent rhythm. Keep trying until you find it. Work to reduce the time and effort it takes for every phase of the process. You’ll be amazed at how efficient you can get!

Mixing Cob: Notes on Tarps

The tarp has become one of the most important tools in cob construction. Finding tarps that suit you is critical to making the work enjoyable and efficient. Below is a partial list of kinds of tarps we have tried, with some of the pluses and minuses for each. Many thanks go to Misha Rauchwerger for his research in this department.

Used Lumber Wrappers: First choice! Lightweight and easy to hold while mixing, often available in large quantities for free from your local mill or lumber store, these woven polypropylene tarps (sometimes with a paper backing) are used to keep plywood and kiln-dried lumber dry during transportation. They aren’t very durable, but the price is right, and if you can get some productive use out of them on their way to the landfill, so much the better. Ask at your lumberyard, or check their rubbish pile.

“Blue Tarps”: These are made of woven polypropylene and are also available in other colors. They are inexpensive and readily available. They shed the cob mix easily. Unfortunately, they are very susceptible to damage by ultraviolet light in sunshine. Store them in the dark if you want them to outlast a single season and don’t drag them across rough ground when loaded, as they tear and puncture easily. Be aware that like many products of the construction industry, they are usually smaller than the advertised size.

Plastic Sheeting: Available in many thicknesses, usually black or clear, plastic sheeting is ubiquitous. Rolls of 8 or 10 mil black polyethylene are inexpensive, but hard to find in small dimensions. Most kinds tear and puncture easily and are slick and difficult to hold on to, so look for plastic with fiberglass reinforcement. Clear plastic photo degrades rapidly, except for UV-resistant types made for greenhouses.

Housewrap: The woven polyester sheeting used to wrap new construction (also known as Tyvek or Typar) is extremely durable, but is hard to get in small quantities unless you find an offcut discarded on a building site, or beg one from a contractor. If you are using salvaged housewrap, it may be brittle due to exposure.

Canvas: Expensive and heavy, especially when wet, canvas wears out rapidly and tears if exposed to lots of water and rough treatment. It takes a lot of work to keep canvas tarps clean, since the mix tends to stick to them. Oiled canvas, available as expensive trucking tarps or from army surplus, is much more durable but heavier and awkward to use. Canvas is not recommended though it seems ironic to build natural houses with synthetic throw away tarps. Is there a better natural fabric?

Mixing Cob: Tips for Speed and Efficiency

We’ve found that by conscientiously working to increase efficiency and concentrating on logistical details in order to make things go more smoothly, people can learn to mix cob more rapidly and with less effort than they at first imagined possible. A fit person working alone can expect to mix and build half a cubic meter per day (about 15 cubic feet). If you are exceptionally strong and well organized, you might increase that up to a full cubic meter per day.

The following suggestions should help any cobber increase mixing speed and efficiency:

  • Plan ahead to avoid moving heavy materials twice. Position piles of earth, sand, and straw as close as possible to where you’ll be building, ideally within shoveling distance of the mixing area. This saves time and energy in moving cob ingredients to your tarp, and in moving the cob itself to the wall. Make sure you have enough buckets, tarps, and shovels on hand so that you aren’t waiting around for tools.
  • Soak the clay well in advance.
  • Spread out the sandy portion on the tarp first. This way, the clay soil doesn’t get stuck to the tarp.
  • Premix your ingredients. Alternate, sand-clay-sand-clay when dumping materials onto your tarp.
  • Don’t overmix. Pay attention to what you’re doing and know when enough is enough. Mix without conversation until the work is completely automatic, as when learning to drive a car.
  • Mix wet. If you can allow time to wait between mixing and building, mix wet. Water lubricates the cob and makes mixing much easier. Before building, you can leave your whole mix to dry uncovered for a few hours or overnight, or build while it is still muddy and let it dry out on the wall.
  • Pay attention to weather. To avoid getting tired quickly, do most of your mixing when it’s cool (early mornings or evenings during the hot summer), or when there’s a good breeze. Don’t mix in the sun; create shade to work in.
  • Work mostly solo. The most efficient (but not necessarily the most fun) way for a group of people to mix cob is for each person to have their own mix on their own tarp. They can pair up for the initial stages of mixing by rocking the tarp back and forth between them, then complete the stomping, rolling, and straw-adding stages on their own.
  • Develop your rhythm. Treat tarp mixing as a kind of enjoyable dance. Music and drumming can keep your energy up and make mixing more fun.

Custom Mixes

Once you have reached a level of expertise where you can consistently make strong, stiff mixes in a timely and efficient way, you may want to vary each mix according to its intended application. The relative proportion of the major ingredients–clay, sand, straw, and water–can be increased or decreased. Different sizes of sand or lengths of straw can be selected. Sometimes other ingredients can be added for special characteristics. Psyllium husk, horse dung, gravel, flour paste, white glue, chopped baling twine, all have been tried–the possibilities are endless. Use your imagination and your understanding of the functions and behaviors of the different ingredients. Below is a list of some of the reasons you might decide to alter your basic mix, with ideas on what you might change in each case.

  • For hot, dry weather, or slow building: Make the mix much wetter than usual–it will be much easier to mix. Then let it dry out a bit overnight or over lunchtime before building. The tricky part is knowing when you have enough straw in the mix, because your normal clues will be muddied by all that water, and you might go on adding straw until the cob becomes hard to work with.
  • For wet weather, or fast building: Make stiffer, dryer mixes. Mixes with more sand and less straw than normal will tend to contain less water and dry out faster.
  • For sculptural details and around windows: Use a finer mix, screening out rocks from soil and sand and chopping the straw, because long straws left sticking out of a sculptural detail or close to glass can be difficult to trim. We sometimes add a little extra clay for a more cohesive, workable mix, but be cautious; with too much clay, cob can crack.
  • For extra insulation: Increase the amount of straw or substitute perlite, vermiculite, or pumice for sand. Pumice cob is difficult to mix and hazardous to the skin because of all the tiny, glassy fragments. You might also try adding sawdust or wood chips, which increase insulation but reduce strength and slow the drying.
  • For extra hardness or thermal mass: Increase the amount of sand as much as you can, and reduce the proportion of straw. To improve thermal performance, you could build exterior walls with an inner layer of high-density, sandy cob with better-insulating, straw-rich cob on the outside of the wall.
  • For straps, corbel cobs, arches, and shelves: These all need extra long straw for tensile/shear strength.

This excerpt has been reprinted with permission from The Hand-Sculpted House by Ianto Evans, Michael G. Smith and Linda Smiley and published by Chelsea Green Publishing, 2002. Buy this book from our store: The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage.