Building With Cob From the Ground Up

With walls like rock that endure centuries, cool in summer, warm in winter, cob provides a comfortable, accessible home for those of modest means. According to Becky, “if you are good at scrounging, your cob house will cost between $7 to $15 a square foot, not counting labor, but counting gas for transporting materials.” With a few whitewashed walls or natural pigment plasters that even Martha Stewart would love, cob has often appealed to those of a more bourgeois tendency as well. Perhaps its greatest appeal to all is that the design of a cob house can be unique, encouraging curvy walls (which are inherently more stable than their straight counterparts), alcoves, cob sculptures, and mosaics of glass, tiles, seashells, and so on.

The Workshop

When I met the Oregon cob aficionado Becky Bee, she was stomping around in a clay pit, wearing a wide-brimmed straw hat, and holding the comers of her flowered cotton dress above her knees away from the thick orange liquid earth. She looked blissful, stomping rhythmically, as though she had the finest vintage of organic Oregon grapes beneath her toes. Her arms and legs covered in clay, she joked, “You probably don’t really want to shake my hand.” I’m not sure I would have made it through Becky’s seven-day natural building workshop in the Ozarks without the Little Buffalo River nearby to go and jump into every few hours when I began to feel too much a part of the earth. Summer was heading toward Arkansas, and though the thick forest canopy holds the heat at bay, it seems to hold the humidity in and sends you leaping regularly for the cool moving water.

With wild hair and the body of a 20-year-old, crouching here and there to roll a cigarette, Becky Bee is the kind of eccentric teacher who inspires a certain fascination from her students. Self-sufficient since running away from home and becoming a teenage mom (she’s middle-aged now), she has seldom had what she considers a proper home. It’s ironic that she fell in love with building and helping others make homes for themselves. She discovered cob at a workshop in 1989 while living in New Zealand. When she returned to live in her birthplace, Oregon, she attended one of the first cob workshops given by Cob Cottage Company of Cottage Grove, Oregon, in 1993, and she became an “unofficial building instructor” for them for a season. (Oregon is the hot place for cob in North America.)

In 1994, Becky started Groundworks and began to hold her own workshops, many of them for women only. Attending her workshop is a bit of a cult experience, with the emphasis as much on building community as on building homes. You’ll have a better time at her workshops if you are partial to drumming, singing, and new age rituals. I’m not particularly, but I did end up enjoying the diverse set of characters the workshop attracted, aside from getting what I came for in terms of learning a great deal about putting up a natural earthen building. I also enjoyed some great music from High Tea, a trio of women folk musicians who attended the workshop, and from some neighborhood bluegrass and folk musicians who dropped by our campfire some nights after we stopped working and started sipping the blueberry wine from the organic blueberry farm down the road. Murray Valley is full of people living an alternative life.

Massage therapist Ann Lasater moved to this Arkansas valley from Houston 25 years ago and started River Spirit. Her wooded property on the Little Buffalo River has facilities for small retreats. Some of the people who came to learn about cob and help her build the retreat center’s hermitage by the river live fairly near Ann in “communities,” meaning they own land in a group and decide by consensus how to use it and live on it. I met a woman named Joy Fox who moved to the area because she saw an ad in the back of an issue of MOTHER EARTH NEWS in the ’70s advertising cheap land. Another woman I met, Mollie Curry, edits a newsprint staple-bound sustainable-living magazine called the Permaculture Activist.

You can see why this land attracts MOTHER EARTH NEWS readers who want to get back to the land. The beautiful canopy of hardwoods encourages a unique neo-tropical ecosystem where dozens of species of plants, insects, and wild animals thrive. The locals revere the chiggers, ticks, and copperheads, for they believe these critters keep Murray Valley from being over-run by ecotourists, nature enthusiasts, and burned-out urbanites seeking a greater quality of life.

We headed down a steep hillside to our building site on the river promptly at nine each morning, after pulling up the flaps on our tents to keep them from becoming saunas. We worked on the building in the morning and had what Becky called “school” in the steamy afternoon. We held school at the building site, sitting around on straw bales in the protection of the trees, asking questions and getting hands-on advice. A lot of what is covered in school is also covered in Becky’s book, The Cob Builder’s Handbook (Groundworks, 1997).

Picking a Site and Design

After spending the mornings mixing cob and getting a feel for the material we would build with, we spent the first couple afternoons of school backtracking. Ann had chosen this site and planned her design for the hermitage long before we arrived. Of course, these are the first steps in building anything. We talked about criteria for site and design selection and finally built models of different home layouts. From these, questions such as “Which room catches the best midmorning sun?” were quickly answered.

Everything you read about how to place and design a natural, energy-efficient building — like putting the north side into a hill and facing angled windows south for passive solar — are especially important with cob because cob is all about thermal mass. The big earthen walls can do wonders at slowing down changes in temperature. There are, however, quite a few intangibles you may not anticipate when it comes to site selection. Ann explains, “While the location I chose near the Little Buffalo River couldn’t have been more beautiful or perfect for the purpose of a hermitage, the choice did present many difficulties. One of the main difficulties was driving many, many truckloads of materials down a long steep undeveloped dirt road, accessible only in dry weather. The site had no power, thus we had to bring in a generator for a few days for the foundation and roof building. It required pumping water in from a long distance. I was truly unprepared for the number of trips up and down this steep hill my old 1979 red farm truck would have to make.”

Becky’s fond of saying, “Remember, if you put your house on your favorite place, your favorite place won’t be there anymore.” Placing the hermitage at River Spirit by the river is a nice idea because the river is an ideal place to swim, walk by, sit by, and generally be near. But people are forever placing their homes in the very jaws of nature and will never be aware of it until severe weather floods them out. Becky warned us: “Watch how the storms affect your site. Test the wind, and think about what water will do if it arrives in abundance.”

This remarkably steep valley, at the bottom of which sits the hermitage on the shore of Little Buffalo River, would fill up like a bathtub in a flood. Nonetheless, placing the hermitage by the water is probably OK. There will be little to no furnishings, furniture, or possessions there. It is a simple place of sanctuary with built-in benches. It is small enough to be easily sandbagged, and extensive repairs after a flood are manageable for a building of this nature. The many hands that came together once to make this building can come together again to save it, if need be. Ann Lasater feels the good of being near the river outweighs the potential threat the water poses to this kind of space.

Foundation and Roof

The foundation work was completed before I arrived. Ann recounted the process for me: “We officially began when a group of friends gathered on the spring equinox, six weeks prior to the workshop, for a groundbreaking ceremony and to dig the footer. Then we began bringing in truckloads of rock and rubble for the underground footer. Next, we brought in more hand-selected river rock for the beautiful, 16-inch high rock wall/foundation. The rock wall is one example of the different mentality of building with cob. Instead of finishing the wall with a smooth, level surface for a sill board to be attached to, we intentionally left the wall uneven and rough with a trough in the center, so the cob would have a good surface to bind to.”

Cob is very heavy and needs a substantial foundation. Determining the appropriate width for the foundation depends upon the intended height of the walls; a wall will need to be nine or ten inches wide at the top, the thinnest part. The outside of the wall should taper up two inches for every three feet of height.

Conventional buildings are built with synthetic moisture barriers on top of the foundation. But placing a moisture barrier between foundation and walls prevents the walls from breathing. It may also weaken the connection between the cob and the foundation. Becky’s philosophy, a common one among natural builders, is that the best way to keep moisture out is by creating a really good drainage system. The Cob Builder’s Handbook explains with illustrations how to make drainage ditches and berms and gives ideas for making natural barriers. Becky encourages future builders to use their drainage system to provide a way for water to soak into their land or gardens. Not only can you keep water away from your house, you can direct it where you want it to go. “Think like water,” Becky says. It comes as a nice surprise to me that in figuring out a good drainage system, imagination is as important as calculation.

Foundation shapes for natural buildings can pose some challenges to linear minds. Curves are softer on the eye and a very pleasing shape for earthen materials, but a curved wall is also naturally more stable than a straight wall. if you’re pouring a foundation, you might wonder how to create a curved form for the foundation. The ever-useful straw bale can be bent into the shape you want and then staked into place for curved forms.

Cob easily facilitates the addition of wiring and plumbing through the insertion of four-inch pipes through the walls or foundation. Ann’s hermitage will be lit by candles and kerosene lamps, and there’s an outhouse in the woods not too far away, so wiring and plumbing installation was not an issue.

For Ann, the foundation and the roof were the only parts of the house she decided to hire help. Building the roof included selecting, harvesting, and preparing ten large cedar poles. Because of the hired labor, the roof was the most expensive part of the building. The hermitage roof is a good example of how any kind of roof can work on a cob structure. Ann designed it to be shaped a bit like an octagonal shell, and conceptually it is quite interesting and unusual. The back slopes downward at an angle that takes it almost into the ground. The front sweeps upward and a separate but lower piece forms a rectangular roof over the entryway. While it is interesting to look at, it posed many challenges for the stooping cobbers who worked on the back wall, bending over very low to place the wall under it, bumping heads, shoulders, and bruising backs. The roof may also prove problematic later as the builders bring the walls up to create some strange angles where the two sections of oddly shaped roof come together. Simplicity is a matter of taste, but may also be a practical consideration for inexperienced home builders.

Though Ann’s roof was built before the walls went up, you can add your roof last. Since cob is load bearing, you can simply build your roof right on top of your walls. I liked working with the roof up. We knew where we were headed, and the roof gave the shelter an identity early. Roof insulation, ceilings, and sheathing can easily go the way of conventional buildings. Some interesting natural ceding materials Becky mentioned during school include fabric, cob plaster, and woven bamboo mats.

The Mix

“Imagine the cob as a miniature mortared reinforced stone wall. The sand particles, like the stones in a stone wall, provide strength. The clay serves as the mortar. The straw does the reinforcing job of the rebar,” says Becky.

The Cob Recipe:

50% to 85% sand
50% to 15% clay
straw to taste
water

Cob is made from sand, clay, and straw. Every cob mixture is a little bit different, however, and you may vary the ratio of ingredients to suit what your soil naturally has in abundance. You’ll want to get to know your soil and try out a few sample mixtures before you can come up with the right recipe for your land.

Several people brought soil from their land and had Becky help them do a test to find out if their soil was sand-rich or clay-rich. To see the proportions of clay to sand in the soil, we put a little in a glass jar, added some water, shook it well, and then let it settle. The sand goes to the bottom and the clay hangs around on top. Silt is in the middle. Silt weakens cob, so if your test shows it in abundance, you’ll want to dig down further in search of a better clay/silt ratio. It is possible, if you have a lucky natural soil ratio, that mixing clay and sand might not even be necessary. You can simply take your dirt, add water and straw, and put it on the foundation and start shaping it into a wall.

If you do have to haul in either clay or sand, you can go heavy in the mix on the one you don’t have to haul, to a point. To figure out what ratio makes a workable cob mix for Ann’s soil, Becky made some test bricks. The test bricks that held up the best after they dried basically turned to stone. We couldn’t even break them if we clubbed them with a rock. Ones with too much clay cracked slightly, and those with too much sand crumbled slightly. The ratio we settled on was three buckets of sand to a three-quarters-full bucket of clay. We added straw until the mix wouldn’t take anymore without clumping apart.

“Because we were building near the river, the soil didn’t have the required amount of clay in it,” Ann says. “This meant the bringing in of pure clay which we harvested alongside the roads, and river sand that had fine gravel in it that we had shoveled into the truck. All of these building materials were free, and the expeditions to gather them were fun, but very time consuming.”

They placed the sand in a pile fairly near the hermitage so we wouldn’t have far to go to get it. (“Use your brain to minimize work,” Becky often implored.) For the clay, they built pits out of strawbales, a dozen or so of which they had also hauled in earlier, simply draping a tarp over six bales arranged in a square you could stand in. They mixed the clay with water and over the next week people spent lots of time stomping around in the pits getting the clay suspended in water to add to the cob mix. (If there is a naturally rich clay soil at the site, you can just add sand to it without suspending it in water.)

We put one cob mixing area inside the building and a couple others outside near the foundation. To make the mix, we dumped the buckets of sand in a wheelbarrow, wheeled it over, and plopped it on the mixing tarp. Somebody would then step in and spread it out a little with their feet. Then someone would bring over a bucket of clay and spread it over the sand. We added a little water and pulled one edge of the tarp up to mix the ingredients. When we released the edge of the tarp, three or four pairs of feet followed each other in circles to step the cob down flat. Becky would wander by and pick up the comers of the tarp and hand them to us. This was her way of saying, “You are not turning your cob mixture in the tarp enough.” Rolling it saves stomping work and speeds up the mixing process. Eventually, when we pulled up one edge of the tarp, the mix would stick together in a loaf. Then we would begin sprinkling straw into the mix. We’d add straw as we flattened our mix with our feet, and then we’d roll it by picking up one side of the tarp. We did this three or four times. When each strand of straw seemed to be covered in the mix, and we couldn’t get it to hold anymore, the mix was ready. It is fairly laborious to reach the stage where you can begin budding.

The Walls

As we began to really build in earnest, we must have looked like an army of worker ants. People were continuously mixing cob as others built. Sometimes we rolled cob into small informally shaped bricks that we could throw into wheelbarrows, onto other tarps, or line up along walls for cobblers to grab and place where they wished. Other times we just grabbed handfuls of cob off our mixing tarps and plopped them onto the foundation or wall and shaped the wall with our hands. You squish cob into the foundation with your hands or, even better, by stepping on it. It’s simple. The wall starts out as wide as the foundation. We followed the tapering formula (taper two inches for every three feet of height, assuming a nine-inch width at the top) on the outside of the wall. To figure out the taper, we cut a styrofoam wedge measuring two inches by three feet and taped it to one side of our level. The idea is to lean the wedge against the outside wall (fat end at top), and trim or shape the wall with your hands or trim it with a machete until the bubble reads level. We used the other side of the level on the inside of the wall to keep it going up straight.

We used our thumbs to knead new layers of cob into old layers of cob. When our thumbs got tired, we picked out some good long thumb-shaped sticks we could press into the cob to knead new layers in. As the walls went up a ways, Becky showed us how she uses her feet for forms. She sat on the ground and placed her feet up on the walls while someone pressed down on the wall from the top.

As we continued to pile cob onto the foundation, eventually it began to “oog” in certain places. Oog is a word cobbers made up to describe how cob squishes off the wall when it has enough mass that it can no longer dry quickly enough to hold its shape. Ooging keeps you from rushing cob; it forces you to let it dry hard so that it’s able to hold the load of itself as it goes up. Becky told us when our walls start to oog, this means it’s time to stop cobbing for the day. We punched holes in the cob with sticks to help it dry faster and to give us places to knead a new layer of cob into the old layer. Cobbers who live in a dry climate cover their cob work with wet gunny sacks at the end of the day to slow down the drying process. (It’s not as easy to add new cob to dry cob.) We didn’t bother with gunny sacks in humid Arkansas. The next day when we came back, we took a machete and cut off the oog on the sides of the walls. The cob had dried enough to take on more cob without ooging, yet it was still moist enough that running the machete down the wall cut the excess cob off easily and smoothly. We used a whisk broom to splash water onto the cob wall before we began adding the next layer.

Ann wanted to run a built-in cob bench along the interior walls of the hermitage. We literally sculpted the bench out from the wall. This is called cantilevering, and you can sculpt cob shelves and countertops this way too (this gives you an idea of how strong the stuff is when it hardens). Becky recommends adding a little more straw and clay for strength into a cantilevering mix. We built up the wall a bit under the bench and carefully kneaded new and old cob together. We got a little carried away and built onto the bench too quickly. The other piece of built-in furniture we made was a platform for a bed in an alcove in the back of the hermitage. One of the workshop participants, Nancy Vaughn, very capable of operating a saw, did some measuring and cut a few boards to span the width of the hermitage. She placed the ends of the boards right on top of the walls, which had risen to about chair height. We placed cob on top of the board ends so they became buried in the wall as it went up. Other built-in furnishings that could be buried into or sculpted into a cob house include cob fireplaces, light fixtures, arched niches, counters, shelves, and whatever else you might possibly imagine.

Windows and Doors

We had placed three windows in the hermitage by the end of the workshop. One was in a wood frame and could be opened, so we buried the wood edge of the frame in cob. The other two were non-opening, and we simply buried the glass edges in the cob. Cob provides a good opportunity to use recycled or broken windows because when you bury the edges, you can create your own shape for the window. (Make sure to tape broken edges so you don’t cut your hands.) We also buried cobalt blue glass bottles in the back wall. Though they are not quite windows, they let a little colored light in during the day and out at night when the candies are burning inside. Becky reminded us that the floor would be higher than the ground we were working on, so we placed the windows a little higher accordingly. Because cob walls are thick, we had an automatic inside sill by placing the window near the outside edge of the wall, taking into account the upward taper of the wall. if Ann had wanted her big sill on the outside of the hermitage we simply would have placed the window closer to the inside edge of the wall. You can also cantilever benches, ledges, or containers for wooden flower boxes at the base of windows if you want.

As we prepared to place the windows in, we built a couple six to 12-inch cob steps, stepping up and away from each side of where the window would be. We checked the level of the sill and set the window in. Then we checked the window’s plumb and level as we filled in the cob steps to hold it in place. As the walls proceeded up the sides of the windows, we began to sculpt columns on either side of each window. When the walls reach the tops of the windows, a sculpted cob arch or a wooden lintel will be placed over the top of the window. This will divert the weight of the cob from above the windows down through the columns on the sides. The cobbers will employ the same lintel-and-column or arch-and-column method over the timber doorframe that Ann has placed in the front of her hermitage.

Mixing and Applying Plaster

On the last day of the workshop, Becky showed us how to make plasters, and we sampled them on small sections of the walls, just as a learning exercise. The real plastering of the hermitage will be done during spring workshops. Plaster will protect the earth building and can also add beauty. Cement-based plaster or stucco does not allow natural buildings to breathe and can actually cause it to deteriorate faster. Becky recommends a basic earth plaster. To make it, we took the same ingredients in the same ratios as if we were making cob and we sifted it all through window screens until it had a fine texture. Straw takes a little effort to sift if it is long and coarse. There are many alternatives to straw to add fibrous strength to plaster if straw isn’t your favorite texture. One of Becky’s favorites is the fuzz from cattails. Africans and Central and South Americans often add various plants and or manure to earth plaster. Once you’ve got your plaster mixed, you can also add natural pigments or make whitewash out of builder’s lime to go over the plaster once it dries. Interior whitewashed plaster walls are particularly beautiful. (Interior plaster usually goes on after the roof and ceiling are done, but before the floor.) We used our hands and plastic yogurt container lids to apply the plaster, but you can also use a trowel or paintbrush. This is a good time to embed tiles and your shell collection in the mantel or along a doorway, sill, or countertop.

Laying Floors

The Cob Builder’s Handbook gives some good basic directions for putting in earthen floors. For more detailed descriptions of methods for making earthen floors, see Earthen Floors by Athena and Bill Steen. (Bill and Athena wrote about building straw bale homes in “Builing a Home Using Straw Bale Construction“) You can surface your floor with any surface you want, including wood or tile, but an earthen, stone, or brick floor can increase your thermal mass greatly.

You can make a one-piece earthen floor from cob mix. Becky says make your cob mix for your floor as dry as possible while still being wet enough to trowel, so it will dry quickly and crack less. Using a little more sand in the mix helps avoid cracks. A few additives can be helpful — Elmer’s glue or flour to harden the floor, psyllium seed husks to give the floor a rubbery quality that might be easier on your feet. Becky recommends making two-foot round test samples of your floor mix. If your sample cracks, you have too much clay, but compensating for cracks with too much sand will result in crumbling floors.

To make a floor, level your ground, figure out your desired floor height, and mark it. Then pound your floor with a tamper for as long as you can stand it. Next put in a gravel base to raise it up off of the ground. This will keep your floor drier, and the base provides stability and a little insulation. This is another common place for a moisture barrier in a conventional house, but natural builders tend to avoid them altogether. Finally, dampen the base, and trowel-on the floor. Let it harden for a few weeks and trowel-on your next layer. Your level should play a role, but there’s no real reason a floor has to be 100% perfectly level. After a cob floor has dried a few weeks, you can put on a sealant like linseed oil.

Final Thoughts

The hand-built cob hermitage at River Spirit is a strange and wonderful place. Despite the ticks, copperheads, and biting bugs, I did not want to leave the forested spot by the river on the last day of the workshop. I have spent a lot of time in coniferous forests, but this week I spent living under a canopy of Ozark hardwoods made me understand how a person can fall deeply in love with trees.

Given the housing problem in America today, in cities, suburbs, and in poor rural counties like this one in the Ozarks, it was at once inspiring and frustrating to see a cob house go up so simply and intuitively. Cob is cheaper, more comfortable, and far more durable than the prefab throw-away homes that millions of us live in. Yet just a handful of people are devoted to cob in this country.

MOTHER EARTH NEWS would be more than proud to spearhead a cob movement that would take home building out of the hands of the contractors and return it to the people who need it the most. Please let us know of any cob projects you decide to put your hand to. We’ll publish some photos of the hermitage at River Spirit when the finishing touches have been completed.

Tools List:

Level, shovels, buckets, machete, rubber tubs, strawbales, truck, wheelbarrow, trowel, tarps, gunny sacks, cobbing sticks, window screens for sifting materials, whisk broom, tamper

Materials List:

Clay, sand, straw, water, tiles, bottles, decorative objects to bury, windows

Resources List:

Becky Bee’s web page

The Cob Builder’s Handbook, (MEB264), is available through the MOTHER EARTH NEWS bookshelf.