Cob

by Khoke and Ida Livingston of Davis City, IA

When Khoke and I married, we moved into a cob house that was originally built by his parents. Living in a cob structure for the first time, one thing I noticed was how regardless of the severity of a storm, our house never shakes in the wind. Southern Iowa has a lot of wind but our house is as planted as a boulder.

What is Cob?

Cob is a raw earth building style, free formed by hand. It is built and shaped like a giant pottery vessel. Only instead of coil, cob is shaped and stacked in carryable amounts as its Old English root suggests; meaning “a lump or rounded mass.” The clay has straw, sand and water added to it for strength, crack resistance, and to make it easy to handle.

Cob is a popular European building style that has some similarities with the adobe of the American Southwest. Adobe, in oversimplified terms, is essentially cob that is shaped into large bricks, dried and then stacked.

Our home is a small (22 ft in diameter), 8-sided, 2-story cob and stone house that is built into the hillside. The walls are 18 inches thick. Our floor is made of cob as well, only a different ratio of clay, sand and straw that is more tolerant of foot traffic.

The thick cob walls actually offer no insulation. They accomplish the same goal with different means. Temperature changes conduct themselves slowly through the wall at a rate of 1 inch per hour. Before the winter cold nighttime temperatures can move all the way through the cob, the sun comes out and the solar passive effect lessens the incoming chill. This incoming chill is also meeting the heat that is slowly working its way out through the cob from the inside as well.

Likewise, in the summer, the sun smiles down and sends energy in the form of heat through the cob. She finds the cob more patient than day length allows the sun. Before there is enough time to push the heat all the way through 18 inches of cob, she has slipped down over the hill.

In the meantime, I have heavy wool curtains on all my south facing windows that stay drawn all day. At night we pull the downstairs curtains aside and open the windows to let the cool air in. At dawn, these windows are closed to keep the cool air in and the south facing curtains are drawn again.

Our downstairs feels like it is air conditioned all summer long. The warm air goes up our stairs to be caught and moved around by the cross breezes from our open upstairs windows. This helps blow any rising warm air out.

Building With Cob

A little over 20 years ago, Khoke’s father, Jeff Livingston went with friends to a cob house workshop in Ohio led by Ianto Evans. At this event they helped build a cob house. They came home to Iowa and each enthusiastically built their own cob cottages.

To build a cob home, once the site has been leveled it is recommended to do the foundation, frame and roof first. The roof is necessary to protect the cob walls over the weeks it takes to build them. Cob walls will wash and disintegrate over time in wet rainy conditions.

The foundation is dug and is either poured cement, or built with block, or stone as ours is. This foundation must be high enough to prevent the cob wall that sits on it from ever getting wet, 18 inches above ground level is the minimum recommendation. If the cob gets wet, it softens and cannot support the weight of the walls above it. Even worse, if it got wet and then froze in the winter it will crumble apart when it thaws.

With the foundation prepared, it is ready to start stacking cob. Tarps are laid out and measured amounts of moist clay, usually from a local source, are dumped on the tarp. The clay, sand and straw ratio will vary somewhat depending on the type of local clay source you have. Often local clay is already somewhat sandy and depending how much it is will affect how much sand must be added. In the book The Hand Sculpted House, Ianto Evans tells how to home test your soil to find the sand and clay ratio naturally in it.

Once you know your clay/sand ratio, you add your measured amount of moist clay, and additional sand to the cob tarp and the stomping begins. Volunteers strip off their socks and shoes and stomp the sand into the clay, using the tarp to fold the mixture to the middle and stomp it again. This is repeated as necessary until well mixed. There are people who have come up with various ways to machine mix the cob but we have manually stomped ours.

When the clay and sand are evenly mixed, a measured amount of straw is added. The stomping resumes until it too is thoroughly mixed in. This mixture is the cob and is about cookie dough consistency. It is rolled into balls to be stacked and smoothed into walls aboard the foundation, building higher and higher until it meets the roof.

The beauty of this building style is the ability to build things right into the cob. Many cob houses I have seen have a round or oval window where a large, single pane window was set in the wet cob and an oval of glass was left free of cob as it was built around.

Shelves can be built into the walls, whether cubby holes are shaped out of the wet cob as it is built or a board is laid on the cob for the platform of a builtin bookshelf. A desk can be built into a wall, or a filing cabinet can be cobbed around. There are many possibilities.

Building a cob house is a relatively inexpensive building style but it is very labor intensive and can be time consuming. Ianto Evans estimated that it takes 6 months to 2 years for 2 people working on it full time. There are a lot of factors that can affect how long it takes. One is the fact that it can only be done during the warmer months as wet cob is ruined by freezing. Once dry (this can take weeks) however, let the weather do what it will and the cob will be fine as long as it is under roof. It will stand up to rain or snow, but flood water is fatal to cob walls. To help cob walls dry faster, builders will plunge a “cobbers thumb” into the wet cob to increase the surface area so the walls dry faster. This is just a wooden peg used to make holes in the cob so it dries faster. Once dry this can just be cobbed or plastered over for smooth walls.

Some people leave the cob walls as they are naturally without a lime plaster. Harsh weather from the direction of the prevailing winds can pit and erode the outer walls over time. For this reason, often those who live in areas with a lot of wind and storms will opt to lime plaster the outer walls of their cob cottage. A lime plaster creates a hard surface that takes the beating of weather better than raw cob.

It is highly discouraged to use any type of concrete as a plaster for the walls. A lime plaster will breathe, meaning moisture can travel through it and thus be wicked away. Concrete does not breathe. Any moisture that happens to collect under its shell, whether from condensation or seeping through a crack, cannot now escape. As this moisture accumulates, it soaks the cob which causes serious problems instead of allowing it to dry out.

These dense cob walls that are invulnerable to any wind, have an Achilles heel, and it is seeping moisture. Dry cob will revert to mud if soaked sufficiently. Once the cob is built and thoroughly dry, it takes a long time to resoak. Again, it is not particularly vulnerable to precipitation but floodwater would be a serious concern.

Here in southern Iowa, our home sees a lot of wind. Our cob house has a lime plaster coating to protect it from driving rain. Our north and west facing sides are assaulted by the weather the worst and over time, the plaster there wears away as the wind and rain pick at it. We expect to replaster our house every 10 years or so, but better maintenance would probably look like every 3-4 years. If we did not have as much wind as we do, it would not have to be replastered nearly as often. Khoke’s grandfather, William also built a cob house, only instead of lime plastering the cob, he put a rock facing on it to protect it from the weather.

Cobbing a Floor

A cob floor can look similar to a concrete floor but it is not as hard. Although it bears foot traffic, furniture can dent it. A concrete floor will outlast it.

When laying a cob floor one needs to account for plenty of depth space. The layers that add up to the floor will total at least 12 inches deep. This starts with 6 inches of 3-inch gravel. Then with a commercial 2×4 (it actually measures about 11/2 x 3½ inches), strips of a stomped cob mixture are laid in sections across the floor. This cob mixture is a blend of 1-inch gravel, clay slip, sand and some straw chopped into short 3-4 inch lengths. Turning the 2×4 on edge, a 3½ inch layer of this stomped cob is laid in strips with the 2×4 board boundaries leapfrogging each other as the cob moves across the floor. The cob is laid the thickness of the 2×4 width and scraped with a screed board to level and smooth it.

Once this layer of floor has dried then the 2×4’s are laid on their side and another layer is laid, only 1½ inches thick this time. This is laid in like manner all the way across the floor. Once this too has dried then the final coat is laid which is only ¾ inches thick. The final coat is a mixture of sand, clay slip, and horse manure. Yep, horse manure. This is for the fine fiber in it. It also has enzymes in it that help plasticize the cob. The final product shows no detectable evidence of it, no odor or visible fiber. The horse manure is sifted through screens before adding it to the stomped cob.

Once this final layer is dry, a blend of turpentine and linseed oil is brushed over the floor just as thick as the floor can soak it in. As soon as it soaks in, another layer is applied. This is repeated until it will soak in no more. Then it is left to fully dry which can take at least a couple weeks.

Variations

The cob building style is often combined with other building styles. Cob blends easily with timber frame. Sometimes straw bale construction is incorporated for its insulative effect in far northern climates. Our home is a blend of stone, wood, and cob construction.

Lime plaster typically dries to some shade of eggshell white. Khoke’s parents originally added a concrete dye to the plaster to give their home a red clay instead of the white plaster. Shortly after we moved in the house, Khoke and I replastered the outside of our house as the plaster by this time had pitted over the years from the weather. Only we let it stay the natural white lime plaster color.

Limitations

Cob structures have immense thermal mass. Location is key to harnessing its benefits. There are places however that would not work well to incorporate cob. Climates that have long, cold, and dark winters where you cannot employ the benefits of a passive solar effect. In such a case the thermal mass can work against you. Thermal mass is describing, in this case, the buildings’ ability to absorb, store and release heat.

Although a cob house could likely stand up to a seismic event, it would be unlikely that it could handle any kind of regular seismic activity. Cob can handle wind without a shudder, but earthquakes will definitely make them shake and would likely crack the walls. Though earthquakes do tend to have that effect on structures in general.

You may also want to reconsider cob construction if you are in one of those few locations with no local clay sources. Whatever expense one may have saved by building with cob would soon be negated by sourcing and shipping clay.

Points to Consider

There are both benefits and drawbacks to about anything worth doing. Cob is a low cost, durable form of housing that grants an incredible range of artistic freedom in design. Building with cob can also be very time consuming. One may need to measure which commodity, time or money, one has in more abundance when deciding the building style they wish to use.

While time-consuming may be a real factor, this building material is family friendly. It is something the whole family can participate in. Children can help stomp cob and make cob loaves to smooth onto the rising walls. Cob and other natural buildings are also popular for the non-toxic materials that have no outgassing of toxic fumes.

Often people lay the foundation and then build the roof with the framework necessary to hold it up. The cob walls then rise to meet the roof. Cob however, shrinks as it dries and will leave an air gap where it once met flush with the sill of the roof. Some plan for this and have the roof jacked up a little so it can be released to set full contact on the walls once they are dry. When Khoke built his cabin he chanced the weather and built his walls without the roof first, cobbing the rafters into the wall.

Khoke has a very tall cousin, 6’6” in his socks, who built a cob cottage and wanted to maximize space efficiency. He built the walls no taller than necessary for his height. But by a minor miscalculation he forgot to figure in for shrinkage as it dried. Tall people must duck for everything, but it is a pain to have to do so in one’s own house.

Resources

There are now a number of books that can be found that tell how to build a cob house. We have built with cob using methods taught by Ianto Evans. He cowrote a book with Linda Smiley and Michael G. Smith called The Hand-Sculpted House. This is a lovely, well written book that is very thorough. When considering the depth of commitment one puts into building a home, having multiple resources is always a good idea. Those who have enough building experience to write comprehensive guides on how to do so will have encountered challenges that other authors may not have and you may benefit from what they learned.

Other books you may benefit from include (but are not limited to):

Building with Cob: A Step-by-step Guide (Sustainable Building) by Adam Weismann, and Katy Bryce

Earthen Floors: A Modern Approach to an Ancient Practice by Sukita Reay Crimme, and James Thomson

Essential Natural Plasters: A Guide to Materials, Recipes, and Use by Michael Henry, and Tina Therrien

The Art of Natural Building Compiled and edited by: Joseph F. Kennedy, Michael G. Smith, and Catherine Wanek

Essential Cob Construction: A Guide to Design, Engineering, and Building by Anthony Dente, Michael G. Smith, and Massey Burke

An Ambitious Experiment

When Khoke turned 15 he decided that he wanted to see if he could build a cabin with little to no money. He had just finished helping build his parents house the year before and decided cob was the most cost effective way to go. He used only hand tools and no power tools for the project.

He selected and leveled a site and dug his footers. He gathered large field stones and dry stacked his foundation. He sourced local sand and gravel, and shoveling it onto a wagon he brought it home with his dad’s team of horses. He gathered his materials and prepared the site that fall but had to wait until spring to cob the walls.

The cabin he laid out was a 12×18 oval. In late spring when it was warm enough to cob he had all his materials assembled, he put in long hours to raise the walls. He got the cabin walls up in about two weeks. He had to hurry because he did not build his roof first and if the weather turned wet it would dissolve his walls.

The poles that Khoke cut for rafters were cobbed in place along the top of the walls. The roof was then thatched with rye and his cousin Zach Miller, then 16, helped him thatch it. This was overseen by their grandfather, William Livingston as he taught them how to thatch it.

Once the roof was done, he put a cob floor and a loft in the cabin. At the end of the project he had spent only $10 and that was for nails. He would have had to buy the turpentine and linseed oil for the floor except his parents had leftover from their house. The poles he had cut and left round. He used repurposed lumber and windows and unused leftover material from other building projects. The cabin took easily double the amount of work since he hand sourced and gathered all his own materials.

Naturally once he had the cabin built, he moved into it. It just meant he had to walk across the yard to Mom’s for breakfast instead of down the stairs. He had a number of friends who bunked at his cabin off and on over the years. They had plenty of stories to tell after I married Khoke. I had to laugh when Caleb Torres informed me that bunking at Khoke’s bachelor pad “was not a democracy.”

In hindsight, Khoke will say that building that cabin was perhaps overly ambitious for someone as young as he was. A lot of work that made a short road to growing up quickly.