Cob building gets its name from the Old English term for “lump,” which refers to the lumps of clay-rich soil that were mixed with straw and then stomped into place to create monolithic earthen walls. Before coal and oil made transportation cheap, houses were built from whatever materials were close at hand. In places where timber was scarce, the building material most available was often the soil underfoot.
Building with earth has a long and successful history. Cob construction is particularly easy to learn, requires no fancy equipment, uses local materials, and can be done in small batches as time allows – making it extremely accessible to a wide range
In the U.K., tens of thousands of cob buildings are still lived in, some of them more than 500 years old. When the British immigrated to the United States, Australia and New Zealand in the 1700s and 1800s, they brought the technique with them. In Africa, the Arabian Peninsula, parts of Asia and what is now the southwestern United States, cob was developed independently by indigenous people. In Yemen, cob buildings stand that are nine stories tall and more than 700 years old.
However, with the industrial age came factories and cheap transportation in the West, making brick, milled wood, cement and steel readily available. Mass production led to mass marketing and the promotion of these new materials as signs of progress. The perception of cob as “poor people’s housing” led to its near demise. By 1985, there hadn’t been a new cob building constructed in the U.K. for more than 60 years, or in the United States for at least 120 years.
Modern Cob Buildings
Today, building your own house is the exception to the norm, and it is almost unheard of to build with local materials. Instead, houses are built by specialists using expensive tools and expensive, highly refined materials extracted and transported long distances, often at great ecological cost. Industrial materials have many benefits — performance, predictability, speed and ease of installation — but they have in common that they must create a profit for the companies that manufacture them. The average number of members in U.S. households has dropped by more than half in the past 50 years. Yet, over the same time period, average home sizes have more than doubled. We are more comfortably housed than at any point in history, but practically enslaved by the payments (the word “mortgage” is French for “death contract”). Fortunately, we have other choices.
Anecdotal evidence and recent testing show cob walls are highly resistant to earthquakes. Unlike cement or adobe, which tend to shake apart in an earthquake, lumps of cob are woven together in the building process to form one large mass reinforced by straw fiber. Also, unlike cement, cob is easily repaired with the same material it was built from, and if torn down, there is no waste to be disposed of — only earth that can be returned to the ground or soaked in water and reused to build another room or house.
Cob Pros and Cons
• Uses local, generally inexpensive or free materials
• Can include creative, beautiful detail
• High thermal mass helps temperatures stay consistent and comfortable
• Negligible environmental impact
• Compatible with other natural materials (wood, stone, lime)
• Fun to build with friends
• Needs additional insulation in cold climates
• Will be unfamiliar to building code officials and insurers
DIY Cob Building Technique
Prepare the mixture. Cob is a mixture of approximately 1 part clay, 4 parts sand and 1 part straw. You can buy bagged clay in powdered form and masonry sand from your local supply store. However, most soil is a mixture of some part clay, some part sand and other materials, so you’ll save money if you use your own soil. If you are paying for soil, ask the supplier for your local variant of “compactable fill” or “road base.”
The “soil” we refer to is not the rich, black stuff in your garden or the top few inches of your lawn where everything grows. It is the subsoil below that, with little or no organic matter in it.
Build a foundation. Cob walls are heavier than those made of light timber. Before you build with cob, you will need a foundation that will keep your work up and away from damp ground and a roof that will keep it out of the rain. Natural builders call this having a good “hat and boots.”
Remove all vegetation and topsoil from your site, and mark out the perimeter of your cob construction with paint or pegs.
During rainy weather, water must move away quickly to keep cob dry. Dig a trench that will encircle the structure and lead water away. Backfill the trench with compacted gravel.
Stone or reclaimed concrete chunks (urbanite) make an excellent foundation. Lay your stones on tamped, solid ground within the gravel trench perimeter. You are finished when the foundation material feels absolutely solid beneath your feet.
Mix. Break up soil manually and soak it at least overnight in enough water to turn it into pudding. Pudding is a lot easier to mix with sand than clods of clay are.
The simplest way to mix cob is with your feet. (Music and friends make this a lot of fun.) Spread a 5-gallon bucket of sand on the center of your tarp, then a bucket of wet clay over that, then another bucket of sand.
Mix the sand and clay to form a pile, adding about two more 5-gallon buckets of sand as you mix. Sprinkle straw over your pile. Keep using the tarp to flip your pile, and mix it until the straw is distributed throughout the pile.
Break the mixture into balls — called “cobs” — of whatever size feels manageable, typically about the size of a softball. Make some test sections by molding some cobs together and letting them dry about one week to see how they hold up. You do not want a crumbly mix.
Build and sculpt.
Place the wet cobs where desired and smash them into one another to form a single unit. If building a wall, place one cob for the outside face, one for the inside, and one for the middle. Meld together. Continue this process until you’ve formed your structure.
As you work, use your thumbs to knit the straw fibers from one cob lump into the cobs around it. Leave a rough surface for your next layer to stick to by covering the freshly laid cobs with thumb-sized holes, and wet the surface before applying new cob.
Everything you build is an excuse to get artistic. Improve a blank expanse of wall with a carved niche for a candle or special object. Sculpt an alligator or dragon bench to sit on. The steps are the same whether you’re making something big or small; it’s just a matter of scale.
To finish the structure, apply plaster in at least two coats: a “brown” coat to even out imperfections, and a final coat, which can be polished or sealed with linseed or hemp oil. A standard lime plaster consists of 1 part lime putty, 3 parts sand and a quarter-part fiber.
Mix the plaster by foot or paddle in a bucket or trash can. Chop straw into tiny fibers using a weed trimmer inside a garbage can (wear eye protection and a dust mask). Cellulose insulation also makes a wonderful plaster fiber. Apply plaster to a slightly damp surface with a firm pushing motion. Plasters will often crack at the line where you stopped or started, so pick unobtrusive points for breaks. Seal plaster by painting it with several coats of linseed or hemp oil.
Store leftover plaster in a sealed jar for repairs. Scratches can be rubbed out with rough sandpaper and re-oiled. Large cracks or breaks should be filled with reconstituted plaster, polished, and then oiled after they’ve dried.