The method we use in most of our projects is known as cob, monolithic adobe, or more specifically the Oregon Cob method. In areas where earthen building traditions are alive and well, it is a great idea to work with an experienced local builder. But if there are no living traditions in your area, the Oregon Cob method can help you create an effective method in a relatively short time.
Good building dirt is bad garden dirt, and vice versa. For a good earthen project, you want subsoil (the lighter or single-color layer underneath the dark brown topsoil); and you want mostly hard grit, sand, and sticky clay - not the soft, velvety silts and loams that make fluffy garden tilth.
For our typical earthen building projects, here are roughly the amounts of materials involved.
Any good earthen building project needs a good hat and a good pair of boots - a roof with adequate overhangs, and a well-drained foundation or dry footing.
We typically build on compacted gravel footings in primitive sites such as earthen buildings or outdoor kitchens. For urban settings, a typical floor might be a 4" concrete slab, or a suitable foundation or reinforced flooring. The finished mass of a cob oven may be about a ton; a rocket mass heater is more like 2-5 tons.
Roof overhangs should be sufficient to prevent much weather from reaching the earthen material - cob can easily handle the occasional wetting, but must dry out between wetting so that the core moisture level stays below about 15%. The clay on the surface will swell with moisture and seal out any direct water leaks for a while, as long as it has time to dry thoroughly between wettings it's easy to keep the moisture levels stable.
Earthen building traditionally is much more popular in areas where the soils are 'ready-mix', and wet earth can be made into bricks or pitchforked onto walls with little fuss. The US Southwest has many such geological areas. European earth-building traditions generally just make the walls as thick as necessary for strength, rather than amending the local earth. Traditional cob houses typically have walls between 18" and 2 meters thick, depending on height, purpose, and local soils.
Here's a general introduction to the process from a recent set of workshop notes:
Earthen Building:
Cob LoveIf you lived in factory housing all your life, hearing scornful references to 'primitive mud huts', you might wonder why we prefer earthen masonry. Why not use modern materials like concrete?
It's certainly possible to build a masonry heater using modern materials – including concrete mortars in the exterior masonry. On the inside, you will find expensive refractory parts, surrounded by an expansion joint to protect the brittle casing cements from cracking (or powdering away from the heat of the flames). Masonry heaters are built twice: once with heat-tolerant ceramic and fireclay, and then a separate layer of 'modern' materials to store warmth and please the eye. These heaters cost from $5,000 for an inner-core kit (finish masonry not included), to five or six figures for a master mason to build a custom installation. They can be beautiful and efficient, but for many families they are a multi-generation investment.
What is ceramic made from? Fired clay. Where do we find clay? … In the earth.
With the right proportions of aggregates, clay, and reinforcing fiber, earth offers great performance and durability. It won't burn; exposure to heat can even strengthen it. Earth is sturdy; plentiful; more comfortable and resilient than concrete; easier to shape than wood; has low embodied energy and is rarely toxic; is easily maintained, repaired, or altered; and naturally regulates temperature and humidity. Only in ongoing damp conditions is concrete a better material – concrete makes good basements, but earthen masonry makes a healthy house.
As cob cottage founder Ianto Evans likes to say, 'Bugs won't eat it, and it's dirt cheap.' If you prefer a haughtier endorsement, look at the mansion across the street from the White House.
If you still prefer to live in a glorified tent (a framed building with insulative paneling), you can explore some of the benefits of earthen masonry by visiting traditional monuments, or adding features to your garden or outbuildings.
Building with Earth: the Oregon Cob Method:
Cob is an old English word meaning 'loaf' or 'lump,' like cobblestones. 'Cobbing' is earth building with wet lumps, just as adobe is earth building with regular, pre-formed blocks.
Soils and subsoils vary widely, so local earthen building practices or 'recipes' may be very different from region to region. Traditional cob homes in the UK and Europe are built of wet local earth, matted with straw, and pitchforked onto walls whose thickness and taper is dictated by centuries of local experience. Other places such as Yemen have their masonry methods so refined that they can build traditional earthen 'skyscrapers' 12 floors tall.
The method known as 'Oregon cob' combines international building and field-testing methods to revive earthen building in areas without current local best practices to follow. Soil analysis, amendment, and consistent batch-testing help get reliable results in any location.
It's most effective to learn earthen building at a local building project, or attend a workshop in a climate and geological region similar to yours. Some people have also gotten started by reading and practice on small outdoor projects, to develop good techniques for local conditions. Masons, geologists, potters, bakers, trackers, and farmers may have an advantage in learning cob. Good garden soil is bad building dirt, and vice-versa.
You sculpt it like clay, but cob is mostly sand or grit. Finished cob (in traditional cultures) may be 6-15% true clay, 30-70% sand/gravel aggregates, with the remainder composed of other minerals such as chalk, silts, organic materials, and rocks. Fiber is usually added by volume or texture, not by weight.
Thermal cob or cob 'starter' mix is like fireclay mortar: 3-5 parts aggregate to 1 part clay-rich soils (or 1/2 part fireclay slurry). Add water to make a cookie-dough texture.
As with any masonry, sharp sand and mixed-size aggregates work best. Sandy grit with a small amount of less-expansive clay gives best results. The sand's job is to jam and hold the clay apart, as the clay dries and shrinks. Highly expansive clays can be diluted with silt or extra aggregate. Good mixing is essential – unmixed clay makes cracks, not bonds.
Unlike cement or lime masonry, the clay bond is not a chemical set. Cob 'cures' only by drying, and can soften again with water. Old cob can be re-wetted and re-mixed. There is no age of cob that cannot be recycled into a new batch; with minor amendments, shoddy old material can become great new material. (Use dust prevention or protection measures when working with any dry masonry material.)
Fiber is added last, after the clay is thoroughly dispersed. When re-working a batch of older cob, more fiber is added to replace decayed material. Straw serves as both aggregate and tensile strength, and sometimes also as insulation. Typical structural cob might be up to 1/3 straw by volume, or a flake or two per batch; 'thermal cob' has no straw and extra sand; insulation might be half a bale of straw coated in a wheelbarrow full of clay-soil slip (no sand). Well-integrated natural fibers form a multi-directional mesh in the wall. (Besides straw, other non-nutritive fibers are also used: hair, animal dung, ceramic fiber, hulls and stems of local crops, even pulped wood.)
Most finished cob, like adobe, weights about 95 lbs per cubic foot. Wet cob can weight up to 3 times more. A 4" concrete slab footing can handle dead loads equivalent to about 2-3 feet of cob height – 180 to 275 lbs/sf. Self-supporting cob walls generally taper by an inch every 6 to 10 feet.
Any building needs a good hat and a good pair of boots (roof overhangs, and well-drained foundations) to ensure the long life of natural materials. With breathable weather protection, earthen buildings last many centuries.
We never use vapor-blocking 'band-aids' like plastic sheathing or concrete stucco. There are too many sad stories about a stout earthen wall (sometimes over 200 years old) that gets 'protected' with concrete stucco, and collapses within a few years from moisture trapped inside. Vapor barriers do not account for moisture from within the home.
Cob can handle occasional surface wettings: the clay swells to slow water penetration. The core moisture levels must remain below about 15% (like dry firewood) to ensure structural integrity. Breathable coatings such as lime plasters, dung plasters, or ventilated wood shake can help protect walls – but keep weather off the walls in the first place. Surface coatings are no substitute for a decent roof.
Cob Cottage Company in Oregon (www.cobcottage.com), Graeme North in NZ (www.ecodesign.co.nz/), and Mike Wye in the UK (www.mikewye.co.uk/) are among our favorite English-language references for earthen building practices.
Almost every continent has historic and beautiful earthen buildings, some haunting ruins, some cheerfully painted and maintained throughout centuries of appreciative use. We are also inspired by World Heritage Sites like Shibam in Yemen; the North American pueblos; the Great Wall of China, temples and villages like Kune Tambo in Mali, Africa; the resilient cathedrals and ancient architecture of Cusco, Peru; the Pise de Terre work of France and French-influenced Carolinas; and English-style cottages - historic and Colonial farmsteads; and modern Cob revival homes.
Books and References:
The Hand-Sculpted House, by Ianto Evans, Linda Smiley, and Michael Smith (www.cobcottage.com)
The Cob Builder's Handbook, by Becky Bee
Cottage Building, A Renaissance - Clough Ellis, 1919
Rocket Mass Heaters, by Ianto Evans and Leslie Jackson; 503-396-1825, www.rocketstoves.com.
Build Your Own Earthen Oven, by Kiko Denzer
The Forgotten Art of Building a Good Fireplace, by Vrest Orton – Yankee Magazine, 1969
Online: Sweep's Library, http://www.OnlineChimneySweep.com, Wood Burning Stoves Forum at http://www.permies.com/forums, YouTube videos – don't trust unfinished intentions. Paul Wheaton links some good videos from http://www.richsoil.com/rocket-stove-mass-heater.jsp
Ernie and Erica's online photo albums: http://picasaweb.google.com/eawisner and ~/eritter.
Blog: ErnieAndErica.blogspot.com
Back to main page: http://www.ErnieAndErica.info.
Other pages: Shop (lists some materials sources at bottom of page)
Rocket Stoves, Natural Building, Natural Paints and Plasters