The Building Codes
Societal Impact Matrix
Return of The Village
Habitat For Humanity
Curves of Breath & Clay
Overview of Techniques
Nature, Earth & Magic
History of Cob
Cob Q & A
Compressed Earth Blocks
German Clay Building
Earthen Plaster & Aliz
Solar Water Heater
The History of Cob
Because of its versatility and widespread availability, earth has been used as a construction material on every continent and in every age. It is one of the oldest building materials on the planet; the first freestanding human dwellings may have been built of sod or wattle-and-daub. About 10,000 years ago, the residents of Jericho were using oval, hand formed, sun dried bricks (adobes), which were probably a refinement of earlier cob. Even today, it is estimated that between a third and a half of the world's population lives in earthen dwellings.
Earth construction takes many forms, including adobe, sod, rammed earth, straw-clay, and wattle-and-daub. "Cob" is the English term for mud building, which uses no forms, no bricks, and no wooden structures. Similar forms of mud building are endemic throughout Western and Central Europe, the Ukraine, the Middle East and the Arabian peninsula, India, China, the Sahel and equatorial Africa, and the American Southwest.
Exactly when and how cob building first arose in England remains uncertain, but it is known that cob houses were being built there by the 13th Century. Cob may have evolved from earlier techniques like wattle-and-daub, where mud is plastered over a framework of woven branches. One possibility is that, originally, mud was used as an intermediate filling between double wattle walls, and that when the wattle decayed, it was found that the building remained sound without it. Another theory holds that cob evolved from the mud mixture used almost universally in Medieval times to mortar stone walls and to fill the cavity between two stone faces. When the valuable facing stones were pirated or fell away, cob walls remained.
However it happened, cob houses became the norm in many parts of Britain by the 15th Century, and stayed that way until industrialization and cheap transportation made brick popular in the late 1800s. Cob was particularly common in Southwestern England and Wales, where the subsoil was a sandy clay, and other building materials, like stone and wood, were scarce. English cob was made of clay-based subsoil mixed with straw, water, and sometimes sand or crushed shale or flint. The percentage of clay in the mix ranged from 3 percent to 20 percent, with an average of 5-6 percent working well. It was mixed either by people, shoveling and stomping, or by heavy animals such as oxen trampling it.
The stiff mud mixture was usually shoveled with a cob fork onto a stone foundation, and trodden into place by workmen on the walls. In a single day, a course or "lift" of cob would be placed on the wall between 6 inches and 3 feet high, but usually averaging 18 inches. It would be left to dry as long as two weeks before the next lift was added. Sometimes additional straw was trod into the top of each lift. As they dried, the walls were trimmed back substantially with a paring iron, leaving them straight and plumb, and commonly between 20 inches and 36 inches thick. In this way, cob walls were built as high as 23 feet, but usually much less. Openings for doors and windows were either built in as the wall grew, or else lintels of stone or wood were set into the wall at appropriate heights and the openings carved out after the cob had settled and dried.
Many cob cottages were built by poor tenant farmers and laborers, often working cooperatively. A team of a few men, working together one day a week, could complete a house in one season. A cottage begun in the spring would receive its thatch roof and interior whitewash in the fall, and its inhabitants would move inside before winter. Often they waited until the following year to plaster the outside with lime-sand stucco so that the walls would have ample time to dry. Cob barns and other outbuildings were sometimes left unplastered.
But cob buildings were not reserved solely for the humble peasants. Many townhouses and large manors, built of cob before fired brick became readily available, survive in excellent condition today. Among them is Hayes Barton, the birthplace of Sir Walter Raleigh, who had so much affection for his childhood home that he offered to buy it from its then owner for "whatsoever in your conscience you shall deme it worth." An estimated 20,000 cob homes and as many outbuildings remain in use in the county of Devon alone. It was common for well-built cob homes to go for a hundred years without needing repair.
British settlers to other parts of the world took the technique of cob with them, transplanting it in Australasia and North America. Early colonists of New Zealand found the clay soil and tussock grass common on the South Island to make excellent cob, and constructed at least 8,000 houses there, of which several hundred survive today. Cob was less popular in Australia, where mud bricks and rammed earth were the preferred earth building techniques, but a few cob buildings survive in New South Wales, in Queensland, and in the vicinity of Melbourne. Cob buildings in North America dating from the same era are few and far between, but include a house built in 1836 in Penfield, New York, and a church in Toronto.
By late last century, cob building in England, considered primitive and backward, was declining in popularity. During the 20th century, however, public attitudes slowly evolved until traditional cob cottages with their thatched roofs are now valued as historical and picturesque. As there was virtually no new cob construction in England between WWI and the 1980s, traditional builders took much of their specialized knowledge with them to the grave. But enough information survived to allow a cob building revival in the 90s, fueled largely by historical interest and the real estate value of historic cob homes.
The English Cob Revival
The English place great value on tradition, and take good care of their historical buildings. In recent decades many long-neglected cob homes have needed repair, causing a resurgence of interest in traditional building techniques. The people involved in the restoration of ancient cob buildings have become the greatest advocates for the reintroduction of cob as a current building technique. The first new construction project of the English cob revival was a bus shelter built by restorationist Alfred Howard in 1978. Since then there has been an increase of new cob built in England, particularly in Devon. Kevin McCabe received a lot of press in 1994 for his two-story, four bedroom cob house, the first new cob residence to be built in England in perhaps 70 years.
The building technique of these revivalists closely resembles that of their ancestors. They mix Devon's sandy clay subsoil with water and straw and fork the mixture onto the wall, treading it in place. Walls are generally 24 inches thick and straight, applied in lifts up to 18 inches high. The machine age has altered the traditional process in only minor ways: McCabe and others use a tractor rather than oxen for mixing cob, and often amend the subsoil with sand or "shillet," a fine gravel of crushed shale, to reduce shrinkage and cracking.
In addition to construction and repair, there is a fair amount of research going into English cob. Alfred Howard, for example, has built experimental walls using a variety of subsoils. Larry Keefe, a former building conservation officer, has catalogued hundreds of old cob buildings and become an expert on why cob walls succeed and why they don't. Larry is the co-founder of a unique program at Plymouth University called "Out of Earth," dedicated to furthering earth architecture, which has sponsored earth building workshops and several international conferences on earth building.
The Development of "Oregon Cob"
Concurrent with the renewed interest in cob in England, there has been a parallel revival in the United States, led by the Cob Cottage Company in Western Oregon. With less access to (and less dependence on) traditional knowledge, the building system that has arisen here is sufficiently distinct from British cob that it merits a separate name, "Oregon Cob."
By 1989, Cob Cottage Company founders Ianto Evans and Linda Smiley recognized the need for inexpensive, healthy, bioregional housing. Ianto grew up surrounded by cob in Wales, and later witnessed earthen construction in Africa and Latin America. They were particularly interested in earthen building because of their experience developing and promoting Lorena stoves (fuel-efficient cookstoves molded from packed sand and clay). Experimenting with earthen building in rainy Western Oregon, Ianto and Linda chose British cob as a model because of its demonstrated durability in a cold, extremely wet climate.
When they started their first cob structure, Ianto and Linda were unable to locate anybody with first-hand experience. They relied entirely on their explorations of existing cob structures in Britain and a very sparse literature on the subject, much of it inaccurate and contradictory. The system they developed involved making loaves of stiff mud, called "cobs." This loaf system had at times been used in Britain ("cob" itself is an Old English word for loaf) as well as in Germany, France, North Yemen, and the American Southwest. Its advantages are that the mix can be made at some distance from the wall and easily transported by tossing the cobs from person to person like a bucket brigade. As construction progresses, cobs can be thrown to a builder much higher on the wall than a pitchfork can be raised.
Another way in which Oregon cob differs from traditional cob is in the attention given to the quality of ingredients and to the proportions of the mix. While cob builders in previous centuries had to use whatever soil was on hand with little or no amendment, we can now cheaply import as much sand or clay as is necessary to make the hardest, most stable mixture. Furthermore, whereas grain straw was formerly a valuable resource for animal bedding, thatching and the like, it is now an underutilized waste product available in huge quantities for little cost. Oregon cob is characterized by both a high proportion of coarse sand and lots of long, strong straw.
Better ingredients, more precise proportions, and thorough mixing allow the construction of stronger, narrower, and more sculptural walls. Using Oregon cob, exterior walls are typically between 12 inches and 20 inches thick; non load-bearing partitions taper to as little as 4 inches (but more commonly 8 inches). Most Oregon cob buildings have curved walls, niches and nooks, arched windows and doorways. By adding extra straw in the needed direction, The Cob Cottage Company developed a system for corbelling arches, vaults, and projecting shelves, beyond the capability of traditional cob.
After inhabiting their first cob cottage for four years and finding it well suited to Pacific Northwest conditions, Ianto and Linda were ready to share their experience with others. I joined them in 1993, when the Cob Cottage Company was formed and the first workshops taught. Since then, we have taught over 60 workshops, mostly week-long, throughout the Western States and Canada. We have also worked in Australia, New Zealand, Mexico, and Denmark. We have trained at least 700 people in cob construction, some of whom have gone on to build homes for themselves or teach workshops of their own. Increasingly, we have made alliances with other natural builders, and a system of hybrid natural building is emerging which utilizes cob for its best qualities in combination with stone (for foundations and thermal mass), lime putty (for plasters, mortar, and whitewash), straw (in bales or straw/clay insulation, as well as in plasters), wood (including unmilled roundwood, for roofs and other structural elements) and many other natural materials.
Cob is well suited to a sensitive house design process, based on careful observation of the site, and placement with respect to slope, microclimate, and ecology. It offers exceptionally flexible opportunities for passive solar heating/cooling strategies. Rather than the industrial geometry of straight lines and right angles, cob buildings can use organic continuous curves, variable wall widths and structural buttresses. Ianto Evans has pioneered space-saving designs which reduce building size and cost, including built-in perimeter furniture, personalized spaces to enclose particular activities, and the sculpting of volumes rather than areas. We have observed that curvilinear spaces are perceived as much larger than rectangular ones with the same measured area. The experience of living in these buildings stretches our ideas of what's possible with ecological building.
The cob revival is still in its infancy. Every year we learn more: how to improve our efficiency at mixing and building, how to use a wider range of soil types, and new applications, techniques, and designs. It is impossible to predict what direction the cob building revival will take in the years to come, but a few general trends and issues are likely to remain prominent:
First, as more and more cob homes are built in diverse climatic, geographic, and cultural conditions, the technique is likely to become increasingly regionally diverse. For example, strategies will be developed where it is necessary to deal with earthquakes, extremely cold winters, and the unavailability of good quality sand and straw. Just as traditional Welsh cob differed in many specifics from that in Devon and from "torchis" in France, one would expect that eventually cob homes in Texas will both look different and be constructed differently from those in Oregon or Massachusetts.
Second, there will be increasing interest and pressure to bring cob into the building "mainstream," both in the United States and elsewhere. This will not be as easy as with strawbale or rammed earth, since cob represents a greater departure from conventional industrial building practices. There may be attempts to further mechanize the mixing process, to use forms, or to add unnatural stabilizers like cement or asphalt to the mix. Eventually, some kind of cob building code is likely to be adopted, but whether cob will continue to be as owner/builder friendly remains to be seen.
Finally, the increasing collaboration between natural builders during the last few years points to the development of an integrated natural building system of which cob is only a part. Expect to see more hybrid buildings incorporating earth, straw, wood, stone, and other natural materials. Hopefully this will place less emphasis on individual materials and techniques and more on finding the best, most sustainable solution to regional building situations using the natural materials that are close at hand.
The lessons from cob building have been surprising in their abundance and profundity, yet clearly we are only beginning to learn, and should record more carefully what we do learn. Demonstration structures, particularly houses, should be monitored and analyzed. Much more could be published, particularly about the psychological and spiritual effects of natural buildings. We should acknowledge that cob building is more than a cheap, environmentally benign way to build. It can be a significant tool for the development of a more sane and sustainable culture.
Michael Smith was a founding director of the Cob Cottage Company. He is the author of The Cobber's Companion and teaches natural building and permaculture throughout the western United States, Canada and Mexico. His also a founder and three-time organizer of the Natural Building Colloquia. See page 16 for contact info. firstname.lastname@example.org
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