The most recent revival in natural building has focused on cob, a combination of earth with straw.  This natural construction method is possibly the world's oldest and most ubiquitous building practice. The renaissance in cob building has occurred mostly in Oregon.  These structures, with their emphasis on curved walls, have gained their own name, "The Oregon Cob Style". Ianto Evans is one of the main figures of this movement, and since its inception his Cob Cottage Company has taught thousands of people how to build with mud. 

The evidence suggests that with cob building we may be tweaking the tip of a big and very solid iceberg, the while impact of which has yet to be seen. Cob construction seems to satisfy its builders in very profound ways. Our files are stuffed with letters of encomium, extravagant appreciation of how good it feels to build with a house of mud pies, to involve yourself in building in such a primal way. You don't get ecstatic about building with concrete blocks or drywall, but with cob there seems to be universal enthusiasm. As a specific remedy for what ails our buildings, cob is unlikely to cure the epidemic, but it seems to be having a catalyzing effect as an inspiration and a tool for considering all the crucial issues.

"What is cob? Cob is a structural composite of earth, water, straw, clay, and sand, hand sculpted into buildings while still pliable. There are no forms as in rammed earth, no bricks as in adobe, no additives or chemicals, and no need for machinery"

                                                                                        .- Ianto Evans

Cob is made by adding a wet clay-slip to a bed of sand on a tarp. These ingredients are then mixed by pulling the corners of the tarp and stomping with the feet to create a homogeneous mixture; straw is then scattered and mixed with the feet. The ingredients are balanced so that the mix isn't too wet or dry (too much clay or sand), with as much straw as it can hold without loosing its mud-like feel. This mixture is then formed into small pies (the word "cob" is Old English for "loaf") and tossed to a cobber who sews the straw in the individual cobs together using his hand or a stick. Cob is heavy and working with it is slow compared to conventional construction. Each foot thick layer of cob has to dry sufficiently before the next layer is added or the wall will deform, thus cob favors a different pace of building. The steady pace of building and the singular enjoyment of mixing cob are suited to owner-builder projects that allow for a building to arise slowly from the combined efforts of friends and family. There is nothing complicated about cob, yet it is a material of such synergy between common materials that has been in active use by people across the globe since the dawn of shelter. It is a durable building material that is readily available nearly everywhere on the globe.

The structure of cob is created by the interaction of its composites. The clay binds the small particles of sand together, much likes stones in a mortar, the straw is then used to 'sew' the individual cobs together, adding tensile strength, and creating a monolithic straw/clay wall. The best sand for cobbing is coarse, since these edges allow for the particles of sand to interlock and support each other. Almost any clay, as long as it is semi-sticky, can be used for cob, but as a rule, the stickier the better. At a certain point, very sticky clay has a higher tendency to crack, but it is important that the clay be sticky. Building with a monolithic material lends itself to unique geometries, allowing builders to easily create curves, nooks and arches, forms that are very difficult using standard methods. Clay is a miraculous substance, pliant when wet, rigid when dry, able to transport water as well as repel it. Clay's versatility is created by its molecular structure, consisting of plates that expand when wet and bind together when they dry. These plates align in structure when dry, when exposed to moisture these plates expand, locking together, and blocking the further passage of water. Clay is a natural material that truly surpasses our modern inventiveness, far surpassing our synthetic membranes that often allow too little water to escape creating mold problems. Clay, due to its ability to expand and contract, naturally tends to crack as it dries. To prevent cracking, sand is the natural ally of clay; as clay dries the sand particles spread the shrinking effect out such that it no longer destroys the internal structure. The clay particles fill the gaps between the sand, drawing it together as it dries forming what has been called a 'stabilized micro-adobe masonry wall' . Cob is created by adding straw to this mix, which, when cemented in place by the clay, provides internal structure and tensile strength to the wall.

The "mass-effect" is the tendency of a dense, massive wall to delay the variation of external temperatures, storing the heat of the day for use during the cool nights and vice versa. Ianto Evans performed an experiment on a cob addition to a wooden cabin, measuring the internal temperatures of each structure as they fluctuated with the external temperature.  The massive walls of the cob cottage smooth out the swings in external temperature, heating up slower, but maintaining that heat more than twice as well as the wooden cabin. This makes cob, or other forms of massive construction, most useful in structures that are continuously inhabited, such as homes and hospitals, since the initial energy used to heat the mass is retained constantly. In schools or supermarkets, structures that are not continuously used, the heat absorbed by the mass is merely lost during the hours when the heat is turned off.

The monolithic nature of cob lends itself to geometries other than those of component materials. Cob is a composite system; the wood frame of a suburban house is a component system. In a cob wall the structure is created by the unity of all the parts, mixed together.  However, in a conventional wall the structure is created by the combination of parts, each an entity unto itself. The strength of each component determines the strength of the whole, regardless of the shape of the whole in the second case, where, with cob, the shape of a wall is integral to its structural capacity. The shape of a cob wall is designed to internally buttress itself, as such curves and tapered walls are the most useful forms. The use of component materials leaves corners that are very difficult to utilize successfully, but with a monolithic materials such as cob, such abrupt changes in the geometry of the wall can result in structural weaknesses. The foot-print of a home is no longer made up of individual walls but one sweeping gesture that encloses the space. Where industrial geometries are rectilinear, cob geometries are curved, tending to round the corners as added buttresses and supports for the wall. This allows natural building to defy the expectation for houses, and mold every space to fit its exact purpose. The builder is able to hand sculpt their home, decreasing the need for additional unused square footage, and giving the builder the opportunity to personalize every aspect of their abode.

Due to the complexity of this structural interaction, like load bearing straw bale construction, cob is often difficult to pass through building departments that require calculable systems of compressive strength. Further complicating matters is that cob is always different depending on the characteristics of the soil used, this makes for a nearly impossible standardization process. The strength of cob has been thoroughly proven in its long history, even in the constantly wet and cold climates of England, cob houses have stood for almost a half a millennia. This is one of the earth's oldest building forms, and one of the most common forms of construction to date. The myth about cob is that it is mostly used in Africa, in the mud hut, primal dwelling, but this is not the case, as Ianto describes:

What is the earth's most common building material? Why, the earth itself of course! Even today between a third and a half of us humans live in houses of unbaked earth… lavish adobe haciendas in Latin America, rammed earth mansions in France, earth brick palaces in china. They're ten-story apartments in Yemen, old fortified monasteries in the middle east, puddle and hand shaped and press bricked and foot-stomped earthen buildings from near the arctic circle in Norway to the tip of Chile and the polar end of New Zealand, millions of them

This is the most convincing argument for the soundness of cob that any builder could present, that this method is proven by centuries of use across the globe. The date plate reads 1539 on one particular cob home in Devon, England. There is no credible debate about the strength and durability of cob, it is a proven building method, yet code departments are slow to accept such a precedent. Although much work still remains to study the structural properties of cob walls, cob's usefulness is not lost on modern construction.

 Works consulted and cited

Bee, Becky. The Cob Builders Handbook: You Can Hand-sculpt Your Home.  Groundworks, 2008

Evans, I; Smith, M.G.; Smiley, L. .  The Hand Sculpted House. Chelsea Green Publishing Company. 2002

Guelberth,  Cedar Rose .  The Natural Plaster Book: Earth, Lime and Gypsum Plasters for Natural Homes New Society     Publishers 2003

Cob (material) http://en.wikipedia.org/wiki/Cob_(material) May 20 2009

Last update: June 09, 2009
02:28 pm

Published in : Articles, Green Tech

Keywords : cob, natural building, low cost, earth, mud, house, home,

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