Natural Building Colloquium


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The Art:
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History of Cob
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Questions and Answers about Cob

What is cob?

The word cob comes from an Old English root meaning "a lump or rounded mass." It's a traditional building technique using earth mixed with water, straw and often sand. Cob is easy to learn and inexpensive to build. It dries to a hardness similar to lean concrete and is used like adobe to create self-supporting, load-bearing walls. Cob has been used for centuries throughout Western Europe, even in rainy and windy climates, as far north as the latitude of Alaska. This ancient technology doesn't contribute to deforestation, pollution or mining, nor depend on manufactured materials or power tools. Cob is non-toxic and completely recyclable, which is important in this era of environmental degradation, dwindling natural resources and chemical contaminants. Since cob adds thermal mass, it is appropriate for use in passive-solar buildings.

What makes "Oregon Cob" unique?

What began in Western Oregon in 1989 as an attempt to recreate English cob has undergone huge transformations, to the point that Oregon Cob now only partially resembles the traditional medium. Influences from Africa and Latin America, and the conscious attempt to make cob relevant to the third millennium, have created something quite different from other building techniques. Here are some distinctions:
·  Oregon Cob is not simply a building material. We have developed an integrated system of siting, design, wall geometry, materials and building techniques.
·  We have developed a range of application techniques to suit different parts of each building: pise, "Gaab cob," cob loaves, etc.
·  Oregon Cob is very strong in compression and shear because carefully chosen ingredients are used, mixing a high proportion of coarse sand to clay, with lots of straw. The clay is pressure bonded onto every facet of the sand grains. The long-fiber straw, sewn from layer to layer, creates monolithic structures with high shear strength and walls without cracks.
·  The sculptural qualities of the technique encourage built-in furniture. This, together with rounded corners, eliminates useless space and keeps buildings compact. Oregon Cob buildings actually seem bigger than their "square" footage would suggest.

Why haven't I heard of cob before?

Cob is a recent arrival in the United States. In other parts of the world, cob and similar techniques have been popular for millennia. Throughout Western Europe many of the picturesque stuccoed or whitewashed buildings are made of cob. In England today there are thousands of comfortable cob homes, some of which have been continuously inhabited for over 500 years. The durability and comfort of these valuable houses has sparked a renaissance in traditional cob centers such as Devon, where beautiful cob homes are again being built.

How is cob different from adobe?

Cob is only one of many methods for building with raw earth, the world's commonest construction material. It surpasses related techniques such as adobe, rammed earth, pise, and compressed earth bricks both in ease of construction and freedom of design. The most basic difference is that adobe is formed into rectangular bricks which are dried in the sun before being used, while cob is built wet. Cob therefore lends itself to organic shapes: curved walls, arches and vaults. Building with cob is a sensory and aesthetic experience like sculpting with clay. You can add on, cut out, or reshape at any time, even after the cob is dry. Cob is well suited to cool damp climates like its native Britain; its resistance to rain and cold makes cob well suited in all but the coldest parts of America.

Why doesn't it wash away in the rain?

Cob is very resistant to weathering and can withstand long periods of rain without weakening. However, too much exposure is best avoided by building roof overhangs and gutters to protect the walls and constructing a high impervious foundation. In windy areas a lime-sand plaster is traditionally used to protect exterior cob walls from wind driven rain.

What about earthquakes?

No building system is earthquake-proof under every seismic condition, but a cob mansion in Nelson, New Zealand has survived, without a crack, two major earthquakes which destroyed the town around it. In South Yemen, in a fault zone, there are Medieval earthen houses 13 stories high. Since a cob building is one monolithic unit reinforced by straw, it has no weak straight-line mortar joints, making it stronger than brick or block. The carefully regulated geometry and scale of Oregon Cob buildings gives them high strength. This is achieved by building mostly curved walls, by tapering and buttressing them, by adjusting wall thickness and material mix, and by keeping roof spans short.

Isn't it cold and damp inside?

Winter visitors to cob buildings in Oregon often comment on how warm and dry they feel. Cob walls one to two feet thick provide immense thermal mass and adequate insulation in that climate, ideal for passive solar construction. Cob structures in chilly (not cold) climates require little additional heating in winter and remain cool and comfortable on hot summer days. Because cob is fireproof, it can be used for building ovens, stoves and chimneys. One of our favorite designs is a cob bench or bed heated by the flue of a wood-burning stove.

Can I build it myself?

Cob construction is a sculptural craft that is easily learned and is built without forms or machinery. Building cob is easier and more enjoyable with a crew, so it lends itself to community projects, building parties and workshops. Many cob structures have been completed by small inexperienced crews during a single season.

How long will it take to build my house?

The rate of building depends on weather and the size of your workforce; in a dry weather workshop setting we can build up to a foot of height per day. In wood construction, the frame is a tiny part of the work, but a cob wall once built is finished apart from the plaster. Pipes and wires are laid directly in place and there's no need for sheetrock, tape, spackling, sanding, painting, sheathing, or vapor barriers. But racing to build fast is missing the point and half the fun. Unlike conventional modern building with its frenetic pace, power tools, and scope for errors and accidents, cob-making is a peaceful, meditative and rhythmic exercise.

How much will it cost?

Cob is one of the cheapest building materials imaginable. Often the soil removed from foundation and drainage trenches and leveling the site is enough to supply all construction needs. The owner-builder can take great satisfaction in supplying the labor, building little by little in leisure time, or inviting friends to join in the excitement of hand sculpting a whole house. With inventiveness and forethought, the costs of other components (doors, windows, roof, floors, etc.) can be extensively reduced. The Cob Cottage Company works primarily with found and recycled materials and hand-worked lumber. Using local materials such as poles, bamboo, native stone, and cedar shakes, our second cob cottage was completed for about $500, and has been Ianto's and Linda's home for four years.

What about building codes?

North American codes today protect the industrial manufacturers of building components better than they do homeowners. Not surprisingly, there is no code for cob (though nowhere is earthen building prohibited) so many cob builders choose not to involve building officials. Legally permitted cob buildings are beginning to appear, though any permit involves expenses, paperwork and delays.

Ianto Evans and Michael Smith, with Linda Smiley, are cofounders of The Cob Cottage Company. Ianto Evans is a landscape architect, applied ecologist inventor, writer and teacher with building experience on six continents. Cob is traditional in Wales, his homeland. Michael Smith is the author of The Cobber's Companion: How to Build Your Own Earthen Home, and has taught over 20 cob workshops in Canada and the US.

The Cob Cottage Company
PO Box 123, Cottage Grove OR 97424
ph 541-942-2005

This article has been excerpted with permission from The Cobber's Companion by Michael Smith (available from The Cob Cottage Company at the address above).


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