Cob Construction

Cob construction uses sand, clay and straw. Mixed well this special mud is applied to the foundation in continuing layers. Each layer must dry so that it can support the next, and the wall is tapered in as you build up. When it is dry, the walls are very hard and load bearing. The roof is built directly on to the walls, as the walls themselves are the support structure.

This technique requires almost no money or skill , and involves very little environmental destruction while at the same time allowing for creative and organic designs which blend into the surroundings.

Cob allows itself to be shaped and moulded while you build, allowing bas-relief, shelves, alcoves and even furniture to be built right into the walls. Cob being earth, is totally fireproof, so even a fireplace can be built into the design.

Regular working windows and doors are embedded in the cob along with their lentils while you build the layers up. If you want a fixed window you can use any kind of glass embedded directly into the cob. This allows for using broken windows ( you cob over the broken part) or creative things such as glass bottles or a car windshield.

These buildings are incredibly durable and don’t worry as they do not wash away in heavy rain. Usually an aesthetic earth or lime plaster is applied to the finished cob. There is no joint or seams. Cob houses have been known to last for centuries.

Because cob walls are between 1 and 2 feet thick they are nice and warm. The cob has a high thermal mass which allows it to absorb the sun’s energy during the day to keep the interior cool and radiate the energy back at night to keep the interior warm. In this way cob acts as a temperature regulator. Because of this a cob house needs very little additional heating in winter. As cob is fireproof one of the older designs inside the house is a cob seat or bed heated up by the flue of a wood burning stove.

If you are serious about building in cob there are a few places on the web that will supply you with cob bricks but if you go on one of the courses that are available you could just as easily make your own.

Our thanks to Jack Stephens for the following information

The Natural Building Network
http://www.naturalbuildingnetwork.org

Questions and Answers About Cob

At Cob Cottage Company we have found that many visitors and workshop participants have similar questions about cob. Here are some of the most common questions.

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 hand formed lumps of earth mixed with sand and straw. 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 nontoxic and completely recyclable, which is important in this era of environmental degradation, dwindling natural resources and chemical contaminants.

How is cob different from adobe?

Cob is 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. Since you don't need straight forms or rectilinear molds, cob 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. Unlike adobe, cob can be built in cool damp climates like Ireland’s: its resistance to rain and cold makes cob well suited in all but the coldest parts of the world.

Why doesn't it wash away in the rain?

Cob is very resistant to weathering. Because of its porous nature, it withstands long periods of rain without weakening. However, too much exposure is best avoided by the "boots and cap" strategy: wide roof eaves to protect the walls and an impervious foundation. In windy areas a lime-sand plaster is traditionally used to protect exterior cob walls.

What about earthquakes?

No building system is earthquake proof under every seismic condition, but part of the Great Pyramids and the Great Wall of China are made of earth. The oldest inhabited structure in the U.S., Taos Pueblo, is earthen construction. 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 curve and taper given "Oregon cob" walls make them even stronger.

Isn't it cold and damp inside?

Winter visitors to cob buildings often comment on how warm and dry they feel. Cob walls one to two feet thick provide immense thermal mass and adequate insulation, ideal for passive solar construction. Cob structures require little additional heating in winter and remain cool and comfortable on hot summer days. As it is fireproof, cob can be used for building ovens, stoves and chimneys. A favourite design is a cob bench or bed heated by the flue of a wood burning stove.

How fast can I build with cob?

The rate of building depends on weather and the size of your workforce; in dry weather we 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 sheet rock, 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. Building cob is easier and more enjoyable with a crew, so it lends itself to community projects, building parties and workshops.

What about building codes?

Codes today protect the industrial manufacturers of building components better than homeowners. Not surprisingly, there is no code for cob, though nowhere is earthen building prohibited. Many cob builders choose not to involve building officials and have had no problems. Legally permitted cob buildings are beginning to appear; there's considerable expense and paperwork involved, as with any permit.

What does a cob cottage cost?

Cob is one of the cheapest building materials imaginable. Often the soil removed during site work is enough to build the walls. The owner-builder can supply the labor, inviting friends to join in the excitement of hand sculpting a 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 recycled materials and handwork lumber. Using local materials such as poles, bamboo, stone, and cedar shakes, our second cottage was completed for $500US, and has been lanto Evans’ and Linda Smiley's home for 3 years.

Does cob building require an expert?

Absolutely not! Once the basics are understood, cob building is amazingly simple. In a week long workshop you can learn how to select materials, prepare a mix, and build a wall. You can learn everything you need to build your cob cottage: site selection, foundations, windows and doors, attachment of wood and other materials, detail work and finishing. Many course graduates with no previous building experience leave a weeklong workshop feeling confident and enthusiastic about building their own cob cottage.

How can I learn more about cob?

The only proven way to learn about cob building is to try it. The Cob Cottage Company offers hands-on cob construction workshops in Ireland each year. Contact us for our current schedule. We also supply videos and slides of finished cob buildings . Our web site is www.cobcottage.com. We distribute a practical guide book, The Hand Sculpted House and an informative illustrated reader, Earth Building and the Cob Revival. Our newsletter, the CobWeb. is available by subscription.

The Cob Cottage Company is a member of the Natural Building Network. Find more information about them and other local Irish resources at www.naturalbuildingnetwork.org.