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
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.