Practice

Cob building

Also known as: Cob, Cob construction

A natural-building method using a hand-mixed material of subsoil, sand, straw, and water — sculpted into walls by hand without forms. Originated centuries ago in Britain (Devon, Cornwall) and across the Middle East and Africa; many British cob buildings are 300-500 years old and still inhabited. Revived in the 1990s by Ianto Evans and others through the Cob Cottage Company in Oregon. Cob is uniquely sculptural — walls can curve, niches and built-in benches integrated, hand-built character impossible in conventional construction. Thick cob walls (16-24 inches typical) provide thermal mass and modest insulation.

Method

Cob is mixed by stomping subsoil, sand, straw, and water on a tarp until it reaches a workable consistency (similar to thick clay). The mix is then applied by hand in layers (lifts of 4-6 inches), with each lift allowed to firm up before the next is added. Walls grow at ~1 foot per day per crew.

Material proportions (typical):

Subsoil with 15-25% clay content
Sand (sharp, angular)
Straw (long, golden, dry)
Water to workable consistency

The straw provides tensile strength and reduces cracking. The clay binds; the sand reduces shrinkage; the water makes it workable.

Properties

Thermal mass — high; walls absorb day heat and release at night, smoothing temperature swings
Insulation — modest (cob alone has R-2/inch); often combined with insulating elements
Sculptural — walls can curve, integrate seating, niches, sculpted relief
Carbon — very low; subsoil and sand are inert; straw is agricultural carbon; lime or [[natural-plaster|earth plaster]] finishes
Cost — very low material cost; very high labor input
Permit-able — increasingly accepted; specific code provisions in some Oregon jurisdictions

Tradition and revival

British cob: documented since the 1500s; thousands of cob buildings 300+ years old in Devon and Cornwall. Cob revival in the 1990s through Ianto Evans, Linda Smiley, and the Cob Cottage Company; their book The Hand-Sculpted House (2002) is the canonical modern reference.

Sources

The Hand-Sculpted House, Ianto Evans, Michael G. Smith, and Linda Smiley (Chelsea Green, 2002)
Cob Cottage Company published materials
Building with Cob, Adam Weismann and Katy Bryce (Green Books, 2006)

Rooted in life.

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Practical

shares approach with

Natural plaster cob walls are typically finished with natural plasters — earth or lime; the wall material and the finish both manage moisture together
Rammed earth cob and rammed earth are both earth-building methods; cob is hand-sculpted, rammed earth is form-built and compressed
Straw bale building both are natural-building methods using local agricultural and earth materials; often combined

3 inbound links · 3 outbound