The Cob House — Sturdy Mud Architecture

Simple Earth Architecture, Traditional Results

man in foreground in large work gloves forming tan-colored balls, looking at light brown house with many windows, tatched roof, and stone foundation
Cob House Under Construction in Greyton, Western Cape Province, South Africa. Mike D. Kock/Getty Images (cropped)

Cob houses are made of clay-like lumps of soil, sand, and straw. Unlike straw bale and adobe construction, cob building does not use dried bricks or blocks. Instead, wall surfaces are built up with lumps of damp cob mixture, compressed, and sculpted into smooth, sinuous forms. Unlike rammed earth or even poured concrete construction, cob walls are generally not built with wooden frames — instead, special tools are used to scrape a thick wall into the desired shape. A cob home may have sloping walls, arches and lots of wall niches. In Old English, cob was a root word that meant lump or rounded mass.

A cob home is one of the most durable types of earth architecture. Because the mud mixture is porous, cob can withstand long periods of rain without weakening. A plaster made of lime and sand may be used to windproof the exterior walls from wind damage.

Cob architecture is suitable for the desert and some people claim cob is even good for very cold climates — walls tend to be very thick, even two feet especially at the base, above the foundation. Small cob structures, like tiny homes and garden sheds, are very inexpensive Do-It-Yourself (DIY) projects. It's also the architecture of choice for survivalists and preppers.

How Do You Make Cob?

Anyone with even a little experience in the kitchen knows that many of the finest foods are put together with simple recipes. Homemade pasta is simply flour and water, with an egg added if you want egg noodles. Shortbread, that rich, crumbly cookie confection, is a simple combination of flour, butter, and sugar. Ingredient amounts vary with each recipe — the "how much" is like a secret sauce. The mixing process is the same — make a well (an indentation) in the dry ingredients, add the wet stuff, and work it together until it feels right. Making cob is the same process. Mix water into clay and sand, and then add straw until it feels right.

And that's where the expertise comes in. When does it feel right?

The easy way to make cob is with a portable cement mixer, which does all the labor-intensive mixing of the clay, sand, water, and straw. But a sturdy mixer can cost hundreds of dollars, so "natural builders" like Alexander Sumerall at This Cob House use what is called the tarp method. The process of mixing is like making pasta, but on a larger scale. The ingredients (clay and sand) are placed on the tarp, which is used to help mix the ingredients. Folding the tarp moves the cob ingredients, and the movement mixes it. Add water, and the fun begins. Sumerall's logo, a footprint with the outline of a house in the arch, makes a lot of sense when you watch his video on How to Make Cob — use your bare feet to mix in the water and eventually the straw. Put most of your energy into the heel of your foot to flatten the mixture like a pancake. Then use the tarp to roll the mixture into a form. Repeat the process until it feels right.

Clay is an abundant natural resource in many parts of the world. It is inexpensive and has been used to build "mud huts" since architecture began. Clay will have different moisture contents, which is why different amounts of sand are used to create cob. The straw acts as a fibrous binder. To build a cob wall, balls of the mixture are thrown together and sculpted atop a pre-made base — a foundation that is usually made of stone and rises above grade by a foot.

How strong is a cob house? When you examine the geology of bricks, you discover that clay is the main ingredient of the common building brick. Just like cob.

The Cob and Thatch Homes of England

The Dorset birthplace of British author Thomas Hardy is a fine example of the English cob and thatch type home. The thatch, of course, is the bundled reeds and rushes that are sculpted to conform to and protect the roof. On the Hardy cottage, the thatch is cut above the second story windows, just as the cob walls themselves would have been cut and shaped. Cob and thatch homes are most commonly seen in the West Country of rural South West England.

Owned and run by the British National Trust, what is now called Hardy's Cottage was built in 1800 by Hardy's great-grandfather. Thomas Hardy was born there in 1840. The future literary icon was trained as an architect and didn't turn to writing full-time until he became an established novelist in his 30s; his poetry wasn't published until he was nearly 60. The writings of Thomas Hardy are heavily influenced by place, and a childhood raised in a cob and thatch home is not soon forgotten. Touring in this part of England will take any visitor back in time.

Cob is Trending

Building a small cob structure is a cost-effective adventure — especially if you live in an area with the proper natural resources. Plenty of books have been written (and continue to be written) to get you on your way: Building with Cob: A Step-by-Step Guide by Adam Weismann and Katy Bryce; The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage by Ianto Evans, Linda Smiley, and Michael G. Smith; and The Cob Builders Handbook: You Can Hand-Sculpt Your Own Home by Becky Bee are just a few of the many DIY guides.

Workshops in the United States and abroad will give a participant hands-on training before you take the personal plunge. Aprovecho in Oregon is a non-profit organization offering "hands-on, experiential education programs for youth and adults." Their aim is "to inspire a sustainable culture.

So, cob is not as corny as it sounds.

FAST FACTS — Definitions of 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, The Hand-Sculpted House, 2002, p. xv
  • cob "A mixture of straw, gravel, and unburnt clay; used esp. for walls." — Dictionary of Architecture and Construction, Cyril M. Harris, ed., McGraw- Hill, 1975, p. 111
  • cob wall "A wall formed of unburnt clay mixed with chopped straw, gravel, and occasionally with layers of long straw, in which the straw acts as a bond. "— Dictionary of Architecture and Construction, Cyril M. Harris, ed., McGraw- Hill, 1975, p. 111