First permitted cob structure in Berkeley could pave way for more green building

Jessica Tong adds roofing to a shed she built in her parents’ backyard, the first permitted cob structure in Berkeley. Photo: Natalie Orenstein

Berkeley native Jessica Tong was not used to the scorching heat she encountered in Hopland, California.

While interning at the Solar Living Institute there, the environmental builder could only find refuge in the structure where she slept, which was built out of earthen materials. The place kept her cool in the heat, and insulated her from the cold at night.

The experience inspired Tong, who studied environmental design, to spend more time working with natural materials, like cob — a mixture of clay, sand and straw that is a lot like adobe, but is not formed into bricks.

Now, the 27-year-old is nearly done constructing the first cob structure to receive a building permit in Berkeley. The shed, in her parents’ backyard on Fresno Avenue, stands about 12 feet tall, with a slanted shingle roof and lively blue trim. On a recent morning, bits of straw were still sticking out of the walls, waiting to be smoothed over with plaster.


Cob is not only appealing to builders like Tong for its temperature-controlling capacities, but because it’s much cheaper than conventional materials, and more sustainable. Building with cob uses six times less energy than regular wood-frame construction, and a quarter of the lumber, according to the Cob Research Institute in Berkeley.

“There’s evidence that cob is structurally superior to adobe, which is really an issue in California because of seismic conditions,” said John Fordice, architect and director of the Cob Research Institute. According to his research, cob construction is thought to have originated in North Africa in the 11th century. Fordice was first attracted to cob because of the creativity it affords builders, who can more easily create curved structures.

“You don’t need a lot of tools to work with it,” said Tong. “It lends itself to people not building in a conventional, industrial, first-world context. You don’t need a mortar mixer.” Plus, cob is “a more relaxing material to interact with as a builder” than cement or another substitute, because it’s non-toxic, she said.

Using soil from El Sobrante, which had to be strength-tested, Tong mixed her cob by hand in a wheelbarrow. She built the shed with consultation from longtime earthen-material builder Massey Burke, and enlisted a crew of family members and friends to help her with the construction.

Building with cob is labor-intensive since no machinery is used — the “opposite” of conventional construction, in which materials are pricy but the work goes quicker, Tong said. And only about a foot of wet cob can be laid each day — “too much taller and it can start to slump over on itself.”

The main challenge for builders like Tong and Burke, however, is not the labor but the legality. Cob is not recognized in the international building code, the basis for U.S. building codes. Some along on the West Coast and elsewhere have built cob structures under 120 square feet, in many places small enough to avoid the permitting process. Others, often in rural areas, have built cob structures illegally. Some undeveloped parts of California, including areas of Marin and Mendocino counties, have relaxed building standards that do allow some cob. England has also legalized cob construction based on the historic popularity of the material there.

“One of the goals from the beginning was to permit cob — that seemed exciting,” Tong said. At least in Berkeley, “no one’s done it before, as far as we know, and it didn’t occur to me there might be a reason.”

While many builders are lured into cob construction by the promise of cheap materials, they don’t realize that labor is costly and, without a cob code, the permitting process takes time and money, Fordice said.

Cob is a mixture of clay, sand and straw, known to be inexpensive but labor-intensive. Photo: Natalie Orenstein

Tong and Massey, along with Anthony Dente, principal with Berkeley-based Verdant Structure Engineers, went through the Alternative Materials and Methods Requests (AMMR) process. The process is designed for experimental builders who want to prove their materials or practices meet the intent of the building code.

“The state building codes as adopted by the city of Berkeley does not provide specific provisions for a cob structure but does not ignore the fact that there are innovative ideas being considered in the construction industry,” explained the city in an email to Berkeleyside. “The code does not intend to inhibit innovative ideas or technological advances.”

Cob structures have passed a “shake test” in Australia subjecting them to the force of a magnitude 7.7 quake

Using the AMMR, the Fresno Avenue builders went through a “very unique” engineering and permitting process, Dente said. They referenced, among other international standards and an earthquake test they conducted with the University of San Francisco, New Zealand’s building code. The country has standards for earthen materials and is similarly prone to earthquakes. According to the Cob Research Institute, cob structures have passed a “shake test” in Australia subjecting them to the force of a magnitude 7.7 earthquake, like the 2001 quake in El Salvador that killed hundreds.

Applicants using the AMMR must work with a structural engineer, who in turn usually needs architectural drawings, Fordice said.

“The costs of that are prohibitive right from the beginning. It stops people when they realize they’re going to have to lay out $10,000 to $15,000 for drawings to get permits,” he said.

Tong and Burke were fortunate to receive a donation helping pay for the permitting process, which took almost three years. The structure was ultimately permitted as “load-bearing,” meaning it is deemed safe enough to support itself and the roof.

“As far as I can tell, this permit is the first time that cob has undergone this sort of scrutiny in the U.S.,” said Tong.

“The permit process was far from easy, with much pushback yet, at the same time, interest in deep green structural wall systems by the city of Berkeley,” Dente wrote in an email to Berkeleyside. He said he knows of one other permitted cob structure in California, but the standards there were not applicable to the Berkeley building.

City staff confirmed that shed is the first permitted cob structure they are aware of in Berkeley. The building department did require the project to include metal reinforcements, in compliance with seismic standards, and the structure has a conventional concrete foundation.

Others looking to build a cob structure in Berkeley, and certainly anyone who wants to build a habitable structure — the 162-square-foot Fresno Avenue studio is not permitted for occupancy — will need to go through the same AMMR process. But it helps to have a completed building on the books.

Fordice is working on a permitted commercial cob structure in Shasta County with Dente, who used the same standards he developed for the Berkeley application, Fordice said.

With these projects, and the Cob Research Institute, the goal is “to create a base of education and repetition so it can be used to convince code authorities, yes, this stuff can be permitted,” Fordice said. “There’s a lot of interest” in building with, and legitimizing cob, but not a lot of financial support, said Fordice, whose institute is volunteer-run.

Another popular earthen building method, straw-bale construction — used to build the Shorebird Park Nature Center in Berkeley, for example — is now, after years of advocacy, recognized in the building code.

In Berkeley, recent laws supporting accessory dwelling units — also known as backyard cottages — could prompt more residents to explore these cheap, sustainable construction methods.

But nobody will be dwelling in Tong’s cob structure, which has electricity but no plumbing.

“I suspect it might end up filled with bikes,” she said.