Cob, cobb, or clom (in Wales) is a natural building material made from subsoil, water, fibrous organic material like straw, and sometimes lime. Subsoil can vary, and if it lacks the right mix, sand or clay may be added to adjust it. Cob is fireproof, protects against termites, resists shaking from earthquakes, and uses low-cost materials. However, it requires a lot of work to build. It can be used to make artistic and sculptural shapes, and its use has grown again in recent years because of natural building and sustainability efforts.
In technical building and engineering documents, such as the Uniform Building Code of the western United States, cob is sometimes called "unburned clay masonry" when used for strong parts of a building. In other cases, it may be called "aggregate," such as "clay and sand aggregate" or "organic aggregate," when it is used as a filler between posts and beams in a structure.
History and usage
The word "cob" has been used in English since around the year 1600 to describe an ancient building material. People have used this material to build homes and structures since before written history. In Iran, cob has been used for more than 4,000 years. The origin of the word "cob" and "cobbing" is unclear, but these terms often mean to beat or strike, which is how the material is applied to walls.
Many similar earthen building materials and methods are used worldwide, such as adobe, lump clay, puddled clay, chalk mud, wychert, clay daubins, swish (Asante Twi), torchis (French), bauge (French), bousille (French mud with moss), beaten clay (pahsa in Central Asia), and cat and clay.
Cob buildings can be found in many climates around the world. In Europe, examples include:
– In England, especially in Devon and Cornwall in the West Country and in East Anglia (where it is called "clay lump")
– In Wales, especially in rural Anglesey
– In Donegal Bay in Ulster and in Munster, South-West Ireland
– In Finisterre and Ille-et-Vilaine in Brittany, where many homes built with cob have survived for more than 500 years and are still used today
Some of the oldest human-made structures in Afghanistan are made of rammed earth and cob. Cobwork, called "tabya," was used in the Maghreb and al-Andalus during the 11th and 12th centuries. Ibn Khaldun, a writer in the 14th century, described this method in detail.
Old cob buildings can be found in Africa, the Middle East, and the southwestern United States, such as the Taos Pueblo. Some cob cottages from the mid-1800s still exist in New Zealand.
Traditionally, English cob was made by mixing clay-rich soil with sand, straw, and water. Oxen were used to trample the mixture. English soil often contains chalk, and cob made with a lot of chalk is called "chalk cob" or "wychert." The earthen mixture was then placed onto a stone foundation in layers. Workers stepped on the mixture to shape the walls in a process called "cobbing." Construction continued once the previous layer dried. After drying, the walls were smoothed, and the next layer was added. Lintels for doors and windows were placed as the walls were built.
Cob house walls are usually about 24 inches (61 cm) thick. Windows were built deep into the walls, creating a unique look inside. The thick walls helped keep homes warm in winter and cool in summer. These walls act as a thermal buffer, absorbing and releasing heat slowly. Cob can last a long time even in rainy or humid climates if the foundation is tall and the roof has a wide overhang.
Cob is fireproof. "Fire cob," which is cob without straw or fiber, is a refractory material (similar to unfired red bricks). It was historically used to build chimneys, fireplaces, forges, and crucibles. However, without fiber, cob loses most of its strength when stretched or pulled.
Modern cob buildings
In 1994, Kevin McCabe built a two-story, four-bedroom cob house in England, UK. This was believed to be the first cob house in the country in 70 years. McCabe used traditional building methods, but he used a tractor to mix the cob and added sand or shillet, a type of crushed shale gravel, to help prevent the walls from shrinking.
Between 2002 and 2004, Rob Hopkins, a person interested in sustainability, started building a cob house for his family in Ireland. This was the first new cob house in Ireland in about 100 years. The project involved the community, but an unknown person set the house on fire before it was finished. The house was located at The Hollies Centre for Practical Sustainability in County Cork and was being rebuilt by 2010. Other modern cob houses have been completed, and more are planned, including a public education center.
From 2000 to 2001, a modern, four-bedroom cob house in Worcestershire, England, UK, was designed by Associated Architects. The house, called Cobtun House, was sold for £999,000. It was built in 2001 and won the Royal Institute of British Architects’ Sustainable Building of the Year award in 2005. The total cost to build the house was £300,000, and the thick outer cob walls cost only £20,000.
In the Pacific Northwest of the United States, there has been a new interest in building with cob. This method is used as an alternative building practice and is chosen for its appearance, usefulness, and cost. Pat Hennebery, Tracy Calvert, Elke Cole, and Cobworks workshops built more than ten cob houses in the Southern Gulf Islands of British Columbia, Canada.
In 2010, Sota Construction Services in Pittsburgh, Pennsylvania, USA, finished building its new 7,500-square-foot corporate headquarters. The building had cob walls on the outside, along with other energy-saving features like radiant heat flooring, a rooftop solar panel system, and daylighting. The cob walls, combined with these sustainable features, helped the building earn a LEED Platinum rating in 2012. It also received one of the highest scores in any LEED category based on the percentage of total points earned.
In 2007, Ann and Gord Baird started building a two-story cob house in Victoria, British Columbia, Canada. The estimated cost was $210,000 CDN. The home, which is 2,150 square feet, includes heated floors, solar panels, and a southern-facing design to help with passive solar heating.
In the 1980s and 1990s, Welsh architect Ianto Evans and researcher Linda Smiley improved a building method called "Oregon Cob." Oregon Cob uses a special wall-building technique that mixes mud with sand and straw to form rounded shapes. They are also testing a new method that combines cob and straw bales, called "balecob."
Cob building code
In 2019, an appendix for the International Residential Code (IRC) was approved by a vote during public comment meetings. Appendix U of the IRC explains how to use cob in load-bearing walls of single-story homes. Based on test results available now, the appendix sets rules for when cob can be used without needing an engineer's approval, like areas with earthquakes.