What Is a Cob House? Definition and Building Process

A cob house is an earthen structure constructed out of a mixture of clay, sand, and straw. Unlike an adobe house, the cob house usually involves a much larger proportion of straw holding the structure together. Instead of stacking bricks on top of each other, the cob house is built by lumping the mixture together and patting it in place by hand.

Cob houses were very popular in England during the 19th century since the straw made it very insulating during cold winters. The look of cob houses is very similar to adobe ones. They can be shaped into curves or other designs making them very artistically flexible. Cob houses are usually curved with arches and organic shapes.

This building technique is sustainable economically and environmentally. The materials required are low-cost, light, and easily accessible in most places. Clay, water, straw, and sand make up the majority of the cob house.

It first became popular as a building technique for one-story structures as it did not require costly or heavy timber. However, for multi-story cob structures, timber supports are usually needed. The cob design encourages self-building, which cuts down on labor costs and energy expenditure from power tools or other conventional building techniques.

Cob houses are fairly simple to construct and boast a variety of environmental benefits. However, they are not widely accepted around the world as a suitable structure. Namely, North America has not approved building codes for this method despite it being one of the oldest and most viable structure designs created.

What Is a Cob House?

Cob itself is a dough-like mixture of clay, water, straw, and usually sand. This building method has existed for centuries in northern Europe and parts of Asia, Africa, and the Americas. Its durability has been proven, as some cob houses that were built 600 years ago are still lived in today. It became popular as a low-cost, simple, and long-lasting structure even in harsh climates. The capitalization on local, cheap/free materials made it more feasible than a wooden structure during the height of its popularity.

The word "cob" means "lump" in Old English. Cob houses first exploded in use in the United Kingdom and British colonies in Australia and New Zealand, where they reliably withstood earthquakes, fires, insect damage, harsh weather, and the passage of time. In fact, this building technique has been known to endure events such as earthquakes better than adobe houses or other earthen buildings, since cob lacks mortar joints and is thus more flexible.

Cob is never fired and hardens by drying, which can be a long process. Walls are constructed by layering cob material while still wet, with the bottom stiffening while the walls are slowly being built upwards. A traditional cob house does not include any wooden framing. This kind of house ideally uses a majority of locally sourced, sustainable building materials.

Even though cob is an ancient building method, for years this kind of structure had not been approved for building codes in North America. However, just recently cob construction was approved for inclusion in an International Code as Appendix U in the 2021 International Residential Code by the International Code Council. The Council codes structures for the United States, Guam, Puerto Rico, and the U.S. Virgin Islands. Local coding and permits must still be met, but approval from the International Code Council will assist with promoting and approving more cob structures.

How Are Cob Houses Built?

Cob houses are traditionally built starting with a stone base/pinning that will protect the bottom of the structure from moisture or flooding. This kind of structure should not be built in a flood plain since it is very susceptible to water damage. A well-mixed cob mud is then sculpted upwards from the stone base. The mixture is layered, beaten, and trimmed to the desired shape one foot at a time.

As the lower parts of the walls dry and harden, more cob is added on top and the walls are reinforced by the interwoven mixture of the straw fibers in the cob that are beaten into each other as the mixture gets layered. The previous layer must be dried before another layer is added. Cob walls should be built during the warm and dry season in your area. Completing the walls could take months depending on the size of the structure due to the drying required.

This structure is highly customizable and usually takes on very artistic and unusual shapes. The walls must be arranged to evenly distribute any weight. They must be 600 mm (about 23 inches) thick or more to support a second story. However, the load capacity should be checked by a professional since a cob wall's strength depends on the cob mixture. Usually, a cob mixture will involve 1% to 3% of dry material (straw) and 3% to 5% of moisture. Too much moisture will jeopardize its durability. The placement of windows and doors must be planned out while the walls are being constructed.

Once the walls are constructed and dried, a decorative and protective sealant is applied to its interior and exterior to get that iconic smooth texture that cob houses are known for. The walls can be wetted slightly to get a limewash to stick better. Traditionally, the roof of a cob house is thatched and made out of natural materials. It is constructed with very large eaves to protect the walls from rain. England is particularly well known for its thatched cottages made out of cob.

Due to the drying time, it will take around 15 months to complete a cob house. The cost of a cob house can vary greatly. The finishings, appliances, and utilities like plumbing and electricity will cost about the same as a regular house. However, a cob house can save homeowners about 25% compared to the cost of building conventional walls. Implementing reclaimed or secondhand materials can also bring the cost down. Self-building your cob house can save on labor but end up costing you more time, potentially.

Pros and Cons

Pros

Fire and earthquake protection. Cob houses are usually fire resistant and can withstand heat for up to a couple of hours. These structures have also demonstrated high resistance to earthquakes which in turn promote economic and sustainability benefits since homeowners of a cob house won't have to pay and construct a completely new structure following a significant fire or earthquake.

Cost. A cob house can be built for very little money compared to conventional houses. By using local materials found near your building site, you can save on supplies and transportation costs. Also, a cob house, like any build, can use reclaimed materials rather than buying new. However, for a cob house to truly be cost-effective compared to a conventional house, you must self-build the structure to save on labor.

Less energy use. The thick walls of cob homes provide very good insulation during extreme temperatures, using an estimated 20% less energy to heat compared with a conventional house.

Cons

Time and labor-intensive. This building method is very time-consuming since each layer of cob must be allowed to dry before additional layers are added on. Layering more cob prematurely can affect the structural integrity and durability of the house. Cob houses are also very labor-intensive, which should be factored in when you are planning a self-built cob structure.

Risk of water damage. Cob is very susceptible to water damage. A cob house requires a sturdy, water-safe foundation (like stone), large roof eaves, and a location not prone to flooding.

Permits. Zoning approval and proper permits can be difficult to secure in suburban and urban areas. Building a cob house in a rural area might have a higher likelihood of approval. However, cob houses have been built in many different areas before and it depends on your local permitting offices and their view on alternative builds like cob.