Cob is a building material generally made out of clay, earth and straw or hay. The straw within the cob prevents the mixture from falling apart as it dries and ages. Cob has many uses however is generally used for the building of houses and traditional wood fired ovens.
Cob Mixture Ratio
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History of Cob
The term cob comes from the Old English word for “lump”. It is used with reference to mounds and lumps of soil rich in clay, which after extraction is mixed with straw and used to build walls and houses. Until coal and oil made transporting materials economical, and cement and bricks became the commonplace building materials, cob was the best choice for construction, since it was the only option available to people living in areas where timber was hard to get. However, the usage and history of cob dates back to prehistoric times. Some of the oldest structures known to man were made of soil acquired by scraping the topsoil and accessing the clay below. Ibn Khaldun, the Arab Historiographer and historian, described cob, and its related works, in detail in the fourteenth century. Today, the UK is probably the largest continuous area of land where cob buildings are still found and lived in. The concentration lies mostly in Devon and Cornwall in the West, Glamorgan and Gower Peninsula, South West Ireland, Finisterre in Brittany and a few other places. Africa, The Middle East, and even the United States have houses made of cob.
Ingredients in Cob
The traditional mix of cob had clay acquired after scraping away the subsoil, straw, and water. It was trampled under oxen feet, and used in building earthen walls. The mix, after preparation, was laid gently onto a foundation made of stone intermittently, a process defined as cobbing. The basic principle when it comes to building with cob is to let each layer dry in its own time. A new layer can only be caked on after the previous one is completely dry. The walls, or he respective structure being built, can then be trimmed, and openings for doors and windows can be made as the structure progresses.
Advantages of Cob
- Today, while the number of members in each house is dropping, the size of the abode is increasing. People are inclined towards using concrete and other processed materials to build houses which not only tend to crack but are also expensive.
- The main advantage with cob is that it comes entirely from the earth. All you need to do is dig up the topsoil, and you will have access to pure clay. Straw will be available easily on any farm or store.
- Additionally, studies and experiments have shown that cob is more resistant to earthquakes than are cement or adobe. While cement tends to crack, cob is essentially a large mass of clay, held together with straw and water, thus making it stronger.
- Structures made of cob can be repaired easily, since they are made of clay and straw. Any cracks can be covered with a new layer whenever convenient.
- Cob is a better option for construction from an ecological point of view. When torn down, cob can be returned to the earth due to its raw materials.
Concerns about Heat Retention
The thermal performance of cob depends on the place where you are living. While it may be sufficient in some places to light a small fire in the morning to keep the place warm throughout the day, others that are chillier might need a little more effort, since the heat gains might be lost quickly. In order to keep cob houses warmer in colder areas, another additional wall may be built inside the house in order to retain the heat, whereas better insulation techniques may be used on the outside of the wall.
Cob is highly resistant to the brunt of the weather. It is porous, and can thus stand wind and rain. However, since it is basically made out of earthen materials, it is best not to take chances. Thus, cob materials should be protected by the "Boots and Cap" strategy.
- Roofs should be built over cob made ovens and other small structures. This will protect them from rainwater.
- Additionally, the structures, even houses, should be built on a platform, or at least at an elevation of the ground. Cob structures should never be built in flood prone areas, since too much water will destroy them.
- Furthermore, it is best to build cob houses or ovens in a place surrounded by trees, since they would provide a break from the wind.
- Buildings should have roof eaves and a strong foundation, resistant to the weather. Houses built in windy areas can be protected by stucco plaster or whitewash, but it might make the houses too hot.
Characteristics of Cob Homes
- Cob homes are built more for efficiency than for size or show. They will be small and just the size that the owner has need for. Thus, they do not take up much land or even resources.
- Cob homes are built entirely from raw materials obtained from the Earth. Thus, they are natural, and local, since you will get all the materials necessary in your city or town.
- Artistic Freedom
- You do not have to be a professional designer to build a cob home, but they do allow for more artistic leeway than do concrete homes. Cob homes are not limited to the conventional ninety degree angles. Additionally, cob can be molded into different shapes, which makes room for creativity to nourish.
- Many people might consider living in a cob house low standard. However, cob homes are every bit as comfortable as are concrete homes. On the other hand, the conventional concrete and adobe homes might be considered uncomfortable and even harmful to the environment. Additionally, cob homes come without the added pressure of a loan or a mortgage.
- Sound Absorbent
- Cob has superior acoustic properties as compared to concrete homes. It can be protected from both exterior noises and well as internal din.
- Termite and Insect Repellant
- Since they are made mostly of clay, straw, and sand, cob homes are not considered as delicious treats by insects. To this date, they have not been identified as breeding grounds for insects.
- Fire Resistant
- Cob does not catch fire. However, if you cook in your house, make sure to install a chimney, since the roof might not be as fire repellant as the rest of the house.
- The natural materials used in making cob homes make them inherent fighters against pollution. Additionally, cob walls are porous. They are said to "breathe". Thus, living in a cob home gives one the added bonus of taking in naturally filtered air.
Cob is commonly used as a building material for the construction of traditional ovens. Cob is readily available, cheap, and an easy material to teach inexperienced individuals to build with. These ovens are thus very common among DIY builders and within poorer communities.