FREQUENTLY ASKED QUESTIONS
1. What is cob made out of?
Cob is made out of three ingredients: clay, sand, and straw. Clay and sand are to be found underneath the first few inches of topsoil, almost anywhere. Straw is simply grass that has been dried out. All three of these are easily procured for little or no money. The three ingredients are mixed with water, either by humans, animals, tools, or machinery. Once the water evaporates and the cob dries out, it is rock solid, and makes an incredibly strong wall.
2. What’s the difference between cob and adobe?
There is almost no difference between cob and adobe and rammed-earth walls. They are all made from the same materials, clay, sand and straw. The only difference between them is how they stacked together to build a wall. Adobe is dried into individual bricks, then stacked and mortared with the same material that the bricks are made from. Cob is sculpted while still wet right into the shape of the wall. Rammed earth is the same as cob, except that wooden forms are used to keep the size of the walls exact.
3. Is cob similar to straw bales?
Cob is not the same thing as straw bales — but they are ecological cousins. Both of them can be used to build houses, both of them make for thick external walls, and both of them are natural and non-toxic. Straw bales are made exclusively from straw that has been bundled together, while cob is less than 10% straw, it is mainly clay and sand. Because of this, straw bales are relatively light, while sections of cob are incredibly heavy. Of course, the weight of a wall greatly affects its performance.
4. Is cob a good insulator?
Cob provides some measure of insulation from the elements, but it is not a superior insulator on the scale of straw bales, for example. And yet, homes made from cob walls can provide their inhabitants with very comfortable living environments, all year round. The way they do this is by being excellent storage units of thermal mass, by absorbing heat — from renewable heat sources like the sun, and from non-renewable heat sources like furnaces — and reflecting them back into the living rooms of the house.
5. Are cob homes dusty and dirty on the inside?
No, cob houses are not dirtier than any other kind of house, it all depends on how often you clean it. Clay and sand do not rub off the walls and fall onto the floor. Floors in cob houses can be from any material, including hardwood and ceramic tiles, but they can also be made from cob itself, and covered with oils and waxes for protection. These cob floors do not get any dirtier than any other kind of floor, either.
6. Won’t cob structures melt in the rain?
Cob is incredibly resilient, and even if it is left exposed out in the rain, for years on end. But eventually, without any protective layer, it will deteriorate, returning to the earth. So in order to keep the cob intact, we give it “good boots and a good hat” — a stone plinth foundation layer that raises the cob above the ground level, and a roof that overhangs the cob wall by a foot or more. In very rainy areas with horizontal winds, some people apply a layer of natural lime plaster for extra protection.
7. How high can you build out of cob?
We know that cob structures can safely be built over a dozen metres high, at least. In the United Kingdom, there are thousands of buildings, some brand new, some hundreds of years old, that are two and three stories high. In Yemen, in the Middle East, there are hundreds of highrises, skyscrapers six, seven, even eight stories high! Of course, in those instances, the walls must be built about a metre thick. The master masons of Yemen have been building in that way for over a millennium.
8. Can cob buildings withstand earthquakes?
Yes, cob structures can even withstand earthquakes! A few years ago, the University of British Columbia conducted a series of tests to determine how a cob house would perform under earthquake conditions. The Chief Building Official of the City of Vancouver determined that it would take an earthquake three times as strong as the maximum possible earthquake that could occur in Western Canada for its test cob building to start to crack up and come apart.
9. Do cob walls have to be really thick?
No, cob walls do not have to be very thick at all. Generally, external cob walls are built thick, in part to give structural strength to the building envelope and hold up the weight of the roof, when there is no post-and-beam frame. But there is no reason that internal building walls have to be any thicker than a typical wood stud and gypsum board wall, which is generally about five inches or thirteen centimetres wide.
10. Can you use cob to build perfectly square angles?
Technically, it’s perfectly possible for cob to be built into ninety-degree angles: either special care can be taken to ensure that the walls go up at right angles, or else the wall can be squared off with a special saw after it has partially dried. Most cob houses in the United Kingdom are quite conventional in size and shape and look identical to stone houses and brick houses that are also plastered over on the outside, concealing the actual materials that they are made from.
11. How much do cob buildings cost?
This question cannot really be answered, because the price of a house is dependent on so many more things that just what the walls are made out of. But if a company is contracted to build the cob walls, then it will probably be equivalent in price to a conventional wall system, because the money saved on materials is spent on long hours of labour. However, if you build the cob walls yourself, then you can save a lot of money.
12. Are cob structures legal?
In some countries with long traditions of cob construction, they are enshrined in law. In other large countries like Canada and the USA, each region has its own building code. On the West Coast, people have been building from cob for the last 25 years, and there are numerous counties that specifically permit cob structures. In most areas, if you have a post-and-beam frame that is made out of wood or steel or concrete, building officials do not care what you fill the walls of the house with.
13. Are there any cob structures in Toronto?
There are a small handful of cob structures in the City of Toronto. The first cob bench in the city was built by a team that consisted of Green Apple employees. (Since then, that private home has been sold, and the new owners of the house did not keep the cob bench). The best example of a publicly accessible cob structure is the outdoor kitchen facility at the Dufferin Grove park, off of Dufferin, just south of Bloor.
14. What else can you make out of cob?
Because of cob’s ability to be molded into almost any shape imaginable when it is wet, and then turn rock-hard when it is dry, it can be used to make almost anything. Truly, it is only dependent upon the skill of the sculptor. In the last few years, people have built benches, fireplaces, ovens, and whole houses out of cob. If you can dream it, then you can probably build it out of cob.
