Cob 101 – FRESH OFF THE COB

Anyone who builds with cob knows it doesn’t take long for people to start asking all sorts of questions about it. Below is a list of the some of the most common questions we get asked along with our answers to the best of our current understanding. If you have a question that isn’t addressed here, contact us and we will add it to the list!

What is cob?

Cob is an earthen building material made from the soil on which you walk. Not the garden soil, full of life and organics, that feeds your veggie garden but the sub-soil below that which, depending on where you live, may contain variable amounts of cob’s secret ingredient: clay. In its raw form, cob is similar to plasticine in consistency and is sculpted free-hand to form self supporting, strong, durable, and wholly renewable structures from small ovens, to entire multi-storey buildings.

What is cob composed of?

Cob is composed of 4 basic ingredients:

Clay subsoil
Aggregate (sand/gravel)
Fibre: fresh straw (never hay!)
Water

To get the strongest, most durable structures most sources say that cob should contain roughly 15-25% clay and 75-85% sand/aggregate.

What is the purpose of the various cob ingredients?

Aggregate: aggregate is the stoney component of cob. It is sand, or gravel, or other sharp rocky fragments. Smooth pebbles, silt, and other rounded stoney particles work poorly as aggregate as their smooth contours prevent them from locking together tightly. Rocky aggregate is inherently stable and does not shrink or expand to any appreciable extent with varying moisture and temperature. It is what makes cob strong, however, to be useful as a building material, it requires a binding ingredient to lock all the individual grains together…

Clay: clay is inherently unstable. It expands when wet or hot, and contracts when dry or cold. This property of clay is related to its microscopic association with water, which essentially coats each clay particle in a film causing the particles to be pushed away from one another when moisture is added, and to be sucked very close together when water is scant (hence the expansion and contraction). It is for this same reason however that clay is a superb binding agent as the water film is what promotes the tight association between the individual clay particles.

Fibre: all different sorts of fibre have been used in cob mixes however the consensus seems to be that fresh dry straw (not hay) is the best. It serves a number of purposes ranging from providing tensile strength to the structure, much like rebar in concrete, to stabilizing and dispersing cracks, to absorbing and holding on to excess water when the cob is being mixed. Some believe that the air spaces created by the trapped straw enhances the insulation properties of cob but we aren’t so sure about this.

Water: water transforms the dry constituents above in to a wonderfully sticky, gooey muck that forms your structure. It is what activates the clay and allows it to coat the individual aggregate particles ultimately gluing it all together in to a homogenous mash.

Why build with Cob?

The reasons to build with cob are infinite and vary with each builder. For us, we build with cob because it is low impact, sustainable, and reusable (yes, cob walls can be knocked down, remixed with water, and reused). It is non-toxic, all natural, locally soureceable (for many), and affordable. There are few places on Earth where clay sub soil can not be found making it among the lowest impact building materials, from a transport perspective, available. It also consumes scant amounts of water (compared to concrete and other engineered materials) and returns what it consumes purified and resuable via ground water and evaporation. If you plan your cob timing right, with moderate rain fall, you can make cob without adding any water at all beyond what comes with the rains.

You can learn how to cob the first time you try it. If you do it wrong, or do something you don’t like, it is easy to repurpose what you have used and start over. It allows for infinite amounts of creativity and craft and encourages an intimate relationship between the builder and the structure being built. We have yet to run short on ideas when it comes to earthen building detail.

Clay is naturally deodorizing and air purifying creating fresh homes. It is also a phenomenal sound buffer creating an interior stillness like we have never experienced in industrially constructed buildings. Cob is fireproof and rot proof. Insects do not chew it to the ground and rodents don’t nest in it. Since we live in Vancouver BC, earthquakes are a part of life here. Cob performs exceedingly well when challenged on shaker tables, such as at the UBC Engineering Earthquake Laboratory.

It passively adsorbs, stores, and radiates back heat which can be capitalized on for heating efficiency by making cob-rich south facing walls and incorporating wood burning rocket mass heaters in to the structure. It likewise buffers high heat temperatures much like concrete parkades and sub ground housing.

Finally, if a day ever comes that we no longer want our cob structure, we can smash it down back in to the very earth from which it came, leaving no trace of its existence.

Is cob a good insulator?

No! However, cob homes are still very thermally efficient. This is because cob, like rock, is capable of absorbing and storing immense amounts of heat that it can then radiate back. Think of the days when stones were heated in a fire and then transferred in to a bed to keep people toasty warm overnight. Cob does the same thing. Cob, like rock, takes a long time to heat up and a long time to cool down. If built strategically, cob can passively absorb all sorts of heat from solar energy to the heat of wood fires and rocket stoves and then slowly radiate it to back to you, such as overnight when solar energy disappears and the fire has gone out.

Is cob a cheap building material?

Yes and no. Cob itself is cheap in cash (free really) but expensive in time, energy, calories, and sweat. It is also only the walls of whatever structure you are building. Like any other structure, cob structures require a foundation, a roof, windows, doors, appliances, etc, much of which can not be made from cob. Building with cob is a creative endeavour however, and anyone interested in giving it a try will no doubt also find joy in the hunt for the salvaged fixtures that can greatly offset the cost of building a hand made home. There are lots of examples of people who have built beautiful cob homes for very little money. Here is one.

Doesn’t a cob home disintegrate in the rain?

No! Clearly cob can not tolerate being left out, unprotected in the rain, but very few building materials can. Wood homes for instance, would not last long if left out in the elements. Just like all other houses, cob structures are fitted with a roof and plastered in a protective coating. Cob homes in the UK (one of the rainiest places on Earth) have stood for hundreds of years! Cob, once dry, can tolerate shocking amounts of wetness. For instance, a cob wall we were constructing got left untarped (unknown to us) for over a week in heavy North West Coast rain without missing a beat. We now leave out dry, unfinished walls in the rain deliberately a few days before we begin work on them again to re-activate the clay. When it comes time to plaster a cob wall, we soak the bananas out of it with a full blast hose prior to applying the first coat. You have to be committed to the task to destroy an established cob wall with water.

Where can I learn more about cob?

There are countless resources available for those who prefer to learn on their own. We are partial to books and have found the following list to be particularly useful to us. This is by no means a comprehensive book list, simply the ones that we have read in full and found to be helpful.