Cob Construction

I've had two different experiences working with cob, after having read about it over several years. I enjoyed Ianto Evan's book “The Handsculpted House”, especially his challenge to visitors to estimate the square footage of his and Linda's home. I was also intrigued by the method Linda used to determine the size of her kitchen space – stand in one spot and swing her arms around slowly. Talk about everything being within reach! Ianto also introduced a term I have found very useful – desking. Primarily used as a verb, desking is quite simply those tasks generally done at a desk – writing, paying bills, reading (mail, magazines, whatever), and so on. Many people will use their eating table (usually, but not always between meals!) for these activities, but it is an activity to take into consideration when designing your home. Personally, being the information collector that I am, a desk space is one of my top priorities. In fact, the desire for a larger desk (and more shelves!) was one of the motivators for building a larger cabin. Plus I miss having a comfy chair to curl up in with a good book, something I could not manage in the 8x12 since I had no loft to stash my bed in.

But the primary impact of this book was kindling my interest in cob construction. So when I heard of a volunteer-based cob house being built a little over an hour's drive from where I lived at the time, I leaped at the opportunity to experience hands-on training.

There was a small river near the building site where the clay was mined. The property had a well and electric. In discussion, I learned the hardest part was sourcing the straw, which is not to be confused with hay.

Straw and hay are two very different things. Straw is the stalks of plants from which the seedheads have been removed. It might be wheat, rice, rye, or another grain. Hay, on the other hand, is usually grasses, ideally with the flowers or seedheads still intact. Straw had little to no nutritional value, whereas hay is specifically harvested for its nutritional value. It is straw that can be a valuable building component (it is also used for animal bedding and roof thatching). Although really anything long and fibrous could probably be used to bind cob, you really don't want animals nibbling at your walls!

I never had clarified what their trouble with straw was – local farms not producing it, the cost of the straw itself, or possibly the logistics of transporting a large quantity of the bulky bales. Availability, cost, transportation – important considerations in any project!

The basics of cob construction are incredibly simple. Make a slurry of clay and water (depending on your clay, some sand to temper it may be needed), add a binding agent, usually straw, and mix well. Mash lumps of this mixture into a wall, let dry. Repeat. Cover with a roof with adequate overhangs to prevent direct rain contact with the walls.

Many people think cob can only work in dry areas, similar to adobe, but it has been widely used in the moist environments of the UK and Pacific Northwest of the US. Cob seems to shrug off humidity, but does last longer with protection from direct rain.

My first experience cobbing was a group of largely college students in shorts and t-shirts, stomping barefooted in the mud, arguably the best way of getting an even consistent mixture. It was then scooped into buckets and hauled to the building in progress. I was astonished and dismayed at the amount of water the endeavor required! It seemed an impossible task without a well.

My second experience with cob was on a much smaller scale, where I helped a friend build a cobwood shed.

Cobwood is a variation of cordwood masonry. Both methods use firewood type logs, whole or split, cut to a standard length. Cordwood masonry uses concrete between the wood pieces, whereas cobwood simply uses cob.

On this project, the owner had cut the wood from standing dead and down trees on the property, dug the sand near the mixing site, but brought in clay from a construction excavation site a reasonable driving distance away. He also had to haul in the straw, and according to his research rye straw would be best, a little more challenging to source. Being only a two-person crew, we mixed much smaller batches, and accordingly used less water. I began to see how the size (and speed) of the project affects, and is affected by, the availability of resources. The water for this project was simply rainwater collected in 50 gallon barrels, the rain being sufficient to keep up with the smaller and slower pace.

Each of these experiences left me with some interesting insights, some not even my own! The fellow in charge of the house project laughed about forgetting they could use garbage materials in the middle of the cob walls. I gathered crushed aluminum beverage cans were common fillers! He ruefully noted that this house probably had the purest cob walls of any. Had they thought to use filler, be it cans of river stones, that would have been that much less mixing needed.

On the shed project, the owner concluded nearly the opposite – the firewood displaced much of the cob needed for the structure, but unless you already have a surplus of cut logs, it would have been just as easy to make a pure cob structure. The additional labor of felling, limbing, and cutting the trees didn't strike him as a beneficial trade-off.

I really enjoyed my cob building experiences, and for quite some time had expected that when I found my place, I would also build with cob. Alas, my place turned out to be in an area of sand and glacial till, and what little clay is available is of very poor quality. It's a good thing there are many different ways to build a home!