Monday, November 26, 2012


By Robyn Cornwell, CSBA
OK, Im sorry, but I just had to write that title. And no, cob has nothing to do with corn-on-the-cob. It has everything to do with green building just about the greenest building, in fact, that you can imagine. Not only that, it can cost you from $100 to $3,000 to build your own home using cob dirt cheap! You heard me right. And, green in this case means that the clay has the property of drawing impurities OUT of the air instead of adding toxic substances to it.
That is because cob is a material made up of clay, sand and straw all of which can be either dug from your own land or bought very cheaply, or obtained from a friend who might give it you. In fact, if you are observant you might notice construction going on where they must dig up a lot of clay and then find a place to put it, which might just as well be your yard, saving them umpteen tipping fees if you know what I mean.
In my plan, almost one quarter of the circle is enhanced with closet space (something all you women out there might appreciate), which I think doubles rather effectively as insulation something cob is not. In fact, since part of that is the north side of the house, I might just add some straw bales for insulation on that side, or some other material in between two layers of cob for that purpose. The south side is a long stretch of windows ideal for passive solar, but may pose a problem in terms of structure: windows must have a lintel above them unless they are arched so this is something I will need to research. The w/d is really just a washer as I would utilize a solar dryer (read clothesline), and the toilet may very well be a composting toilet even if code requires a septic system.
The shower is made of cob with a special type of plaster coating it to make it waterproof as well as quite attractive. And yes, that is a two-burner stove since that is all I ever use. Note the kiva against the southwest wall which is a cob oven that works extremely well for steady radiant heating in addition to baking bread. Of great consideration to me was designing the home with all the water use concentrated in a small area, hence the kitchen and bath back-to-back. Also, putting the kitchen and bed in the middle of the space alleviates having to deal with the traditional problem of how to break up a round design (pie shapes really do not work well).
Ordinarily, you would dig a trench going below freezing depth generally two feet and add gravel along with conduit to wherever you want water and utilities to go, and along wherever you will be installing a foundation for the cob walls. At the cob workshop, we were able to skip this step as we were building on a ledge of limestone, plus there was no water or electricity to bother with either since this structure is only meant for outdoor use (namely, spirit worship). So we built the foundation of large stones directly on the ledge mortaring them together with concrete, not letting it show from the outside (especially since concrete is definitely not green due to its huge embodied energy). We leveled this off at about 18 high, and it is two feet thick, by the way, typical of cob walls.
Next, we mixed the clay, sand and straw by stomping on it with our booted feet on numerous tarps. As soon as a group of mixers got their cob mix ready they shouted cob toss and we all lined up to throw their balled up mix (what cob really means) to each person in the line just like a water bucket brigade until it gets to the wall where other people are waiting to pack it into the wall.
Only later did we discover that there is a much less labor-intensive way to do this, so instead of taking weeks to mix the amount of cob you need by boot stomping, you can utilize a Bobcat to drive over the mixture and have it ready in minutes (sounds really good to me). Then, you can drive it to the staging area, replacing the fun you could have had with the cob toss not to mention all the mud you could have gotten all over yourself and others in that process. In addition, you can use the plow of the Bobcat to stand on when you need to reach higher levels, instead of climbing on less stable bales of straw.
Our instructor, Christina Ott, explaining to us the reasons the wall fell down: In addition to the rain (the least of it), the fact that we built it up too fast (under orders, I might add) as it did not have enough time to dry between layers, may not have been the best formula of clay versus sand versus straw, we did not stitch it in properly,  and was not kept plumb as we built, meaning Get your saws ready. The day of the wall failure the workers were notably quite late getting back from lunch.

Worker sawing the wall to make it plumb (so it wont fall down again). Note the wood blocks stuck in the wall in preparation for a door frame. Note too, the popularity of wearing coveralls the better to protect oneself from the errant ways of the cob tossers.

If all of this intrigues you in the least bit, you might check out a book by the original cob builder and instructor, Ianto Evans, called The Hand-Sculpted House. The cob workshop is offered by Christina Ott at
And, if you have any questions about my house plan (or anything else), you can reach me at 828-758-0880 or

No comments:

Post a Comment