Building with Cobb- A Greenhouse Project

Building with Cobb

By Allison Moore Peel

At the end of the summer I decided that I needed some new wall supports for a small greenhouse I had set up with a large pop-out window that a friend had salvaged for us. Turned on it’s edges, it made a nice greenhouse roof. Located in a sunny spot near the front of the house, I would be able to grow starts early for the garden and also use it for over-wintering house plants that are cold tolerant. Having already used it for this purpose but set on a ramshackle frame made out of some concrete block, boards and plastic, and meeting with some success, I was very motivated to improve upon the design by building new walls with cob. Cob is the name for a mixture of unfired clay, sand and/or silt, and straw all mixed together to a firm texture which you can then hand build into walls and  floors. I already had a floor, so I used some scrap wood and attached to it a light support frame, then chicken wire to that with a staple gun. You don’t need to use chicken wire but I thought it would make it easier to construct the walls thinner than those made for a full sized house. I sent out bulletins through the WCA email List Serve asking for unwanted clay reclaim, and also advertised a free cob-building workshop so that others could learn while helping make my small dream come true!

What I discovered first, was that people have an astonishing amount of reclaim hoarded in their studios! Folks, if you are ever going to use your reclaim you must first dry it out completely and then smash it up with a sledge hammer on a concrete surface. It will then slake very quickly into mush. To make cob, first pour off all of the water from the slurry that you can, then add the sand. I used Mason sand for it’s sharper structure and varying grit size. This acts to create an extremely strong clay body that will support itself as you gain height, especially after you also fold in the straw. Mushy reclaim is perfect because the extra water will quickly be absorbed by the sand and the straw, as it’s mixed in. Roughly chop the long straw into 6″ lengths, and separate the clumps. Straw helps to prevent cracking as the cob dries out and it’s easier to mix it consistently if it’s not too long. I had six or seven people show up for the workshop and we had a blast! It is extremely labor intensive but lots of fun with a group all working together.