How to Build a House With Cob
By Melissa West
Before friends come to visit our house for the first time, I try to prepare them. There’s the long, muddy driveway and the lack of cell-phone service along the way. Then there’s the endless forest and resident bear that might be around. Finally, there’s the house itself.
“We live in a cob house,” I begin.
“A what?” my friend will ask. “You live in a house made of corn cobs?”
I laugh. “Not corn cobs,” I explain. “Cob is the name of a natural building material. My house is made out of sand, clay dirt, and straw.”
Not much further into our conversation, he seems to understand. “So, basically, you live in a mud house,” he summarizes.
“Basically,” I agree.
For my friend, a house made of clay, sand and straw might be even stranger than a house made of corn cobs, but earth homes like the one my husband and I live in have a long history. Earth was once the commonplace building material in many parts of the world; as common and incurious as we now find the vinyl-sided double-wide trailer. Our cob house, like many others being built today, is in the “Oregon cob” style, which descends from the cob-making tradition in the United Kingdom. The name cob is the Old English word for “lump.”
Sky’s the Limit
It’s a simple approach, though more complicated in practice. Making cob is both labor and time intensive. Walls in a stick-built or straw-bale house go up much more quickly than those in a cob house. And, just like in any other building, there’s still the roof, foundation, windows, doors, floors, heating, plumbing and electrical wiring to consider. None of these are any easier simply because your walls are made from cob.
Yet, an increasing number of people are choosing to build with cob. Why?
While building our cob house, we often told others that paying as we built would prevent us from building more house than we really needed or wasting materials. While this is true, perhaps a more accurate answer would be that despite its challenges, cob is simply unlike any other building material available. Beyond constraints of size, its only limits are your imagination and the natural laws of gravity.
As you design and build with cob, you’ll find yourself readjusting your mindset and thinking more creatively about what a building can be. Rigid spaces give way to nooks defined by archways and sculptural elements formed for the uses of the building. Want a chicken coop? Sculpt the nesting boxes right into the walls. What about a playhouse? Your children can join in and build exactly what they want.
If you do decide to build with cob, there are many questions you’ll need to answer. What size will your building be? Small is good, but not too small – you don’t want to skimp on wall thickness (at least 18 inches is best) and you want to make sure you have enough thermal mass inside the house to help buffer any temperature extremes.
What will the floor plan and elevation profiles look like? How about the frame work – will you use a pole barn style, roundwood timber framing, or load-bearing walls? Then consider roofing and foundation materials. Will you be using recycled materials? Will you do the work yourself or use hired labor? Do you need a new driveway, septic system or other site improvements? How about a building permit? After you’ve made these decisions and likely more, it is finally time to begin building your cob house.
Playing In the Mud
Making your first batch of cob can be intimidating. The materials are simple – dirt, sand, water and straw – yet no book can give you set ratios for your particular mixture with the materials in your area. You also need to figure out if you have the right dirt. The idea seems silly at first, but you won’t spend long digging before you find out that not all dirt is the same. Dirt is made up of three elements: sand, silt and clay. A good cob mixture contains just enough clay to bond the sand particles together, and enough sand to prevent the clay of the cob from cracking as it dries. Silt creates instability in a cob wall and is not desirable.
The best mixture of dirt, sand, water and straw is created by feel. Try making several cob mixtures with varying amounts of sand added and allowing them to dry. Test which ones are the sturdiest and resist both cracking and breaking. Your ratio could be as much as three buckets of sand to one of clay subsoil, or you could be lucky and not need to add any sand at all. Our soil already contains a good amount of sand, so we needed only one bucket of sand for every three buckets of subsoil.
When you have your ratio figured out and the foundation of your home ready, it’s time to start building. Here’s a step-by-step guide to making foot-stomping cob.
1. Gather your materials. It’s easiest to assemble the mixture near the building site so you are not hauling it over long distances.
2. Place the sand and clay at one edge of the tarp. Grab the corner of the tarp and pull, rolling the sand and clay into each other. Repeat with the other corner. When the mixture reaches the opposite edge, the sand and clay should be well combined. Repeat this process and continue mixing as you move the dirt and sand back to the opposite side again, pulling one bottom corner at a time.
3. Make a well in the sand-clay mixture, and pour in some water. The amount of water you will need might change daily depending on the moisture of the soil and sand and the weather. Begin with a small amount of water; it’s easy to add more later.
4. Jump in! Either with bare feet or old shoes on, alone or with a friend, it’s time to get messy. As you stomp, you’ll be turning your sand, clay and water into the mud that makes up the cob mixture. When you need to add more water, simply grab a corner of the tarp and pull, turning the mixture so that the dry portion is on top of the wet. Make another well, pour in more water, and stomp away. Continue turning and stomping the mixture until you have a firm mud.
5. When the mixture begins to ball up when you turn it over, it’s wet enough to add the straw. Scatter an armful of straw over the mixture and stomp it into the cob.
6. Next, it’s time to roll your “cob log.” Grasp two adjacent corners, one in each hand, and gently pull toward you, rolling the mixture into a log. If it crumbles instead, you’ll need to investigate. You might need to add more water or straw, or try stomping the mixture more.
7. Your mixture will be very heavy, so you’ll need to break it into smaller lumps, or cobs. The best cobs stay together when tossed from person to person and are made in much the same way a potter wedges clay or a baker kneads dough. To make a cob, break off some of the mixture from the log. With both palms of your hands, push down, and then, in one smooth motion, using your fingers, fold over. This creates a long and skinny piece. Stand it on the small end, then push and fold so the cob becomes shorter and fatter. You may need to repeat this a couple times depending on the size of your cob and how wet or dry the mixture is.
8. When you’ve made the whole log into smaller cobs, use them immediately, or cover with one end of the tarp to prevent them from drying in the sun. Do not store longer than one night or they will become too dry.
Once you’ve made your cobs, you’re ready to start building. Smash them directly on top of the stem wall. The layers of cob will dry as you build, and the new layer will bond with the straw from the old.
To make a strong wall, you’ll need to help the layers bond further, and you can increase bonding by pushing the new layer into the old at intervals with a dowel or stick. Make sure the top of each layer is rough and filled with nooks and crannies so the layers will bind together. Avoid the temptation to smooth the cob, including the sides of the cob wall. It’s important to keep the pores of the cob open to allow airflow for drying. You can always trim your wall with a serrated saw once it’s partially dry; use a level as you go to make sure your wall remains straight.
As you build, incorporate shelves and niches by cutting into the cob wall with hammers or butter knives (depending upon the size of the built-in), or create projecting shelves with cobs that have lots of long straw in them to help them attach more securely to the main wall.
Wooden elements like window and door frames will not stick well to cob, so you’ll need to add protruding nails and screws to your frame so the cob has something to grab onto. Likewise, if you’re joining straw-bale construction, wood framing, or cordwood walls to cob ones, you’ll need to make sure they’re well attached, using rebar for instance.
After a few batches, you’ll find the process of making and working with cob to be fairly straightforward. Inspiration and advice abounds online and in print. Read everything you can, and take workshops if possible. It’s hard work, but don’t hesitate to make it a community effort. Cob captures the attention and imagination of people from all walks of life – take advantage of it.
• Clay-rich subsoil
Our Cob Home by the Numbers
• Building Time: 2 years
• Move-In Date: November 11, 2011
• Space: 340 square feet, plus loft
• Inhabitants: 3, plus 1 dog
• Cost: About $20,000 (includes solar power, driveway, materials and hired labor)
• Number of homes previously built: 0
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