Cob (an old English word for lump) is old-fashioned concrete, made out of a mixture of clay, sand and straw. Becky Bee’s The Cob Builders Handbook (Chelsea Green Publishing, 1998) is a friendly guide to making your own earth structure, with chapters on design, foundations, floors, windows and doors, finishes and, of course, making glorious cob. Enhance your cob construction skills and learn how to build fireplaces, cubbies and more in this excerpt “Sculpting Cob Shelves and Furniture.”
You can purchase this book from the MOTHER EARTH NEWS store: The Cob Builders Handbook.
Building With Cob
Cob is amazing stuff. It’s so strong you can sculpt it out from the wall to create shelves and benches that project into the room (this is called a cantilever). Built-in surfaces for storage and seating maximize your interior space. Make yourself a beautiful window seat by cantilevering a bench out at the bottom of a big window. When you build indoor furniture consider the final floor height, so the seats are at a good position. You can copy the measurements and angles of one of your favorite chairs or couches to help you create a comfy seat. If you want a wide seat or shelves, you may want to build up the thickness of the wall or foundation under the cantilever before you start sculpting it. This will save you time because making a cantilever is time consuming, careful work.
Take extra care to make sure the new addition is well attached to the last cob. Cantilevering takes patience and a little practice. Sometimes the cantilever falls off. Don’t panic, keep at it, just add on a little slower. You can stick little sticks or straw into the cantilever, sticking out into where the next cob will be added. This will help hold the weight until the cob hardens and add to the tensile strength.
If you use something to temporarily hold up the cantilever while it hardens, like a bucket or straw bale, make sure you remove it within a day, or it’ll get stuck as the cob dries and shrinks.
You can always chop away furniture and shelves if you decide you don’t like them later on. It’s possible to add cob furniture after the building is made, but it will be easier and stronger to do it as the building grows.
Fireplaces can be sculpted out of cob. Fireplaces are very beautiful and romantic, but not very fuel efficient. The Earthbuilders Encyclopedia has blueprints for a fireplace and lots of tips for adobe brick fireplaces that can be easily adapted to cob.
Cob Around Wood Stoves
I suggest doing some reading to get an understanding of the basics of wood stove efficiency. A metal wood stove and stovepipe can be surrounded by cob, leaving access to the door of course. It’s a good idea to insulate between the hot metal and the cob with vermiculite or wood ashes. The insulation insures that the fire will burn hotter and more efficiently. After the heat gets through the insulation, it will be stored in the cob, radiating heat into your house after the fire has died down. Stoves can heat up cob benches for toasty bum-warming seats. In many countries, the stoves and pipes are all made out of cob.
Cob Construction: Relief Sculptures on the Walls
Cob is a medium that invites sculpture! It’s super flexible. You can make relief sculpture as you cob. You can also make artistic dips and holes in the wall. Or you can cob a bulge onto the side of the wall and when it’s hardened a little, carve it to the shape you want. You can add a sculpture to a dry wall if you rough it up well, re-wet it, and add some nails pounded part way in to help the fresh cob stick. When adding large sculptures to an already dry wall, use large nails. You can add one part mushed up newspaper pulp to one part cob to lighten the weight.
If you decide you can’t live with your sculpture or you want to change it, go for it. One of the wonderful things about cob is you can chop and change it indefinitely.
Burying in Shelves, Counters, and Loft Floors
Any wooden shelves, shelf supports, counter tops, or wood to support cupboards or lofts can be embedded into the cob as you build. It’s much easier to sculpt cob around wood than to cut the wood later to the organic shape of the dry cob. Everything that you build in as you go will save you a lot of fussy wood cuts later. You may want to keep the wood clean by covering it with cloth or plastic while you build. The counter, loft, and sturdy shelves can serve as scaffolding. Make sure the cob has hardened enough to support the protruding wood with your weight on it before you stand on it.
If you choose to put your counter or loft in later, you can leave a little cob shelf (2 or 3 inches) to set it on.
To support something small like a counter or shelf, compensate by making the walls a little thicker below the ledge. For a loft, you might want to make the whole wall a little thicker from the foundation up. You can cut the counter (or floorboards) to fit and set it on the ledge. Fill any gaps between the counter and the wall with cob or plaster.
Counters may need extra ‘legs’ to support their outside edge. These can be made out of wood or you can build cob pillars. These can also support shelves under the counter. If you want to attach cupboard doors, bury in a piece of wood to attach the hinges to. Remember to add something to help key the wood to the cob.
For two-story houses, you can save space by burying pieces of wood for steps that protrude out from the walls into the room. The cob is very strong and can easily hold the wood in place. This stair system has been used for centuries but is not very safe for little kids and will not pass any building code.
Cob Construction: Making Niches or Cubbies in the Walls
Niches are recessed spaces built into walls to create an altar, bookcase or a place for candles. These are fun to sculpt in as you build. If you finish a wall and then wish you had built a niche, you can carve one out later. This is much easier to do if the cob is still wet.
If you bury colored bottles though the back wall of the candle or light fixture niches, you’ll see the colored light shining through the walls when you’re outdoors.
The tops of the niches can be treated like any other opening. They will need a lintel or an arch. If you make niches less than a foot wide, you can forget about the lintel or arch rule and let your imagination go.
The thinner walls in the back of the niche will be less insulating than the thicker parts of the wall. If you live where it’s really cold, you may decide to forget the niche idea. Books are good insulation so you could make a bookcase niche and keep the shelves close together and full.
Building with Cob Assistance: Scaffolds
You’ll need something to stand on as your walls grow to keep your upper body weight over the wall. Standing on straw bales or on planks that are resting on straw bales works well for the lower part of the wall. Only stack the bales two high; three is too precarious. The straw bales against the walls will slow down the drying so you may want to move them once in awhile to give each part of the wall a chance to dry out.
It’s OK to stand on the partially dried window sills, cantilevers, and built-in furniture. Use your judgment. You can straddle the wall, sitting on it like a horse.
As your walls get higher, pieces of wood can be buried through the walls to support the scaffolding planks, both inside and out. Put the supports into the walls at least a foot from an opening.
Four or five feet off the ground is a good place for the first scaffolding supports. Depending on how tall your walls are and how agile you are, you may want to put more supports every two or three feet. Be safe, rest strong planks on the buried wooden supports and attach them well. Add some additional sticks to help hold up the outsides of the scaffolding supports. You can use a ladder or straw bale steps to get up onto your scaffolding. The cob can be piled into dishpans or tubs and set up onto the scaffold, then lifted up onto the top of the wall.
When you’re finished cobbing, the supports can be left in the walls to permanently carry the weight of shelving or cabinets. In many countries, the scaffolding sticks are left in the walls as decorative ladders and to stand on during replastering later.
If you want to remove them, they can be sawed off and plastered over or they can be knocked out of the walls with a sledge hammer, and you can fill the hole with cob before you plaster.
On the sunny, glass-intensive wall, it’s likely that you won’t have enough cob between the windows to hold embedded scaffold supports. A simple scaffold can be set up using two stepladders with the scaffolding planks running between the ladders, and resting on the ladder rungs (not on the top of the ladders!) Make sure the ladders are well-balanced where they stand.
Installing Electric Wiring Around Cob Construction
If you haven’t put the access pipes for the electric wire and plumbing through the foundation, make sure you set them on top of the foundation and cob them in place. If you haven’t decided exactly where the access pipes should go, just put in extra ones and fill the ones you don’t need later with cob.
Because cob is completely fire proof, electric wires can be buried right into the cob walls or put in conduit pipes and buried in as you build. Bury them at least an inch deep in the wall to avoid chopping into the wires later when you do the final shaping of the walls. Mark where the wires are buried in the cob so you can find them in the future.
To hold the electric receptacle boxes in the cob, get a piece of wood a little bigger than the box, and nail or screw it to the back of the box. Cob it in where you want it. If you’d rather deal with finishing the electricity later, leave a hole and, when you get to it, embed the box into the hole with cob.
Another approach to installing the electric wires is to carve a groove out of the cob where you want the wires to be, either when it’s wet or dry. Lay the wiring in the grooves after you’ve finished the walls and just fill the groove with cob or plaster afterwards.
Keep yourself warm with cob: Learn about the different ways you can insulate a cob home roof in Roof Insulation Options for Cob Houses.
This excerpt has been reprinted with permission from The Cob Builders Handbook: You Can Hand-Sculpt Your Own Home, published by Chelsea Green Publishing, 1998. Buy this book from our store: The Cob Builders Handbook.