Have you ever looked at a cliff swallow’s nest and marveled at their cleverness? Nature once again offers us examples to emulate. The lovely thing about cob is how forgiving it is for the novice. Luckily most of us have experience with it as wee things playing pretend outside with mud pies. In fact I have striking memories of serving up mud patty biscuits to accompany my outdoor tea parties. Can I offer you one or two sugar cubes to go with that…um…cookie?
Cob is an old English word meaning a round mass. It’s comprised of a mixture of sand, soil, straw and water. It differs from adobe in that adobe is dried as bricks or fired in some way to harden the earthen material. Cob on the other hand is mixed and applied while still wet usually by forming it into a lump about the size of a loaf of bread, and stacking one layer upon the other to create the walls. As it dries it becomes almost as hard as stone, and after a coat of lime plaster can stand up to many harsh climates.
Cob is an artists’ dream material.
Curving walls, sculptures built into the wall…
…fireplaces, even furniture can be formed as part of the house, and for this reason it was originally my number one pick to build with… right up until I discovered that it is unsuitable for areas that experience extreme winters. Apparently the heat loss is substantial and where I intend to live can dip to -40 below so…that wasn’t going to fly.
In other parts of the globe however, cob has proven itself given the numerous buildings from the 1500’s and up still in use in England today. One thing is certain; this architecture is HEAVY so an appropriate foundation to hold it up is absolutely necessary. Cement works fine, but stone foundations (masonry) add a touch of class. Either way it is important to keep the walls from having contact with water from above or below so design it with that in mind, including sloping the ground away from your house and putting in French drains. You KNOW what happens when the ocean tide comes in and takes your sand castle away. These homes are fireproof but water will damage them.
Of course cob is mixed in certain ratios to get the optimal effect. Surprisingly topsoil is NOT preferred as it contains too much organic material that can decay thereby weakening the walls, thus subsoil IS preferred. Keep the topsoil for your garden!
Some tips for testing cob are to make them into bricks and allow them to dry. Drop the brick! If the test sample cracks – it needs more sand. If it crumbles – it needs more clay. If it breaks apart easily – it needs more straw.
Mixing cob sounds like an excuse to throw a mud wrestling party to me. Take a group of barefoot people dancing madly on a tarp and le voila, you are either at a college frat party or mixing cob. Of course if you have a lot to mix up you can use cement mixers, but I doubt it will be nearly as much fun. Generally it’s a good idea to mix up a batch of cob at the end of the work day to give it time to set overnight and be the right consistency (cookie dough batter) the next day.
There are three ways of using the cob to make walls each having a set of pros and cons. Flattening cob like pancakes is perhaps the simplest method. The mixture is piled onto a hard surface such as a board and walked on until 1.5 to 3 inches thick, where it is then picked up with a pitchfork and transferred to the wall to be tamped by foot once again, thus bonding wet cob to the semi dry layer below. This works well on the lower part of the wall as the base tends to be thicker, however, as the wall tapers on its way up tamping becomes a balancing act and a lot of work.
The last technique called shoveled cob is a cross between the first two as the mixture is shoveled onto the wall, and then worked in by hand. Many favor this technique for saving on the labour of ‘flattening’ or ‘loaf making’ and going straight to shoveling it on and shaping it in place. No matter which technique you go with one necessity stands true for all three, and that is to create a proper bond between each successive layer. This is accomplished by wetting the old layer before applying the new one, and also employing a tool called a cobber’s thumb which is used to create a depression by pressing down hard forcing the new layer into the old below it, thus knitting the wall together.
How do you know you’ve built enough wall for one day? When it starts bulging and oozing that’s a pretty good sign to knock off. Cobbers call it ‘ooging’ and generally only do up to one and a half feet of vertical wall per day. By all means work on a different section of wall, but no more than 18 inches high in any one spot for the day. The next day any excess that oozed is cut away with a sharp tool, the wall is wet down and the process begins again. Now for my favorite part-finishing plasters.
You’re going to need a hawk and (rounded) trowel to apply plaster to the completely dried out cob wall. If you don’t wait for it to dry thoroughly you will have to repair cracks as moisture finds its way out. It behooves you then to be patient. The first couple layers fill in cracks and dents, whereas the third finishing layer is thinner as the intention is to leave a smooth finish. This is also the coat that you add pigment to and polish smooth. While working on movie sets as a scenic painter I found that dampening a sponge or soft cotton rag enabled me to deal with any imperfections and a plastic object like a margarine lid did the trick to (think Karate kid) -wax on/wax off. Another trick we used on the set was to practice in a spot less likely to be seen (like under a set of stairs) to get the technique down.
You can do all sorts of decorating with cob from ‘carving into it’ or adding shapes on top of it.
You can insert glass bottles in interior walls for a ‘stained glass’ effect, to sculpting your chimney the shape of a dragon…the potential for artistic expression is limitless. Last but not least protect the plaster from moisture by adding three coats of a lime wash with a paint brush. (You can tart up the color by adding pigment here too) however, it is caustic so don the rubber gloves/long sleeves/googles and respirator. If you get some on you (yes I have) rinse immediately…do not wait til ‘after’ when you notice the ‘burning’ sensation (ahem). What can I say? Nice girl…little slow.
• Cheap! If you DIY the labour the materials are ‘dirt’ cheap
• Cob is an artistic medium similar to sculpting with clay. Your home will be very unique and artistic
• This building material is fireproof.
• Insulates noise from outside well.
• Suited for passive solar heating
• In dry desert areas it keeps cool in summer and warm in winter
• Cob is forgiving therefore easy to correct mistakes
• Cob plays well with other kids such as adobe or straw bale
• Cobbing makes for a safer work place for those involved
• It’s fun! A great way to invite community to come out and play.
• It’s labor intensive but requires little electricity and/or tools
• A local resource thus low embodied energy
• Durable. You can huff, and puff all day long Mr. Wolf.
• Easy to learn. Even with minimum skills young and old can contribute.
• This building material lends itself to creating curvy walls which allows a better air flow than 90 degree angles.
• Cob allows you to sculpt furniture, chimneys, shelves right into the structure.
• No out-gasing from the building material
• Stone foundations if chosen are labor intensive and expensive if you hire masons.
• While cob is forgiving and simple to use it is very labor intensive. Depending on the status of your back or your wallet that may be a problem.
• Permits. In England they have about a 500 year track record and about 20,000 cob homes are still in use today, however here in Canada…not so much. The good news is it’s not as hard as it used to be. On the Mayne island cob tour here on the coast of British Columbia we saw many lovely straw bale/cob hybrids so if you look to the bottom of the page we’ll hook you up with the right people to get it handled here in the Great White North.
• It doesn’t meet the R20 requirement for insulation (remember that’s why I’m not using it in the land of the cold)
• How it holds up (or doesn’t) in an earthquake is suspect
Books on Cob Building:
Monolithic adobe known as English cob (The natural builder) by Steve Berlant
The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage by Ianto Evans, Michael G. Smith and Linda Smiley
Building With Cob: A Step-by-step Guide by Adam Weismann and Katy Bryce (2006)
Go to http://www.cobcottage.com/products for the following DVDs:
Cobworks –of British Columbia offers tours, apprenticeships and workshops
Our Eco Village-Vancouver Island, B.C. hosts a slew of workshops on all things sustainable
The Cob Cottage Company – of Coquille, Oregon offer workshops, the Cobweb newsletter, books/dvds
Kleiwerks –of North Carolina offer many sustainability workshops
Groundworks –of Oregon. Women’s workshop Sept.19-25, 2010 during the Equinox and Full moon with author of the ‘Cob Builders Handbook’- Becky Bee
The Natural Building Network – is non-profit membership association which is a great source of information on builders/workshops