«The Cob Builders Handbook You Can Hand-Sculpt Your Own Home Table Of Contents: Acknowledgements and Warning INTRODUCTION What is cob? Why build ...»
Heavy earth moving machinery can be used to mix up big batches of cob. Front end loaders are great! They can do the job of compiling and mixing the ingredients. The finished cob mix can be scooped up in the loader and raised to the height of the wall.
You can climb into the loader and hand-sculpt the walls without having to lift the cob up onto the wall by hand.
Putting the cob on the wall
Drag the mix on the tarp to the spot where you want to build. Anywhere is fine. If the foundation is low enough, two people can roll the mix off the tarp and up onto the foundation. If the foundation or walls are too high to roll the cob up onto, you can make a ramp to drag it up or lift it by handfuls, tubfuls or buckets onto the foundation.
If you haven't already put the pipes in for the electricity and plumbing, now is the time to do it!
Cobbing by foot sometimes called 'pise', (pronounced 'pee say') This is a method of squishing the cob mix down onto your foundation by stepping on it. This pushes the cob into the spaces between the foundation stones.
Get your weight off the cob when it starts bulging out too much on the sides, technically called 'ooging'. Let the cob harden for a day or two, until it can support your weight. You can cut off the oog later with a big knife or spade, before it's hardened completely. If you like foot cobbing and have good balance, you can do it all the way to the top of the walls. Make the walls thick enough so you can come back and tidy them up by cutting off the rough, protruding cob with a machete or spade.
Cobbing by hand Cobbing by hand is more gentle and controlled, creating more finished walls. Hand cobbing will allow you to build up a little further before the cob starts ooging because your weight is not added to the wall.
Thumbs are a great tool for the job but they do get tired, and thumb nails get sandeddown from shoving them into the cob. Give your thumbs a rest by replacing them with a stone or stick part of the time. You can use your palms and knuckles too. One hand can be the form as the other compresses the cob against the form and incorporates it into the cob below it.
It's easiest if you can keep your upper body weight over your arms, using gravity to help you push and massage the cob together.
It's important to make your house one massive hunk of cob, avoiding weak connections between 'layers'. When adding cob to a wall, you may need to re-wet the last application of cob so it's soft enough to massage the new cob into it.
Do not smooth out the surface of the cob too much or it will dry as a hard skin, which will slow the drying in the wall beneath it.
When you're cobbing, you are creating a surface for future cob to be built onto.
Make sure you get into the habit of making a flat surface on the top edges of the wall, not a rounded one, so the next "layer" of cob has ample surface area to sit on comfortably, instead of sliding off.
Make the bottom of the wall wider than the top. I usually make the interior side of the wall pretty straight up and down. It's a matter of taste. If you want your walls to grow organically, use your common sense. You can angle the wall for the first foot or so and then make it straight up, imitating a tree trunk or the interior of a cave.
The wall has less and less weight to support as it goes up, so it doesn't need to be as thick as it gets nearer to the top. You can taper the outside of the wall. Tapering cuts down on the amount of cob you'll need and makes the house that much lighter. The angled shape is a stable shape. Taper two inches for every three feet of height.
Cut a wedge measuring 2 inches x 3 feet out of styrofoam or wood and tape it to one side of the level. Leaning the wedge side of the level against the outside wall, get the bubble to read level. The edge of the wedge will then show you the taper you're after.
Cut off the bumps on the wall to form the plane you want. Use the other side of the level for your interior vertical line.
Keep an eye on the outside planes of the wall as you cob.
As you build, you will be creating the vertical planes on the sides of the walls that you will always see. As you add cob, keep an eye on both the inside and outside plane of the wall. Cob has a mind of its own. It's surprising how hard it is to keep the walls growing the way you want them to by just using your eye. If you don't check a level often, it's easy to build a lot of unnecessary width into a wall before you realize you're going wonky.
When working on a tightly curved section of the wall, work mostly from the outside of the curve. Cob curves tend to lean outwards as the wall goes up.
Ooging If you keep adding a lot of cob at once, it will eventually start to oog or bulge out on the sides, pushed out by its own weight. When that happens, stop cobbing in that spot and move on to a different section of the wall. If you have a big house-raising party and you manage to get to the point where the whole wall is ooging, pat yourselves on the back, make mixes in all the tarps to cure for tomorrow, and call it a day. Unless you live in a very humid climate, the wall will be dry enough the next day to chop off the oog and keep cobbing.
Over build the sides and trim them later It's much easier and stronger to chop off cob than to add it to a dry, or even a partly dry, surface. It's better to make your walls too thick and bulgy than too thin and dippy.
The walls can be checked with the level and trimmed to give you a more exact shape.
Do this in the mornings before you start cobbing. A machete, meat cleaver, or spade are good tools for shaping the walls. Chopping cob will dull your tools, but they seem to work just fine without sharpening. A 2x4 held on its side and scraped across the
The cob that you chopped or scraped off can be re-wet, retread a little, and recycled back into the wall, or thrown into an almost-finished new mix and tread in a little.
Another totally different approach is to build the whole house, leaving the sides of the walls all lumpy and messy. Then after the walls are built, wet them down really well and carve them into their final shape all at one time. A hoe or shovel are good carving tools to use along with your big knives.
Putting the cob to bed at night
At the end of a cobbing day, remember to make sure you haven't left any dips in the sides of the walls and poke holes in the tops of the walls to allow for drying and re-wetting. The holes serve another purpose too. They provide a key for the new cob to hold onto. When you continue cobbing, push the fresh cob down into the holes.
When you've finished cobbing for the day, poke finger-sized holes with a stick in the top of the wall. Make them 3 or 4 inches apart, especially along the edges.
Control the wall drying It is easier to get the previously applied cob and the new cob to stick together if the moisture levels are similar. This means that you are riding the fine line between keeping the top of the wall as moist as possible so the next cob layer will adhere to it, and letting the lower part of the wall dry enough to support the weight of more cob.
Cover the tops of the walls with dampened straw, burlap sacks, and/or tarps and kiss it good night.
If you suspect heavy rain is coming, you can cover the walls with tarps or sacks. To speed up drying, you can place lots of dry straw on top of the wall under the tarps or sacks. If it's likely to be windy, weigh down the covers so they won't blow off. The rain won't hurt your wall seriously but it will, obviously, slow down the drying time.
It's a good idea to keep your fresh walls out of strong direct sun and harsh winds, which will dry the outside surface too quickly, and can cause cracks on the dried surface as the inner cob dries and shrinks. Protect the walls with shade or by covering them. If you're cobbing where the temperature drops below freezing at night, pile straw bales or loose straw on the sides and on top of the walls to prevent them from cracking up. You could use old sleeping bags or anything that insulates. Take the covers off during the day so the walls can dry.
If you'll be back to do some more cobbing in the next few days, stop working on the walls while you still have enough energy to make up some cob mixes to cure. It's a delight to have mixes ready at the start of a new cobbing day. Because these mixes will be curing and drying out for awhile, you can mix them extra wet which is easier and quicker.
I also like to tidy up so when I return to the site it feels welcoming, and I can find my clean tools.
What if the cob dries out completely before you get back to it? It's OK. Fill the holes in the top of the wall again and again with water until the cob is rehydrated. Be careful to really incorporate the first new layer of cob. Use a stick and push the new cob hard into the old. Another trick is to sew the layers together with straw, pushing pieces of straw through the new mix into the holes in the old cob.
If you know that you'll be leaving the partially finished building for a long time, there are a few things you can do to make it easier to connect the next 'layer' of cob.
Leave the top of the walls rough, with lots of humps and holes and valleys.
• Make the re-wetting holes extra wide and deep.
You can push small sticks or bamboo into the cob on the tops of the walls 4 or • 5 inches down, leaving 4 or 5 inches sticking up. Put them every foot or so and poking out at different angles. The sticks will be buried in the new cob when you continue building.
Push straw into the tops of the wall leaving some of the straw sticking out to help knit the next cob application to the previous one.
Patching an already dried wall Plaster adheres beautifully to cob and will cover minor dips and smooth out the overall contours of the walls. But if you want to make a more major change and add new cob to the old, start by wetting the dry cob as much as possible. Drill, carve, or hack with a hammer claw holes in the hardened cob for the new cob to hold onto. You
How to destroy dry cob If you want to add a window or door, or change something in an already dry wall, get out the battering ram. You will soon have a new respect for the strength of cob and a healthy appreciation for taking the time to plan carefully. A 3/4 inch steel pipe can be sledge hammered through the wall many times where you want an opening. Wetting the wall again and again helps. Try an axe or hatchet to chop out what's left. It is much easier to fill in an unwanted door or window than to chop one out!
Sculpting cob shelves and furniture
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.
Add extra straw and clay to your cob mix for cantilevering. Build out a little at a time, leaving a rough holey surface. Let it dry enough to support itself before adding the new cob.
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. See the recommended reading list at the back of the book.
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. See the 'Books to Read' section (page 173) for recommended reading on masonry (cob) stoves.
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.