Cob Building Tutorial

Chapter 6: Cobbing - The Basics

 The recipe for cob is very simple. It’s just a mixture of clay, sand and straw. Although it is a very forgiving material to work with, it works best if the proportions are right. The best formula is one-third clay to two-thirds sand, then add enough straw to bind it all together, and enough water to make it workable.

Clay and sand are in the soil naturally in most places. A simple test will allow you to determine whether your soil contains the correct proportions of clay and sand. This test is called the ‘jar test.’

To perform the jar test, get a clear jar (about a quart) with a lid. Wash it thoroughly. Next, dig down past the top soil until you reach the clay beneath. Take enough clay to fill about one-quarter of the jar. Next, add water to about an inch from the top of the jar. Put the lid on and shake it until well-mixed. Set the jar aside for about 24 hours, until it starts to separate in layers (see Figure 6A). The bottom layer will be sand. The next layer will be silt, and the top layer will be clay. Hold a ruler next to the jar and measure the height of each layer. If you’re lucky, the sand layer will be exactly 2/3 of the clay layer. For example, if the sand clay layer measures one inch, the sand layer should measure two inches. If the clay layer is two inches, the sand layer should be four inches, etc. If the silt layer is larger than either the sand layer or the clay layer, you haven’t dug deeply enough. Keep going until the silt layer is the smallest layer.

If your soil sample doesn’t have enough sand, you can add sand by buying a truckload from a building supplier. It is rare that the soil won’t have enough clay, but if this is the situation at your building site, you may be able to get a load of clay for free from a local excavator. Just make sure to do the jar test on it as well.

How much leeway do you have in the proportions for the mix? A good rule of thumb is that you don’t want less than 25% clay to 75% sand. On the other end, 50% clay to 50% sand is pushing the envelope. It’s always easier to add sand than to add clay, so buy sand if you have to. Note that these proportions are just general guidelines as well. Another test will help you determine if you’ve gotten the proportions correct: the brick test.

To perform the brick test, mix up a small batch of cob, about enough to make a small brick (you can use a shoebox or old Tupperware container as a mold). Allow this brick to dry in the sun. Once it has thoroughly dried (about 24 hours on a sunny day), examine the surface of the brick. If it has begun to crack, you’ve got too much clay. Next, pick it up. If it falls apart easily, you’ve got too much sand. If it has surface cracks and falls apart as well, you haven’t mixed the cob thoroughly.

The next question is, “How much straw do I use?”

The quick answer is, “Enough to do the job.”

The way you know whether or not you have enough straw in the mix, is to make a ball about six inches in diameter from a test batch. Roll the ball out into a cylinder, and then have two fairly strong friends try to pull it apart. If it’s difficult for them to pull it apart, you’ve got enough straw. If it’s impossible for them to pull it apart, your batch is perfect!

For the final ingredient, water, you add just enough water to make the cob mixture workable. Aim for the consistency of modeling clay or cookie dough. If the cob slides off the wall, you’ve got too much water. If it crumbles as you’re working with it, you don’t have enough water. Don’t worry about adding too much water. As long as you have a workable consistency, one in which the cob doesn’t lose its shape as you’re working with it, you should be fine. The water will eventually evaporate.

If you’re fortunate enough to have a large supply of natural straw on your site, consider yourself lucky. If not, you can purchase straw from a lawn and garden or feed and seed store. Make sure it’s straw and not hay. The difference is that hay contains the ‘pith.’ This is the material inside the cellulose. The way to tell the difference is to pick up a stalk or two and look at it. If the inside is hollow, you’ve got straw. The reason you don’t want to use hay with the pith still in it is that the pith rots, and could deteriorate over time, weakening your walls.

Now to the fun part…mixing the cob. This is a fun group activity if you have a lot of hands to help you. Start by laying a small plastic tarp on the ground (about a 5’ x 7’ tarp works well). Next, add the clay and sand, based on the formula you worked out with the test brick. Add a little water. Now, take off your shoes and start stomping! Use your feet to mix the ingredients together, adding a bit of straw every few stomps, until the ingredients are mixed well. When the mixture strays from the center of the tarp, pick up the edge of the tarp, fold it, and re-center the mixture. This is great fun. Many cobbers sing songs while stomping the clay, so this mixing technique has come to be known as the ‘cobber’s dance.’

A word of caution about stomping the mixture barefoot…the occasional pebble will show up in the clay, so to avoid stone bruises, it’s best to screen your materials beforehand. The best way to do this is to Build a frame, about 2’ x 2’, out of 2” x 2” lumber. Once you’ve built the frame, attach some screen material to it using staples. The type of screen that has openings about 1/2” wide works well. Any smaller, and the clay has a hard time passing through. Any bigger, and you get pebbles that can cause painful encounters while doing the cob dance. Just be aware that if you use the screen procedure, you’ll generate a lot of dust, so wear a mask!

You can avoid the pebble problem altogether by wearing an old pair of sneakers that can get muddy, but to me that takes away half the fun. There’s something about being in direct contact with the earth that makes the whole thing an almost spiritual experience. And don’t worry if a few pebbles get into the mix. They simply add strength to the final product, in the same way that gravel adds strength to concrete.

As you gain experience with the cobber’s dance, eventually your feet will grow accustomed to knowing when the mix is right. The feeling gets in your toes. This is one of the reasons I don’t like mixing cob with shoes on. It’s like trying to read Braille with gloves on. With a little practice, you’ll be able to tell when the mix is ready just by feel alone.

Once you’ve got your mixture right, the next step is to start building. You do this by making cobs. Pick up enough cob to make a ball, somewhere between the size of a grapefruit and a basketball. Experiment to determine which size works best for you. After making a dozen or so cobs, you’ll know instinctively what size to make them. Next, just start stacking them on the foundation wall. Press them into place so that they seat properly. Try to complete one course around the entire foundation wall before adding height. Don’t work in sections. Build up around the entire wall a little at a time. Doing so allows the cob to interlock into a more stable construction. If you work on one section at a time, it’s hard to get the cob to interlock. Think of laying bricks, and you’ll know what I mean. It’s hard to ‘match up’ different sections if you don’t go all the way around the foundation, interlocking as you go.

I’d recommend staggering the cobs as you go, pressing them together as much as possible. Once you have the entire foundation wall covered, one layer thick, go back and start again with another layer of cob. If the previous layer has dried out by the time you get back around to where you started, moisten the layer a little bit…just enough so that the next layer sticks to the bottom layer. At this point, you’ll need a cobber’s stick.

A cobber’s stick is just any stick you can find lying on the ground, about a foot or two long. Sharpen the end a little. As you lay the next course, poke through the cobs to the next level, at least halfway through the course below. This process forces the straw from the top layer into the bottom layer, interlocking the courses, in much the same way that Legos interlock. These holes have the added advantage of making it easier to re-moisten the top layer when you return to the site the next day.

As you build up your walls, calculate where the plumbing and wiring will go. It’s perfectly safe to install plumbing directly into a cob wall. If installing wiring, make sure it is contained within conduit first. Work closely with the building inspector to avoid violating any building or safety codes. It’s far easier to place plumbing and wiring correctly as you build than it is to try to tear down and re-adjust placement after the fact, so check and check again before placing anything inside the walls!

If you’re installing outlet boxes, one method of getting a stable surface to attach them to is to drive a 2” x 4” stake into the ground. The outlet boxes can then be attached to the stakes. If you’re using a floor with a crawlspace, you can either install the outlets in the floor, or nail a board to the floor itself. These boards can then be covered with cob. A word of caution about burying wood in cob: There are two schools of thought on this practice. Some building codes say that burying wood in cob can cause rot, so they don’t allow it unless the wood is pressure-treated. I’ve heard this theory, but I’m not sure I agree with it. I’ve seen cob buildings over forty years old that have untreated wood embedded in them. But if you want to be on the safe side, use pressure-treated wood. If you’re a purist, and you don’t want to introduce chemicals into your home (pressure-treated wood contains many chemicals, including arsenic), you might try using cedar posts instead.

As you build up the walls, you’ll want to make sure that they’re straight and tapered inward slightly towards the top. I’ve developed a measuring stick that I use to help in this regard. This ‘plumb stick’ is a 1” x 6” board, 8 feet long. I cut it diagonally from corner to corner. This diagonal is the amount of taper that the finished wall will have. I then tack a short stick to the top of this board, and attach a plumb bob to it so that it hangs down freely. The plumb bob helps to determine that the plumb stick is being held straight. Then by placing the stick against the wall, I can determine whether the taper is accurate or not (see Figure 6B).

If attaching a plumb bob to the plumb stick seems too unwieldy to you, you may also tape a standard carpenter’s level to the stick, and use that to determine whether you’re holding it straight up and down. I prefer the plumb bob, because it gives you accurate readings in two dimensions. You can tell if you’re tilting sideways as well as whether you’re tilting in or out. Granted, it takes a little more patience with a plumb bob, but the extra accuracy is worth the patience.

As a rule of thumb, the smaller the building, the more you can ‘eyeball’ it rather than relying so heavily on the plumb bob. I wouldn’t try to just eyeball it with a wall longer than six feet, though.

While it is not absolutely necessary to build walls with a taper (especially if you’re building a single-story home), I find it helps to distribute the weight more evenly throughout the walls. As the walls rise, there’s less weight to bear, so you can taper. I don’t usually taper the inside walls because it makes the rooms look uneven and is more noticeable from the inside than it would be from the outside.

As you press the successive layers of cob into each other, make sure you’ve gotten all the air bubbles out. You want to press it together enough to make sure the layers bind together.

Don’t worry that the finished product will look a little rough on the outside. Once the walls are completed and dry, you can go back with a handsaw or a machete to even out the rough spots. Cut away the protruding parts, and fill in any low spots with a mixture of cob and a trowel. If these low spots are shallow, don’t worry about it. A certain degree of unevenness helps the final plaster to adhere better, and you can fill in the low spots with the plaster.

Remember as you’re adding layers of cob, to keep the top of the layer rough. This gives the next layer something to adhere to. It’s best to make a slight ‘V’ shape as you work so the next layer settles in better. Avoid the tendency to leave rounded tops as you go. Force the middle in more than the sides.

Unless you have a couple of dozen people working, you’re not going to get it all finished in one day. To keep the cob moist for the next working day, you can cover it with tarps. If you don’t have that many tarps handy, try soaking burlap sacks and laying them over the top of the wall. You can get burlap sacks fairly cheaply at a seed and feed store. You may even be able to get ripped sacks for free! Don’t forget that you can also moisten the walls again the next day. If you’re going to be away from the site for more than a day or two, you’ll probably just have to bite the bullet and buy more tarps. Remember that it’s important to work wet-on-wet so the bottom layers can stick to the new layers.

Before starting to cob, you must first install the doorframes. If you’re using a pre-hung door, position it in place before you begin cobbing by driving stakes into the ground to support it. You can nail supports into the frame itself as long as you don’t drive them all the way through the frame. If your final floor is at ground level, you can support the door by driving posts into the ground on either side of the doorframe. If you’re building with a crawlspace, the door would be framed up just as in a stick built home by nailing it to the sub-floor. Make sure that the door is affixed firmly enough that slamming it won’t knock it down! If you have any questions about how sturdy it has to be, check with your architect or building inspector.

With windows you have several options. The simplest is to imbed a sheet of plate glass directly into the cob. The advantage of this approach is that you don’t have to worry about a frame. The two main disadvantages of this approach are the fact that you can’t open the window if it’s embedded in the wall, and if it ever gets broken it’ll be virtually impossible to replace without tearing down the wall. You can get around this second disadvantage by using tempered glass. It’s a little more expensive, but well worth it to avoid the hassle of a broken window.

Another advantage of simply embedding the glass in the wall is that you can create unusual shapes for your window. This allows for greater artistic expression. If you’re embedding a sheet of glass, you’ll want to rest it on a piece of wood or a metal strip so that it doesn’t sag and start to ‘travel’ down the interior of the wall.

For windows that open, you’ll want to install frames. You can usually find old wooden framed windows at surplus or salvage building supply outlets. If installing a window with a frame, you can either support it by driving stakes into the ground and cobbing over them, or by simply setting them into the cob itself. If you’re choosing the latter option, you might want to add a little extra wood, nailed to the frame, to lend extra support. This extra wood can then be buried in the cob.

A little trick that can help the wood frame ‘grab’ the cob better is to take all your old bent and rusty nails and drive them into the frame at intervals of about 3 inches. The more crooked these nails are, the better! As you cob around the window, just press the cob around the nails. As it dries, the nails set into the cob, making the frame sturdier. If you choose this method, exercise caution while cobbing around the window frame. You don’t want to snag a knuckle on a nail!

A final method of letting light into your cob home is to collect old colored glass bottles and bowls. You can then just lay them into the walls and cob around them. This recycles the bottles while giving you an aesthetically pleasing conversation piece in your new home.

You may want to try a combination of framed windows, salvaged glass, and embedded windows to enhance the artistic look of your home. The framed windows allow you to open up your home to the outside breeze, while the rest take advantage of the creativity that is inherent in cob building.

Once you’ve worked past the windows, you’ll eventually reach the point of the roofline. While cob is load-bearing, and it is possible to install a roof directly on top of a cob wall, building inspectors who are not familiar with cob may balk at the idea. If this is the case with your building inspector, one way around this is to build your roof first, raised on posts or stone piers, then build the cob walls underneath it. The advantage here is that the walls will not then be load-bearing. Such an arrangement will also offer protection from the elements during the building phase, and it’ll keep the sun off of you during those hot sunny days.

If you’re going to build the roof directly on top of the cob walls, bear in mind when designing your structure that cob has about one-third of the compression strength of the average masonry wall. Since most cob walls are about three times as thick as the average masonry wall, the load factor is the same. Just make sure that the weight of the roof is evenly distributed along the entire width of the wall.

If you’re adding the roof directly upon the cob, make sure there are no drafts in the finished product. You can take care of this problem with fascia boards and by cobbing around any gaps you may find.

If you’re building under a pre-existing roof, then you simply have to cob up to the roofline, making sure that you plug up all the drafts. Cob tends to shrink a little over time, so you may have to patch things for a few weeks until it’s all settled in. You can take care of some of this by putting a fascia board on the outside of the wall.

Once you’ve reached the roofline, you’re done! That is, unless you want to take advantage of the sculptural quality of cob by adding relief sculptures to the inside or outside of your new home. When sculpting with cob, don’t worry if you make a mistake. Just get out your machete, chop it off, and start over. Cob is very forgiving! You can always make changes, then cob over your mistakes.

That’s what is so great about cob…it allows you to be creative without feeling that your work is ‘set in stone.’ When it’s ‘set in cob,’ you can erase it!