To see how much sand and clay you have in your soil, do a simple shake test. Dig down beneath your topsoil to get to what's underneath. That's what you want to test. Get a jar, and fill it about a quarter of the way up with your soil. Top it off with water, and put the lid on. Now, shake the dickens out of it. You want to thoroughly separate the sand from the clay. Once it's all shaken up, set it on a flat surface to let it settle. The sand will settle first (unless you have heavier particles like gravel), and will do so pretty quickly. The clay will take a bit longer, possibly even a week or more. Once everything has settled, you can easily see the makeup of your soil. The first layer is sand, then you may have a thin dark layer of silt, and then the top layer will be clay. The best make-up of your soil for the greatest benefits will vary based on the type of clay (some are more sticky than others) As a general rule, you want more sand than clay, but the ratios's can be anything from 1:1 to 1:25. It just depends. So after you know what you have, do a few small test bricks with different ratios, adding just enough water to make it like a yogurt consistency, to see which one works the best. Too much clay, and you'll get cracks as the mix dries and shrinks. Too much sand, and it will essentially crumble. You can always purchase (or scavenge) more of either to add if needed. Beyond that, the simplest thing is to leave the mix as is. It can be used this way. However, most experts will encourage you to add something else to mix for added strength if building a home or, really, anything much bigger than a small sculpture. The most common additive at this point is straw. This will help bind and strengthen the mixture, making it much more durable for bigger projects (like walls). There are some who would suggest to add cement, ash, gravel, and even ground granite. Granted, I am no expert. But I would tend to believe that the straw would be enough for most projects, simply because that is what most experts focus on, and it's easy enough to obtain.
So what, exactly is the difference between the three building styles? Simply put, cob is free-form. It is built up onto itself without the use of forms. It is easy to get creative here with curved walls, organic shapes, and even sculpture built right into your walls. Adobe is the same mixture only formed into bricks, and laid out to dry in the sun. Then they are layered, as you would masonry bricks, using the same base mix as the mortar. If you like the look of a brick wall, but want it to be a little less expensive, this could be your bet. Rammed earth is, again, the same basic mix, only this time, it is packed into wooden frames, one layer at a time, until the wall is completely built. All of these options can be adapted to as thick or thin as you'd like. However, thicker is generally better for supporting heavier loads. Think of load-bearing walls needed to support the roof.
Another thing that all three of these options have in common is the basic care, both in the construction and maintenance. In construction, it is important to remember that these are all made out of earth. While they can very easily last a few centuries if properly taken care of, they will need a little protection to keep them from wearing away. A little moisture won't do too much. However, the idea is to keep excess moisture away from your structure. You must start with a good foundation. In an earthen structure, it all begins with digging a trench around the perimeter of your future structure. Your exterior walls will eventually sit on top of this trench. The bottom should reach down to the frost line. Fill the trench with gravel or small stones to help drain water through. Some experts also suggest laying a polyethylene (pvc) pipe into the bottom of the trench, drilled with holes, in order to help the drainage. Above this, will be what's called a stemwall. This gives the wall above something solid to sit on, and keeps it from having direct contact with the ground. You can use found stones, buy stones, bricks, concrete blocks, or even use recycled concrete pieces if you can find them. Your cob/adobe/rammed earth sits on top of this. Then, after you build the wall, you also need to protect it on the top with a good overhang. There's not really a set size for the overhang, but the general rule is the bigger is better. It needs to come out over the wall far enough to give it really good protection from any moisture that may come in from above. I've seen everything from 10 inches to a few feet. Just make sure it's solid and gives good coverage to the wall.
If you are building in a dry climate, you may be able to stop there. However, it's always a good idea to put a good waterproofing layer on your earthen walls. A lime-sand plaster is generally the most common, and it works very well. There are currently some earth builders who are experimenting with something called waterglass (sodium silicate). It's been known for quite sometime, but for this particular use, it has mixed results. There is also a technique that is commonly used called tadelakt. An ancient technique from Morocco, this is a lime plaster that is sealed with a black olive oil soap. I'll get into this one a little bit more in a later post discussing interiors. A good seal is important to any exterior wall, but remember to add an extra couple of layers to any portion that may have need of more protection. For example, if you are building in an area that snows, you want to make sure that the bottom of your walls have that extra protection. Of course, ideally, you'd want to set up some way in which the snow won't sit in direct contact with the walls. These water-resistant methods can also be used for sinks and bathtubs. Believe it or not, you can even have a cob swimming pool for much less money than a traditional pool. Something to think about. But again, I'll get a bit more into that later on.
You're probably now asking just how long will this take to dry, right? Well, there's really no easy answer to that. There are few factors, such as climate, water in the mix, and thickness of your walls, that will alter the timeline. In a very arid climate, it may dry so fast that you have trouble keeping up. Whereas, in a wetter environment, it can seem like it takes forever. But remember that while it may take longer to dry, this is going to be both the interior and exterior surfaces, and the internal structure of the wall, all in one. Also, plumbing and electrical can be built right into the walls as you go. So yes, it may take longer to build, but when you're done, you've already got all that other stuff already finished, too.
One thing that is very important to remember when using earthen materials for your walls, is that these are great for thermal mass, but are not insulation. Whereas insulation keeps heat or cold in or out, what thermal mass does is it absorbs heat and then releases that heat slowly as the surrounding air cools. It works beautifully in designs that include the use of passive solar.
These are easy techniques to learn and even master. An earthen structure can be done by just about anyone with the will and time to devote to it. Look to see if there are any workshops that you can attend to get good, hands-on experience, and you'll feel like a master in no time. As far as building codes and guidelines, well, those are hard to pin down in one single post. Some areas have strict guidelines, where other areas have practically none in regards to earthen structures. So be sure to look up your specific area to find out for sure.
I will tell you that even if none of these building techniques seem right for you, mixing cob could still come into play in other types of green building. It's great as a outer coat for straw bale and earthbag structures (which will be discussed in upcoming posts), and is also a great mortar for cord wood (also upcoming), and bottle brick walls we talked about with earthships a couple of weeks ago. There are so many uses, that it's just a good technique to learn.
There are a great number of resources out there for more detailed information. The main source that I would highly recommend is here. The lady who does this page is Sigi Koko, and I will tell you that she is so full of information, knowledge and wisdom, that you may not even need another source. She is an expert in multiple types of green building and techniques. Follow her on Facebook, Twitter, and YouTube. I promise that you won't be sorry.
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