September 28, 2013


Why Build a Cob Bench?

A Kaizen approach to climate appropriate natural building

Cob Benches can be simple, elegant, functional, and comfortable. This is the sort of bench I am imagining building. from

One of the powerful ways we go about learning and developing our understanding here is by experimentation. Doing small tests, where the stakes are relatively low, and building up the complexity of our knowledge one step at a time.

Indeed this is the historic path of the apprentice in medieval guilds. And it is our philosophy as Stewards that we are in constant, ongoing apprenticeship with nature.

One of my greatest passions is finding ways to meet our need from the land. And I believe that techniques such as Cob, Strawbale, and round-wood Timber framing can all be simply and inexpensively employed to meet out need for functional structures that have beauty and uniqueness.

As Wernher von Braun aptly said, "Research is what you do when you don't know what you're doing." While natural building is far from rocket science, there are many basic techniques, skills, and intuitions that go along with successfully creating a durable structure out of naturally available on-site materials.

The construction of a Strawbale/Cob bench is a first step in experimenting with these, now fairly commonplace natural building techniques. More importantly, it is a small scale test to see how these material hold up to the particulars of our climate; characterized by cold wet winters with deep snow, and long hot dry summers. These climatic factors are notably different than those of the most successful examples of these techniques.

Building a Structure to House the Bench

R&D on some Roundwood Timber framing basics

The Goats helping out with construction.

Building a good roof over an earthen structure is key to its longevity. For a structure with standing walls, large eaves and gutters are essential to keep wind blown rain and snow from hitting the walls, deteriorating the earthen plasters and potentially working its way into the bench causing decomposition. So naturally, we first had to build a roof structure.

As part of learning and experimentation, I decided to use our native timbers in their natural rounded form for the vertical supports. Round wood timbers are notably stronger than sawn timbers of the same diameter. This is due to the natural structural arrangements of the wood grain being left fully intact, with extra "meat" around knots, and a gentle spiral shape that gives the wood a kind of "spring" to recieve and dissipate sudden forces like gust of wind.

The quaities of round-wood is well described in this European report of small-diameter conifers used for pole construction. (Bending and compression properties of small diameter round timber.)

The timbers used in this building were Douglas fir, taken out of the forest in 2010 as part of forest thinning for tree health and fire prevention. We stacked many good quality pole timbers from Pine, Fir, and Oak in a shady place to slowly dry and cure.

A paramount concern when using organic materials in construction is that, when exposed to water and oxygen, biological processes will begin to break them down. Doug Firs, and most conifers, have very little rot resistance. So I had to figure out what could be done to increase their durability.

It is standard contemporary practice in America to use pressure-treated lumber in any construction conditions where the wood is in direct contact with the ground or with seasonal moisture like snow-drift or rain splattering. The PT wood is protected against rotting by the injection bio-cides, allowing them to ward off hungry soil organisms.

After extensive research into traditional building methods, I found NO examples of long lasting buildings (over 200 years old) with wooden supports that were in contact with the ground.

Not even the most rot-resistant highly-durable woods like red mulberry, osage orange, and Pacific yew , Honey locust and Black Locust (all of which can contain notable amounts of natural fungicide, depending on how they are grown), have a life expectancy in the ground of 70-100 years in normal conditions.

Whether in England, Europe, Scandinavia, Japan, or Russia (the most common places I could find resources), the wooden supports were always up off the ground. Usually situated on boulders sunken partway into the earth.

For several reasons, I decided that I was not quite ready to try to put the shed up on boulders. Primarily because of the added structural stability that digging in the posts provides.

I'd have to add a lot of cross bracing to ensure the building would not fall over if I was putting it up on boulders. And, given how small this shed is intended to be, and how people will be going in and out of it from all directions, I was not convinced the building would have enough mass to stand up against some of our more severe winds.

I could image the whole thing toppling over in some of our late winter gusts which I have witnessed rip whole trees from the Earth. So.... I looked into other, less rot-resistant methods of utilizing conifer poles that are sunken into the ground.

What I found was some obscure references to a traditional Scandinavian method of treating conifer fence poles by charring their outsides upwards of three feet from ground level, and then placing them on top of gravel in the bottom of the hole and back-filling with more gravel.

This method provides rot-resistance in three ways:

  • The layer of charcoal is a biologically dead zone. Bacteria can not directly eat the charcoal, and so cannot eat their way inside the wood.
  • Charcoal is hydrophobic so it repels water that might otherwise soak into the interior wood. Since life needs water to operate, keeping wood dry is an effective preservation method. (think of a mummy)
  • The gravel pit around the post acts like a french drain. Water falls away before it reaches the post, preventing the moist conditions that promote biological activity.
  • To get the poles ready I selected 4 of them that were about the same diameter, and cut them to the roughly the same length (about 10ft). I then removed the bark with a draw knife. This was actually quite a long process, and the wood was still fairly green and the thin bark of the young fir was still tightly adhering to the sapwood.

    After debarking, I started a oak fire and let it burn it down to glowing coals. I placed the poles so the first 4-5 feet were laying directly on the coals. I let them burn until the first half inch of wood had burnt to charcoal. I kept turning them until all sides and bottom were charred to about 5 feet.

    The posts holes were triangulated and dug to 32 inches which is below the frost line here. It's important to dig footings, foundations and posts to below the frost line, or else they can be moved laterally and heaved upwards by the hydraulic action of the moist earth expanding when frozen. Never underestimate the power of freezing water! Just imagine glaciers slowly scalping whole continents.

    Before putting in the posts I put small diameter gravel below each post to help with drainage. And the posts laid in, stabilized with supporting pieces of wood and plummed. In the process I also maneuvered the posts so their outside faces where in a square plane with one another.

    Remember, these posts are round and different diameters, and so you cannot get them to be "perfect" but perfection is not on the menu for such buildings. Instead you are dealing with an adequate approximation, and a custom fitting of the pieces of the building to the uniqueness of the individual parts.

    That is really the art and craft of working with such materials. And over the course of working with the building I developed a greater sense of how to work with them.

    A detail of the doubled up "top-plate". The rafters have corresponding double Birds-mouth cuts to set level on both the top plate boards.

    When filling the post holes, I wedged fist-sized stones between the post and the wall of the hole is all directions. I then filled in around the stones with smaller (less than an inch) stones. I repeated this all the way up the to the soil surface.

    For the rest of the building, I used a pretty standard method of constructing a shed roof, using a double set of home cut true 2x6's for each of what I will call the "top-plate" because I am not sure what to call it when used in this configuration.

    I used the same cut of lumber for the rafters and bracing as well.

    I used 1/2" x 6" home cut lumber spaced 6" apart to form a decking to apply the roofing material. The roof was finished with pieces of aluminum siding salvaged from the demolition of Bravo Trailer.

    The whole structure is only about 10ft square. Sufficient to provide the bench cover from the elements, and make a nice place out of the elements for us to gather before the morning walk.

    Left: The finished shelter with the beginning of the foundation of the cob bench. Right: Detail of the cross bracing.

    Building a Rubble Foundation

    One of the most important parts of any building is the foundation. A poorly built foundation will lead the sinking, heaving, and the ultimate damage or even destruction of the structure.

    A common, simple, effective, and natural foundation can be made out of small stones and gravel dug in below the frost line. A slightly modified version a rubble trench foundation is what we decided to use underneath the cob bench.

    I refrain from calling it a rubble trench, because we ended up digging out the whole underneath of the bench. Because make a trench would have actually been more difficult, since the foot print of the bench is so small.

    The construction of the foundation was pretty straightforward. First we dug a straight sided, level bottomed hole to a depth below the frost line, in the footprint of where the bench will sit.

    Wet the earth and lay in a course of small stones and tamp them in. Then begin to fill the whole with large stone. I used what I call “two handers” stones which I can only pick up with both my hands. Make sure that the stones are places snugly together in a matrix, so that thy cannot move laterally in any direction without running into a stone next to them. This helps the whole of the foundation act like a single unit.

    Progressively lay in smaller stones, making sure that each course is snugly packed together.

    When we got about 6 inches or so from the top of the hole, we began laying in a course of two handers around the edge of the hole. These stones will not only be a decorative feature of the bench, but will also help prevent water from splashing or wicking up the bench, as should also help prevent the earth plaster from being kicked by peoples heels when the sit on the bench.

    Sinking these edge stones into the ground adds to their stability. Because they are sticking 6-8 inches above the soil surface, they also create a container that allows us to continue laying in stones above the surface of the soil.

    The stones will not soak up water and will allow water to fall down to the bottom of the foundation. Since cob has the capacity to wick moisture, we want to make sure that their is not part of the cob that is in contact with the soil. Getting the whole thing up a few inches of the ground is important in preventing wicking and the associated rot/weakening/cracking.

    When we got to the top of the whole, we were working with stones a little large than a fist. From what I can tell, stones of this size were historically referred to as “cobblestone” in England, and were what was used as pavers in cobblestone streets and such. Since I get a kick out of reviving old words, I will be calling these fist sized rocks, Cobblestone from here on.

    As you can see by the above picture of the work site, as we dug the hole, different qualities of soil and sizes of stones were seperated out. Large stones, medium and small stones, clay that had pebble sized stones mixed with it, and the top soil which was removed at the beginning of the dig to be used as a top dressing on the soil around the shed.

    When the cobble was laid a few inches from the top of the eding stones, we stopped adding cobble and instead began sifting the pebble-clay mixture through a half inch screen to separated out the pebble-sized stones (as well as bits of root and sod) from the clay.

    rocks and organic material sifted from the clay.

    This left us with both the sifted clay needed for making the cob, and quite a lot of pebbles which were added on top of the cobblestone. This created a relatively flat surface from which to continue building the bench.

    sifted clay, still with bits of stone in it, but good enough for cobbing.

    At this point, I decided that we needed some kind of level pad before we begin placing the straw bales on top of. I am not sure if this is necessary, but it seemed like a good idea to create a flat, compacted place for the bales to sit on.

    My concern was that, if the bales were put on a relatively jagged rock base, they would sink down over time and the whole bench would begin warping and cracking. By creating a large flat space, the weight of the bench, and those sitting on it, will be distributed evenly over the whole foundation.

    The material used to create to pad was a simple clay-sand mortar composed of 1 part clay to 2 parts sharp sand. The first got to use this kind of mortar at a Barrel Oven Workshop in Portland tought by Eva Edleson of Firespeaking, and hosted by PDX Permaculture Meetup group.

    We used the clay mortar to stick together all the bricks for the oven. It worked superbly, and was superbly simple to make. In the case of the Barrel oven. an important benefit of the clay-sand mortar was it's imperiousness to the high heat conditions associated with the oven.

    The sand is important in the mortar, because it reduces the capacity of the clay to shrink as swell as it takes up and released moisture.

    clay-sand mortar. Note the texture.

    It is analagous to the combination of bricks and cement/lime-based mortar. The bricks are like the particles of sand, and the clay is like the mortar. The clay is sticky and he sand is hard.

    The desired goal of the clay-sand mortar mix is to add just enough clay to coat all the particles of sand with a thin film of clay. The compressive strength of the mortar goes as you add more clay to the mix.

    So you can imagine, the feel of the clay-sand mortar is...very sandy. It is very gritty. When you rub it between your fingers, the sound of the sand rubbing together should be distinctly audible.

    I'll talk more about clay and sand later when I get to the contents of Cob.

    Mortaring in progress, and the piglets out gathering acorns underneath an elder Oak.

    The clay sand mortar was first applied to the rocky surface of the foundation, making sure that the holes were filled in. More mortar added on top until 1-2 inches thick, and completely level.

    Close up of the finished foundation

    The mortar was then covered with a tarp and allowed to slowly dry over the course of a few days. Voila! ready for the bales.