Building the Cob and Green Roof Garden Wall- Part 2: The Cob!

We started laying on the cob to our wall in March 2014. The earth for this project came from a construction site just down the road. It was a beautiful mishmash of grey clay, grit, and stones and required only the addition of straw and water to get a good consistency for cobbing.

We manually mixed and applied all the cob on this wall, bit by bit, aching muscle by aching muscle. It took three months of two of us working on it part time (plus the odd hand from some generous friends) to get the first section (from the T junction through to the gate) up to height. During this time we also cobbed the second section of the wall (from the gate to the other end) but left it extremely rough and unfinished as we have plans to extend the far end of the wall in the spring of 2015. The first layer that went on was both exciting and extremely anxiety provoking as we had no idea what was to come and wondered what on earth we had gotten ourselves in to.



Jen enjoys an end of cobbing day fire.


As we worked we continually had ideas for various details which were incorporated in along the way.

To prevent the wall feeling like a solid barricade, we incorporated circular openings in to it. These were accomplished using 2′ diameter sono tube as forms. We wrapped these in newspaper to facilitate removal, placed them where we wanted them, and then cobbed up and around them.


Sono tubes in place. The opening between them will become the arched bench.


This seemed like a great idea in its conception but proved to be more trouble than it was worth. The sono tube forms are designed to withstand expansive forces from within, not the compressive forces from the outside that we exacted on them. Because of this, the forms, which were cylindrical to start, became distorted out of round as we worked cob around them. When the time came, they were easy to remove as they simply unravel when you peel them apart, but they left extremely wonky ‘circular’ openings that we had to go back and chip at and build up to get a round shape. What a pain! These openings were eventually filled with old metal wagon wheels.

We have since taken to building wooden arch forms and using those to make circles. We build a wooden arch with a circular profile, slightly more than a semi circle in size. We place it arch side down on the wall to begin with, cob around it til we get to the height of it, remove it and then place it back in arch side up to complete cobbing the circle opening. This works MUCH better!


We also incorporated an arched bench opening that is 3′ wide and provides a nice covered sitting area. We built this using a custom arch. form and then carved candle alcoves in to the walls of the sitting nook and Jen designed and cut a metal cutting for the backing.


The left most opening shows how wonky the rounds became. The arch form for the sitting bench is in place and getting cob worked over it.
Jen draws up the design for the arch bench metal backing. Red wing black birds are a large part of our back yard cacophony so they too (along with swallows and starlings) have made it in to our cob structure art.
Arch bench plasma cut metal backing.


Jen carves out the candle alcoves in the arch bench.
Candle alcove in the arch bench. The dark cob is manure plaster. The back wall metal cutting is partly visible.


We decided to incorporate a fireplace in to the wall which is a post in and of itself and flanked the fireplace with circular openings that we ultimately fitted with plywood shelves. We use this as a wood storage and drying area.

Lastly, in the T wall we incorporated a water feature, also a post of its own.

As the wall got taller we began to think about what we wanted its top profile to look like. We decided we wanted a curved profile and first shaped it out in a blocky stepped form. Once we were generally happy with that shape we went back and cobbed the steps smooth.



These curvy curves were sculpted by Ryan. A wagon wheel is visible in the circle opening to the right.




Once at full height with a nicely contoured roof, we took a saw and lawn edging tool to the wall and shaved the walls smooth and flush with the foundation. Any low areas were leveled up with a cob and horse manure plaster. We simply made a wet cob batch, the consistency of icing, and then stomped fresh horse manure in to it. The ratio was roughly 1 part manure to 10 parts cob. Manure makes the plaster stick beautifully, it is essentially entirely composed of finely chopped fibres and is completely (and surprisingly) free of odor when dry.


Shaving the wall smooth


Jen shaving away.

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