Since I used scrap-wood and earth already available to me, I only had to purchase a few items. Total Cost: $30.00
Rather than focus on building the actual structure, this instructable covers the cobbing part of the project.
This project was spaced out over two months to give each step plenty of time to dry, settle, harden and adjust.
Step 1: The Structure
The living roof is around 5 inches deep, with a layer of roofing tar at bottom (dried properly), thin layer of gravel on top of that, a layer of old cotton rug found at the local thrift store on top of that, and finally the soil, I have 2x4's spaced out to help hold the soil in place (you can just make them out in the photo). Also, plenty of drainage along the edges for excess water to run out of. Note that there is plenty of roof hanging away from the house all the way around.
Step 2: Underneath the Cob -stain, Paint and Moisture Barrier
The result: sturdy wire-fencing held 1/2 inch away from the surface. This will be what holds the cob in place.
You can also see that I have brought in rocks and earth around the base of the playhouse. This gives it more thermal-mass, which is how this structure stays cool during the hot summer. Thermal mass will absorb the heat and release it at night.
I also added sand in front of the house for comfortable bare feet playing
In the second photo, grass is already growing on top of the roof. We did not have to plant anything, as the dirt we put on for the top-layer already had the seeds and roots in it.
Step 3: Muddy Muddy Cob
The sandbox was almost impossible for the kids to play in during the summer because it was in the sun, and also had turned into a kitty-litter box. (burning hot sand with stinky cat poo, yck!)
Converting the sandbox into a more fitting play-structure for our climate was the right move.
As there are plenty of websites that cover mixing cob, I will forego that here. I will just go so far to say that most of the websites I read about cob were people building serious houses, so they were very rigid in their requirements of fine-sifting the dirt and carefully measing the mixing amount.
Since I was building a mere playhouse, I was not so hard on myself. I simply dug up the natural clay, mixed in sand and straw, and plopped it onto the house.
It was A LOT OF WORK! Even this tiny playhouse required quite a few wheel-barrows full of clay that I had to dig up. We have a seasonal creek-bed that I can dig as much as I like. It had plenty of tiny gravel bits, clay and sand already, so I just mixed it with the sandbox sand until it felt right.
You have to use your feet to get the mix really blended well. Dont even waste your time trying to do it with the shovel.
Mix the mud, straw and sand until it is like peanut butter. Plop it on the wall starting from the bottom...continue until it is completely coated.
Let it dry for a week or more
Step 4: The Final Layer
By making a mix of some slightly sifted clay, sand and cat-tail fluff,, you can add a final layer that will seal those cracks and provide a lovely light-brown finish. It does not take many heads of cat-tail, as each head will put out a huge amount of fluff.
You can also use horse-manure, which also has a fine fiber in it that will work. I used a mixture of both the cat-tail and horse-manure, since some nearby neighbors have horses and it was easy to get to. Once it is mixed up and dries it DOES NOT smell like horses or poo. Just a clean sort of earthy smell.
Apply your final layer..about an 1/2 to 1 inch thick. let dry..apply another if you need to.
This structure has held up fine for one year. It lasted through winter rains, and 2012 hottest summer on record. Inside the house is cool and dry. The little porch has play-sand (mixed with rounded-smooth river stones to discourage cats but still be comfortable on little bare feet).
Cob structures are also fire-resistant. One other thing we have noticed is that no wasps have made nests here, which is unusual for our area. Generally we have wasps-nests on every structure available. For some reason the wasps have shunned this play-house.
This small playhouse has, in total, over 40 wheelbarrow-full mounds of dirt. The roof, walls and around the base are loaded with dirt and rocks. That is where most of the work is: hauling dirt.
However, the end result is a huge amount of thermal mass that easily absorbs that summer heat, and provides a comfortable place to play for the kids.