Yay! The cob hen coop is finished. There's even a bit of grass growing on that roof. So here it is: How to make a smallish (1.5m diameter) cob chicken coop, including a living roof and limecrete floor (yes my hens live in style).
Note to all the folk who love to read this stuff, but never do it: This is the one time you should quit the procrastinating, and just give it a go. A cob coop/kennel is perfectly doable alone, it costs basically nothing, and you don't need a permit for it. That's it. I've just set a match to all your excuses:) No garden? Make a small cat house in the park or something! An animal shelter is a super way to practice using cob. It’s basically a miniature version of a house, so you get to try out all the techniques on a manageable scale before committing to a more ambitious structure. Yup, it really is time to get your hands dirty!
First, dig a small trench/circle for the foundation. This is a hen house, not a castle, so it really only needs to be 10-20cm deep. Fill it with nice fat rocks. Then fill in the gaps with smaller rocks.
Adding gravel over the top isn’t strictly necessary, but it fills in all the tiny gaps and creates a nice flat surface to build on. This is especially useful if you’re adding a limecrete/earth floor.
Mix up some cob. I used roughly one part clay, one part soil, one part straw, some horse manure ,and two parts sand. But each person’s mix is different, so if you want to understand more about cob and earth plaster mixes, sign up for the free earth plaster course.
Now add another ring of rocks for a stem wall (to raise the cob off the ground so it is protected from rainwater runoff). This is an important feature with all mud buildings, so don’t skip this part unless you especially like watching cob disintegrate in water.
I wanted to test out both a limecrete and earthen floor in my climate, so I made the chicken coop floor half and half. I also wanted to test out whether a vapour barrier was a good idea or not. Hence I laid some plastic between the gravel and the mixture in one half of the coop, but slapped the mixture straight down onto the gravel on the other half. Now that I’ve seen how it functions, I think these natural floors work much better without the vapour barrier (others disagree). What I’m seeing is, the half of the floor where I stuck the vapour barrier doesn’t dry out properly because the damp can’t evaporate.
Take your cob mixture and slap it on top of your stem wall to create your first mud layer. Wait for this to dry before adding the next layer on top. In my climate (damp) I found I could lay about 30cm (height) of cob mixture at one time, after which it began to buckle.
Add your door frames wherever you want them. These need to be properly anchored into the cob mixture. You can see from the photo how I created small lintels above and below the door, which extend into the cob. These were nailed into the cob mixture below. I also added nails (see the red lines) into the side of the door frames so the cob has something to stick to.
Keep building up and around the frames until they’re completely embedded into the structure.
Once you reach the top, add some ‘wall plates’ for your roof to sit on. Again these should be anchored into the cob. I nailed mine in.
Before the roof goes on (if it’s a smallish coop), sculpt the inside. It’s a lot easier this way.
Add your roof joists. I used bits of old wood I had laying around. Make sure they’re level, and that the roof is sloping gently from front to back.
Use some sort of roofing board to cover the joists. Cut it to the shape you want and screw it in place. Then add a water proof membrane of some kind. You can add hens as well at this stage, if you want.
Attach a rim to the edge of the roofing board to create a kind of tray for the dirt to sit in. In this case I used skirting board, but hey, use your imagination and whatever you have to hand.
Remember to add mesh at the back so that water can drain out.
For a living roof you need a root barrier. Bitumen based membranes, Onduline, or roofing felt won’t be enough because roots will soon eat through them. Sadly plastic is the only way here (as far as I know).
Normally on a human house you’d want a drainage layer here (see how to make a living roof here).
Because this is a tiny hen coop, I didn’t bother with that and stuck the dirt straight in.
In about six months the roof should be green. But in the meantime, the hens are happy and snug.
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