Where is this project?
This chicken coop is located at my son's school. Part of their curriculum is gardening; I offered to build one to allow them to keep 24 chickens.
Why was rammed earth used?
I've built a shed and a retaining wall; this gave me an opportunity to refine my skills in rammed earth. That said, rammed earth is durable, cheap and beautiful. It's very forgiving and practically anyone can build with it. In keeping with the vision of the school, it was a natural choice.
How thick are the walls and what size is the structure?
It's 9 ft x 15 ft. The walls are the same width as concrete block: just over 7.5 inches. This is extremely thin I know but since I was not so much concerned with the thermal flywheel effect as being inexpensive and quick (less material and less time ramming more material). I used concrete block in an effort to quickly build a stem wall and that was the width of the block. My utility shed in my backyard has stood for about 3 years with no problems using the same width.There is no reinforcement in the walls.
How did you attach your doors and windows?
The windows were built in place - I don't recommend it. Sure for a chicken coop it's fine but they were not square. I used scrap wood to make a volume displacement box (as described in Easton's book) but instead of removing them, I simply left them in place (I put screws through the boards before ramming to make sure they were secure). Using these 'gringo blocks' - in Adobe parlance - I attached 1/4 inch hardware cloth and nicer trim wood to make "windows". This serves a dual purpose: allow the kids to view the chickens when they are not in the yard and, more importantly, air out the coop!
The door was built from scrap 2x4 lumber culled from disassembling the forms and walers.
I attached hardware cloth to the frame and voila! The door! The door jamb was made from (somewhat warped by then) wider walers attached to the rammed earth wall using concrete screws. I had to use many of these to provide enough friction to secure it.
What was your forming system?
One corner form made from plywood, 2 x 6 and 2 x 4.
One straight form from the same materials.
I used 3/4 inch black iron pipes and pipe clamps.
The corner form, straight form, pipes and clamps were like the ones described in David Easton's "The Rammed Earth House"
How did you finish the top of the walls and attach your roof?I poured a bond beam -not very well I must admit. Like I said above, rammed earth is forgiving. More so than I am! I used metal angle brackets and concrete anchors to secure the wooden roof to the bond beam.
What type of equipment did you use?
I used a 4in x4in x about 3' piece of lumber attached to a 3 ft long 1 inch diameter steel pipe. For corners, I tamped with 2x4s or whatever was available and many work gloves. I used a concrete mixer to mix the cement, dirt, and water. Five gallon buckets were used to lift dirt into the forms.
Since I was trying to do it on the cheap, I did not rent a backfill tamper and air compressor. The tools would have cost greater than $100 a day. I believe it cost much more in time; if I did it over, I'd spend the money on lumber to create a single form encompassing the project and rent the power tools!
How did you come up with your mix?
A few years ago when I was making adobe, I tried out different mixtures here at my house. I found the nearest quarry has at least two mixes that work. I performed 'field tests' suitable for adobe blocks and the data gathered from my 3 years standing structures in my yard.
So now, I can simply have the dirt delivered if I doubt the integrity of on site dirt.
I admit that this was the most daunting task for all my rammed earth endeavors. I had no experience with knowing how good was 'good enough'. I was very cautious at that stage; once one works enough with soil mixtures one gains confidence. For instance, I can tell if a mix has too much silt by the sound it makes when I rap my knuckles on a test block! That took a lot of knuckle rapping and drop tests though!
How long did it take to build the walls?
I found I could mix, and tamp about 6 hours a day (on the weekend). I believe - and it's at least reasonably close - it took 20 minutes to collect and mix, and 20 minutes to tamp 3 five gallon buckets. On occasions when my wife mixed for me, it cut time in half and allowed me to labor longer.
I work full-time and was playing in two bands, so my time was limited to weekends and a vacation. Some injuries and inclement weather pushed this project to two years! My wife estimates 160 hours with 60% of that time was me working solo.
Knowing what I know now I'd:
1) Build a single monolithic form - or cost prohibiting - as large as possible. Forget modular as much as possible. My time and enthusiasm are more valuable!
2) Use tamper and compressor and at least one concrete mixer
3) Take two weeks off from work to prepare foundation and build the form and have dirt delivered.
4) Use a whole day on the weekend (not the whole weekend!)
I believe after the the form was up and secure, actual mixing and tamping using just my wife and me could be done in four days - four saturdays. But I haven't used the tamper so it's a guess.