[Cob] mixing cob with rototiller

[avjyoung at shaw.ca]

Chiming in here because we have experience of mixing with rototiller and 
small digger.... We borrowed a small digger with tracks, an arm, and a 
bulldozer blade from a neighbour for $25CDN an hour and parked it at our 
place so we could use it whenever we needed. We used the scoop as a measure 
for making cob, and ended up with a pit in the driveway to help contain the 
cob so it didn't spread all over, then scraped it out when done so it was 
easier to access. Eventually the neighbour needed it back, so we got a 
second-hand rototiller. We built a platform about 9' x 9' from thin scrap 
wood and that works fine to prevent the tines and cob from disappearing into 
the earth. The tines need cleared of long straw sometimes, and I mix a bit 
more straw into the cob manually as the rototiller doesn't seem able to get 
quite enough in. I think you can blunt the tines with an angle grinder if 
they are chopping through the straw too much. Rototiller cost $50CDN.

Amounts; mixing a small batch with a digger (one scoop clay, two pumice, two 
sand, straw and water to taste), gives a lot of cob. We had layers of tarps 
over it in the shade to stop it drying out and  sprayed it with the hose 
when it felt too dry. It was good for doing walls with, when I was getting 
through a lot of cob very quickly. With the rototiller, we mixed using a 
wheelbarrow as a measure. It ends up quite wet, more so if you don't cover 
it with plastic against the winter rains. I put a load in a wheelbarrow at 
the front and keep a flake of straw at the back, and mix a handful of straw 
into every fork load of cob. I think this helps counter straw rot from the 
longer drying time in winter, and helps to dry it out a bit. Still wetter 
cob, but smaller quantities, which works well for the interior furniture and 
walls I am doing at the moment. I just do multiple thin lifts to let it dry 
enough.

Cost; if you can figure something out like our arrangement with a neighbour, 
it's way cheaper than a hire from the tool rental place. The one time we did 
this, it worked out about $100 an hour for a little bobcat. You only need it 
for 3-4 hours to mix a batch, but still have to pay all the delivery fee 
etc. A rototiller is cheaper again, and doesn't usually make more than one 
person can easily use, so it's not sitting too long.

Overall, I would rather do either than mix by hand, as there are just so 
many other things that need done to build a house / raise a family, and 
mixing by hand is very time consuming. We are keen to get done so we can 
move in, after 7 years building so far, so saving time is getting more and 
more important for us.

Hope this helps your decision-making,

Anna
cob/bale timberframe in Victoria, BC