Rototiller cob

Thank you to whoever first came up with this idea! So fast and easy!

I am not sure who first came up with that idea.

In this thread from 2010 it is mentioned:

https://permies.com/t/5368/building-cob-mixer#46398

Maybe some folks who know will chime in.

I will add a pro tip that greatly sped up the process. I used our battery chainsaw to chop the straw (not the saw shown) so fast!

By the way the cob pit became instant butterfly "garden" to my surprise!

This idea may not be new, but it IS brilliant, especially for folks who have no help, have health issues that would prevent them from being able to mix it by walking in it, etc. For example, my ankles, knees, and hips could not possibly manage the task, at all, nor could John's heart, without doing it over the course of weeks. This is one of the primary reasons that even with our heavy clay, here, we couldn't do cob. But with the electric chainsaw and the rototiller, we could double-team it, and break it down into reasonable sized batches that we could mix one day, cover with a tarp, to hold in the moisture, and build, the next day. It would still be slower, with just the two of us, and the smaller batches than I've usually seen done, but it would be doable. GAME CHANGER!!

Absolutely agree. My wife is limited to lifting 10lbs at any given time because of osteoporosis but this method of building is very "plastic", forgiving and democratic. We are slow but it is amazing how much can get done regardless. She also insists on rubber dishwashing gloves.

Today we added a good pitch fork to move the cob to the wheelbarrow and from the wheelbarrow to a high table I made to make higher work easier. Before I used to struggle with a shovel.

I have found that after cobbing a section adding chicken wire and applying a scratch coat of lime plaster allows the plaster a long slow cure because the cobb takes a long time to dry! Should be ideal. I also add some plastic cement to the first coat for strength but probably not necessary. I mix the plaster only a small amount at a time in a bucket with a plaster paddle on my cordless drill.

When I mixed a large amount of cob, I started with a dry mix of powdered clay, sand, and straw.
Using a larger tarp I just flipped the tarp to mix my ingredients, then water was added slowly,
Keep rolling the tarp and walking on it, (Yes you can wear shoes) until you have the correct consistency.


Currently, with the growing popularity of brick stratification chambers, mixing large amounts of cob is now a choice, rather than a necessity.

I don't think the tarp idea is faster or easier, but it certainly wins for simplicity.

The main reason I need a lot of easy cob is to build the main house structure not a rocket mass heater.

I started building a cob privy back in 2016 (see my thread on the build). The first summer was spent mostly on the foundation and stem wall. I got a couple feet of cob wall put on, but it was slow going.

By the next summer, I realized I needed a faster way to mix cob. I found some YouTube videos, and from what I could see, they were using small front-tine tillers. The downsides they talked about were rocks getting stuck in the tines and the straw getting chopped up. One even mentioned having to mix in the straw by foot, which defeats the purpose. From the videos, the problem I saw was that the rototiller did a good job mixing the clay, sand, and water together, but it didn’t compact the mix at all. And compacting strikes me as one of the most crucial parts of making good cob. The more I thought about it, the more I was convinced that what you needed was a big rototiller. One that would have the power to cut through the hard, dry clay. One with big enough tines not to get hung up on small rocks. One with the weight to compact the cob.

I went to the online classifieds and found a second-hand Troy-Bilt Horse. Reviews of the Horse say this is the gold-standard for rototillers. It’s designed for big gardens, up to an acre in size. It  has wide tires, weighs in at two hundred fifty pounds, has two speeds, goes in reverse, and has adjustable depth. I decided it would serve my purpose.

I laid out a runway for the rototiller cob four meters long, digging down just to the hard-pack. I started by running the Horse through dry ingredients, but that didn’t work so well. It worked much better with water. I tried different techniques: running it forward and then throwing it in reverse; running it in circles letting the wheels spin in place; running it forward, making a U-turn, and running it through again. The latter-most technique worked best. With enough water mixed in, I threw in the straw and ran the Horse through again.

The first batch wasn’t as good as foot-cob. I couldn’t control the mix as well because clay from the runway got pulled in. And it used more water because without a tarp the ground soaked up some. The straw got chopped up a bit, but not so bad.  With practice, I got pretty good at it, and I'm convinced I was right about the weight of the Horse being an asset. One trick that helped a lot was to soak the straw and dump the whole thing, water and all, once the sand, earth, and water was mixed well enough. The biggest down-side is the noise.

Where before, I felt like I was constantly having to start a new batch. I could do one roto-cob batch in the morning that lasted the day. It took a little under an hour, and maneuvering the Horse was hard physical work. But it sure saved time!

On my present projects I don't need nearly so much cob--and I don't foresee ever needing to do that much again--so now I'm content to mix it by foot again. It's more pleasant...