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Rocks in clay for cob?

Roger Brown

Joined: Mar 06, 2012
Posts: 8
Location: NW Arkansas
I live in NW Arkansas. Anyone who has been here knows there are rocks everywhere. But, we have this beautiful orange clay here that I am thinking would be great for cob building, which I intend to do. My first question is how many rocks in your mixture is acceptable? It is easy to get the large rocks out. But, what about the smaller (1 inch to 1/8th inch diameter) rocks? How small is small enough to leave in the mixture? Does it strengthen or weaken the mixture if you leave more rocks or gravel size pieces in the mixture? etc.....any comments?


nadine bal

Joined: Mar 09, 2012
Posts: 2
That was interesting. Its nice to put on natural materials to build a house or building.
Suzanne Cornell

Joined: Apr 19, 2012
Posts: 19
Roger, did you ever get an answer to your rocks question? I too live in an area of many rocks in the soil and have been pondering the same question. I won't start building my house until next Sumer but I am planing on experimenting with a small bench/ cooplia (sp?) this summer. I am making experimental bricks to see what mixture is strongest before I start. I will let you know how the bricks with stones hold up if you like. Or I guess you could make your own bricks with differing amounts of stone incorporated into them. "They" say to throw them around try breaking them, and slam them aginst something. The weakest ones will break. The ones with too much clay will crack, the ones with too much sand will shed sand when rubbed gently. They recommend trying different amounts of soil/sand/ straw mixtures and in your case stone, but caution that you should carve the proportions into the brick so you don't have to rember which mix is which brick. Hope this helps. Good luck on your building!
Drew McGlothlin

Joined: Apr 28, 2012
Posts: 1
Hi Roger, I also live in NW Arkansas, near Winslow. I just started digging the foundation for a cob toolshed ( about 140 sq. ft.). I also have that orange clay, and the clay to sand ratio looks about right. I've read in some of the books that people add rocks to cob to speed up the building process. So thats my plan, remove the big rocks, and build. Not that many small rocks in my soil, most are golf ball sized or bigger. Don't know how the cob stomp will work however- I probably will be wearing shoes. BTW I will be digging and gathering all soil by shovel, so it won't be hard to remove the big rocks, but I'm not planning on straining the soil through a screen. I hope someone who's already built, and knows, replies to your thread.
Kate Nudd

Joined: Dec 09, 2010
Posts: 103
Hi Roger
I have 'some' cob building experience( helped on a cottage build on Vancouver Island,did a cob oven in interior BC and cob bench in south Alberta)
1/8inch size rocks are fine with the occasional one inch one.
I keep coming back to what the structure is.....clay is the bond for the sand granules and the straw lengths. I used to think that it was the clay that was the main building block when in fact it is the sand. So when I put rocks in that equation,I think can the clay that I squish around everything in the mix hold the rocks as building material.
Yes, those one inch ones hurt the feet a bit and then I would just pick them out but the occasional one will not topple the wall.
Perhaps try making some test blocks with a greater and lesser amount of the stones in your clay and see how they dry and hold together.
Also, it would also depend on the quality of the clay it pure clay or how much silt/soil is within it?
And, yes, people do put 'things' in the walls to help fill the space and build up faster...this would depend on the width of the wall you're building.
Ianto Evans suggests in his book that 'to reduce the amount of cob needed,you can add hard,non-compostable objects as filler in the middle of the wall.'
Eg: rocks,chunks of concrete,bottles,ect. He writes more about using these on pg.188 of The Hand Sculpted House.
All the best with your build. It sounds like a wonderful project using your colorful clay.
Roger Brown

Joined: Mar 06, 2012
Posts: 8
Location: NW Arkansas
Hi, again. It's nice to see the response here. Drew I would like to see what you are doing with the tool shed sometime. I'm in Newton County, not too far. Have you started the build yet?

I was reading Ianto's book and they indicate that the random shape of river gravel, sand is the perfect thing for cob. So, I went down to the end of my drive where the creek runs and
gathered some gravel and screened it to 1/4 inch I think and got some of this great orange, almost pure clay along my driveway and made some cob. It turned out really well I think.
It is just a slab about 18"x6"x6" sitting on a large stone. Worked perfect. So, now I'm going to start to test different recipes. This one was about a third clay and I just sprinkled in straw
until it got hard to mix. It actually feels sandier, grittier than I expected. I am going to make a bench also. And, I need to find some cheap or free fire bricks to make a rocket mass heater
test unit. I will update again and hope to hear from any others in NWA area. It would be good to know people to pool resources.
Matt River

Joined: Jun 30, 2012
Posts: 36
I live at a river delta at the end of a long river of clay. Several types on the property, trying to learn to identify them somewhat. I am using my own pit-mined gravel for my cob - it is sandy with some pea gravel in it, with stones up to four inches in size as well although I try to dig the sandier stuff. No screening. Mixing it with an all-terrain forklift in 20 yard batches (the boom extends almost 30'). Good gravel and good clay makes a very hard, very durable cob. Not for cob-dancing, need shoes to compact etc. As long as there is binder, clay, and aggregate, your cob will work. With a quality clay and lots of straw or grass, your cob will work very well whatever your aggregate as long as the ratio is right.
Holly Dozier

Joined: May 07, 2013
Posts: 2
I know this is an old thread ... sorry. Im just learning about Cob and I have a question that I can not seem to find the answer too. Everyone is concerned about heating Cob in the winter ... what about cooling it in the summer for us folks that live in NWA. I live near fort smith and the thought of a house that is going to generate heat in the summer is NOT good. Can someone tell me how this holds up to keeping it cool in our hot arkansas summers?
heidi shackelford

Joined: Jun 08, 2013
Posts: 17
Holly, i live just south of Tulsa and I'm new here too. i have been doing research on all different methods of natural building. i am planning a round timber frame from white oak, cedar and i have some reclaimed utility poles i will use for the largest section of my house. after the frame is up then i will "in-fill" with cord wood, cob, and stone. my problem is I'm not 100% sure what to put where for max heating and cooling efficiency. some say put the thermal mass on the south so the sun will heat it and it will transfer into your house at night and heat in the winter. long eves help give it shade in summer but as you know even the air in this region gets "thick", hot and humid, and i can't help to wonder how cob or stone will stay cool with temps and humidity at our levels even if it's shaded. currently my sight it thick with trees and i will start cutting only where my house will sit and then i can clear the rest when i see how the sun is landing across my house during different times of the year and where i need more or less is a great sight. it will show your exact location and you can type in any day of the year and anytime of day. it will show you the sun pattern as it rises and sets. very neat and very helpful. good luck with your house.
subject: Rocks in clay for cob?
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