Cob and clay brick combination

I saw this picture today and it inspired me. I am interested in using a combination of fired clay bricks and cob for my build. I was thinking at least a brick foundations/stemwall, and perhaps also brick load bearing columns and cob infill. Anyone got some practical ideas or experience to share about doing this? Are there any issues with adhering the cob to the clay brick? Does it shrink and pull away? Does the use of clay fired bricks affect the breathability and moisture movement of the cob wall like concrete does?

Using similar materials (clay or binder type mainly, and aggregates, brick/COB/mortar) of the same density and plastic index(PI) will be key at the interfaces in this hybrid design so they expand and contract at the same rate or are not too far off each other. High density will be less permeable or "breathable" but will be more structural and visa versa. PI determines expansion/contraction. How breathable and how structural will depend on seismic and wind activity an Engineer can determine structural requirements. If they spec out a high density brick, mesh and bar reinforcements, then other less dense hygroscopic materials like an inner plaster can be used to re-gain breathability. In harsh cold climates, double wykes may be required. There the outer wyke can be a structural rated brick to take loads, the inner can be a brick_COB combo for breathability and anesthetics. The center insulation core/vent cap would serve as a thermal brake an ventilation gap. The core r-value would depend on climate. Roxul makes an rigid mineral board IS that is high perm (30 ish) for applications like this and they are rated to be in contact with soils to also be used as foundation outsulation.

What I do when I'm not sure how designs will perform is build a mock-up in the climate zone, add radiant heat from a heater and/or moisture from a spray bottle and see how fast it dries or if it cracks. I'll even soak it in water or spray it with a hose. Temp and moisture meters always help. A pro lab is always best.

... yeah I am sorry but I honestly don't understand any of what you are saying! I think that all just went right over my head.

Terry Ruth wrote:Using similar materials (clay or binder type mainly, and aggregates, brick/COB/mortar) of the same density and plastic index(PI) will be key at the interfaces in this hybrid design so they expand and contract at the same rate or are not too far off each other. High density will be less permeable or "breathable" but will be more structural and visa versa. PI determines expansion/contraction. How breathable and how structural will depend on seismic and wind activity an Engineer can determine structural requirements. If they spec out a high density brick, mesh and bar reinforcements, then other less dense hygroscopic materials like an inner plaster can be used to re-gain breathability. In harsh cold climates, double wykes may be required. There the outer wyke can be a structural rated brick to take loads, the inner can be a brick_COB combo for breathability and anesthetics. The center insulation core/vent cap would serve as a thermal brake an ventilation gap. The core r-value would depend on climate. Roxul makes an rigid mineral board IS that is high perm (30 ish) for applications like this and they are rated to be in contact with soils to also be used as foundation outsulation.

What I do when I'm not sure how designs will perform is build a mock-up in the climate zone, add radiant heat from a heater and/or moisture from a spray bottle and see how fast it dries or if it cracks. I'll even soak it in water or spray it with a hose. Temp and moisture meters always help. A pro lab is always best.

I think what Terry meant was this:

The key to success in this hybrid design will be using similar materials.

Materials of the same density and plastic index (PI) will expand and contract at the same rate. Dissimilar materials expand and contract at differing rates, which causes them to crack, or to separate from each other.

If you use high density materials, they will be less permeable or "breathable" but will be more structural, capable of carrying heavier loads. If you use lower density materials, they will be the reverse: more "breathable", less structural.


Your criteria for choosing how breathable to build your walls versus how structural are seismic and wind activity. An engineer can determine the structural requirements for you.

When you consult the engineer, if they specify a high density brick with reinforcing mesh and bar, then that will be a wall that's very structural and not very breathable. If you want to regain some breathability, then you can use other, less dense hygroscopic materials like an inner plaster.


Masonry is a poor insulator, so in harsh cold climates, you may want to build a double wall system. If you do, the outer wythe can be a structural rated brick to take loads, and the inner wythe can be a combination of unfired bricks and cob, for an example of breathability and aesthetics. The gap between the inner and outer wythes would serve both as a thermal break and a ventilation gap.

The r-value you should try for, that would depend on your climate. Roxul brand makes a rigid mineral board that is high-perm (30ish) for applications like this, and they are rated to be in contact with soils, so they can also be used as foundation outsulation.

What I do when I'm not sure how designs will perform is build a mock-up wall in the climate zone. I add radiant heat from a heater and/or moisture from a spray bottle, and then see how fast it dries or if it cracks. I'll even soak it in water or spray it with a hose.

You can learn good information this way. If you can use temperature and moisture meters while you do it, that's even better. Setting it up and having it tested in a professional lab is always best.

Thanks, that's a little more understandable.
Honestly though, I don't really want to hire a structural engineer... might have to just stick to one material to play it safe.

Thanks Mike. Great job interpreting my post. Professionally I am use to writers such as yourself called "tech publications" putting my Engineering into words for mechanics and builders. I won't be posting much more on this site I can always be reached at my contact info below for professional advice.

I'll be offering pro natural building Architectural 3D modeling and drawing's to include moisture management analysis soon. I'll let you all know when. Similar materials or not without it "natural" designs will fail.

It's be great collaborating with the body of people on this site. Good luck to you all!

Similar materials or not without it "natural" designs will fail

Without what?


Good luck to you too Terry!

Without the proper knowledge & analysis, common Kris you commented on many threads especially COB failures you did not understand. Hey Kris you got a lot of great intuition and post and your heart is in it feel free to moose me anytime if you are struggling answering questions I am surrounded by alot or world class professionals if I do not know I can find out Good luck to you too! Bro!

Hi Fianou,

The photo is striking.

I don't know about the expansion and moisture stuff posted above. They probably are important. I would not consider such a hybrid for other reasons.

When building with Ianto Evans, we did include sizeable pieces of rock and broken concrete, but they were completely embedded in the cob. This was to decrease the amount of cob we had to mix, and make it possible to build more height into the "lift" before we had to wait for it to dry. The wall was still a cob wall, and it was important not to put in too many rocks too close together. The rock had to be the minor part of the wall, with the straw embedded in the cob thoroughly wrapped around each piece of rock, just an over sized piece of aggregate. Probably related to the engineer stuff.

Another thing that I consider worth mentioning: cob does have load bearing capability, but when you use it for infill, you are making something very heavy and if there are no corners or curves in it, what is to hold it up in an earth quake, or if the ground settles unevenly beneath it? (I imagine a card on its side. Hard to balance. If you fold a corner into it, there it stands, and it can't fall over.

When I build with with traditional materials, I believe the safest thing is not to try to mix it with modern methods. I think the traditional materials withstood the tests of time, but they came with instructions and tutelage. The methods were embedded in a culture and the wisdom to use the methods and the limitations and caveats were passed right along.

Good luck with your building process, what ever you decide to do!

Thekla