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Organic Material in Cob

 
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I am digging to get heavy clay soil for cob.  Not because it has a lot of clay, because it's all I have on site.  

Anyway, digging between two trees, I'm digging up a lot of very small, very flexible roots.  Could I use those for part of the straw or should I soft them out for a hugel-type garden bed?
 
pollinator
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Organic material will decompose, eventually.  When it does, it will leave voids in your walls.  Voids are bad.  Will it cause a wall failure?  No one can tell.  Too many variables, but they will be there; and over time may need to be repaired.  Know the risk and use best judgement.  My OPINION is some would be fine, but minimize where you can.
 
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Hi Chris;
What were you hoping to build with this cob?
As a structural build I would be very cautious.
However if you are thinking of building an RMH or a pizza oven then your roots would be no different than added straw.
 
Chris Bright
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Jack Edmondson wrote:Organic material will decompose, eventually.  When it does, it will leave voids in your walls.  Voids are bad.  Will it cause a wall failure?  No one can tell.  Too many variables, but they will be there; and over time may need to be repaired.  Know the risk and use best judgement.  My OPINION is some would be fine, but minimize where you can.



Straw is also organic material.  It was my understanding that the long, stringy fibers in straw helped reinforce cob the way rebar is used to reinforce concrete.  Is that wrong?  It would seen that there would be minimal, if any, decomposition of straw or other organic material once the wall is cured and dry.  Lime plaster to finish the exterior.  Clay slip paint on inside.
 
Chris Bright
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thomas rubino wrote:Hi Chris;
What were you hoping to build with this cob?
As a structural build I would be very cautious.
However if you are thinking of building an RMH or a pizza oven then your roots would be no different than added straw.



I was looking at the stringy roots, any twiggy ones would get composted, along some branches from pruning.  Two of the builds are structural, a playhouse for the kid and a studio for myself.  The third build is sculptural, a hot tub that will appear to be a hot spring.  

There is some dead grass in the soil as well.  I still need to source aggregate, straw and lime.  And test cob, earthen floor and lime plaster mixes.
 
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I've had the same thoughts about roots in my clay and sand. I dug a pond to take clay from, and I get my sand straight from my wet weather creek. So they both have roots and the occasional grass rizome. I've been building for two years now and have notice zero problems. Now that I'm carving out parts of my wall for electrical I have run across imbeded organic material and its completely preserved. The problem with the idea of them breaking down is there is no water to assist. Any water is quickly absorbed by the clay in the outer parts of the wall and subsequently dissipated. So besides really large deposits of top soil or leaf litter, I have a hard time believing the whole "decomposing organic matter void" theory. Good luck.
 
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I'm with Johnny on this one. Other than my own similar experiences of busting into a cob wall after many years and still seeing well preserved grasses and straw I have also read where in Devon England where cob houses are many and built back in the 1500's, there have been the same testing to the walls and the straw is still intact and strong. The word Mummification comes to mind.
 
Chris Bright
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Thank you, Johnny Cobins, Gerry Parent.  So as long as material is “straw like”, I should be good using it to replace part or all of the straw,  

No branches or cut with branch cutter size hard roots, unless my goal is an anchor point for doors or rafters.

If the roof overhang is minimal, use lime plaster or lime paint to keep wall dry during wet times.
 
Gerry Parent
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Straw was somewhere decided on as a good material to use as it was inexpensive, sometimes considered a waste product, readily available almost everywhere, had good tensile strength etc.
but a lot of things can fall into this category too, so working with local materials just makes sense.
"A large hat and a good pair of boots" has always been a key statement used when building exterior cob walls. Don't rely on lime plaster or paint too heavily though, make sure you maximize your sombrero 'hat' overhangs as much as possible.
 
Johnny Cobins
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       If you are trying to keep water out of an unprotected wall, I would give it several coats of a very natural, like not the painters version, boiled linsees oil. I dont really like that most commercial grade boiled linseed is so toxic, and I have been doing some R&D with shellac. I think shellac can be a superior water resistant coating for cob. Neither of these products should be used as a complete solution for whole house due to the lack of breathability, but as large treated area the cob turns into near concrete and doesn't suffer from dust-off. After all the solvent in shellac is gone, shellac is food-grade if using a pure product. Commercially its used in nail polish, candy coatings, and pharmaceutical coatings for pills. You can also mix natural pigment with shellac and paint it directly on cob like house paint.
      Pic below is shellac and pigment paint on top, then pigment and boiled linseed oil and lime and paint , the green paint on black square is boiled linseed oil paint over shellac, bottom design and black square are boiled linseed oil applied directly to cob. Everything is has just one coat.
20191125_085738-2.jpg
[Thumbnail for 20191125_085738-2.jpg]
Top(orange color) is shellac and pigment
 
pollinator
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Johnny

The wood finishing sites do agree with you somewhat viz shellac. In addition here is one quote the raises a good point"

"
By Bob Flexner
Posted February 21, 2018  In Flexner On Finishing, Flexner on Finishing Blog

I’ve written this many times and said it many more. The film thickness of a finish is much more important for preventing water getting through to the wood than the type of finish. For example, polyurethane is more water resistant than shellac. But three coats of shellac is much more water resistant than one coat of wipe-on polyurethane.
"
https://www.popularwoodworking.com/flexner-on-finishing-woodworking-blogs/finish-type-thickness-determine-water-resistance/


Another site  notes that it can withstand water for 4 hours.
"https://www.shellacfinishes.com/introduction/"

There is much discussion out there, but most of it relates to wood. Whether that translates into an effective water protection for cob may require careful verification, though. When it gets applied relative to the dry-out cycle for cob probably matters, also. One quality doesn't  by itself mean a method will survive the real world. It's usually a set of complex compromises.


Regards,
Rufus
 
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In order for decomposition of organic material to occur, a couple of variables need to be present: oxygen and moisture.  If you have one but not the other, the organic material will remain.  This is why straw-bale walls in homes do not disappear.  As long as they remain dry, they don't decompose.  In the same way, old logs sink to the bottom of a lake and remain there for decades or even centuries: no oxygen at the depths of the lake.

I've seen 100-year-old adobe made with straw and mud, and the straw fibers were as complete as the day they were first cast into the bricks.

Once your cob dries, as long as you keep it dry, the organic material therein should never decompose.  The lack of air inside the cob will be a major impediment to either fungi or bacteria, the two primary decomposers of organic materials.

Best of luck.
m
 
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