Organic Material in Cob

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?

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.

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.

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.

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.

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.

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.

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.

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.

       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.

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

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

I agree that you would be fine with the roots.  As a ridiculously obvious reason why straw isn't mandatory as the fiber ingredient, you can toss it in the compost and it will rot just like anything else.

I think insect infestation may be a valid concern when deviating from straw, but once again as long as the wall remains dry I don't think insects will want to bother trying to gnaw through hardened clay.  I think that's part of the reason why you strip a log - it helps to keep the surface from catching and soaking up too much water, which in turn keeps the fibers tough and hard to chew through.  I could be wrong about that, though.

There are earthen walls over 1000 years old in my region that are unprotected and have straw bits visible, undecomposed. After one thousand years. But this region is high desert with infrequent precipitation and generally low humidity.

Normally if you want to keep earthen walls in good shape, you make sure to protect them with a good roof overhang so that precipitation won't hit them. There are some earthen building experts who say that a waterproof coat is not a good tactic in the long term, due to condensation inside the wall. However other people are using waterproof coats successfully, at least for ten or twenty years.

Straw is not a waste product in our region, so we've tried mixing other fibrous plant materials into earthen walls at various times. What we found is that some materials if mixed in while still slightly green seem to shrink or twist or something while drying, and cause surface crumbling until they get exposed. And some chopped shrubby materials seem to keep pockets of air around them and don't mix in as thoroughly as straw does. We find that wood shavings and autumn leaves (the only free biomass waste products we can get) make the earthen material crumbly -- maybe good for infill, more insulating than straight earth, but not good for structural walls. Not all materials work the same as straw in earthen building.

We've also made a lot of rammed earth walls (kind of like cob in long wooden forms) with no fibrous material mixed in at all, and those walls are also excellent, very sturdy, hard to break with sledgehammers, and fairly resilient even where exposed to the elements.

Rebecca Norman:  Have you tried long fibers of wood rather than just wood chips?  I am imagining something like 1/8" or smaller, a minimum of 12" long.

Ben Clurk wrote:Rebecca Norman:  Have you tried long fibers of wood rather than just wood chips?  I am imagining something like 1/8" or smaller, a minimum of 12" long.

No, we haven't tried that, I think. It's not really a waste product here. I have seen occasional little twigs in old earthen walls that seem to be okay. You should test it out in some adobe brick sized pieces of cob, and dry them thoroughly, then see what the results look like. There might be some issue with swelling in the wet cob and then shrinking more or less than the cob does while drying. Or it may work great. There may be a difference if the twigs are fresh wood or have been dried already.

I would have preferred to stick with straw because it is proven to be a great match, but I don't have access to nearly enough to do anything with.

I was thinking specifically of shaving down some of my most crooked logs, taking care to get nice, long shavings with my drawknife, and then further splitting them with my pocketknife.  A slow approach, no doubt, but I am afraid to trust rammed earth because the walls will be going on a very crude stone foundation that is bound to heave all over the place in the winter.

I did a small test block of pure subsoil without fiber a while ago and was impressed with the results.  I'll take a larger sample and do some more blocks with the wood fiber.

Thanks for mentioning the swelling issue; I hadn't really though much about it.  I will try saturated, green (fresh-cut tree, ambient moisture), and seasoned to see if there is a difference.  Saturated, flexible fibers seem like they would be best, if possible.  I think I'll also try it with heartwood, for extra decay resistance.

As soon as I have a dry day I'll go out and dig some more for the tests.

"a very crude stone foundation that is bound to heave all over the place in the winter"

This is the place to focus your work on. No matter how good the cob or earthen structure, an unevenly shifting foundation will destroy it. You need to make your foundation deep and wide enough to be below the frostline. If you have expansive clay soils that swell when wet and shrink when dry, there is probably nothing that you can do to make a rigidish structure last for the long term. In that case, if you have to build there, I would go with something like a timber frame that can flex a bit, and infill with mass/insulating material in smaller panels that can shift independently a bit.

If your foundation is shallow, you may be able to use a Scandinavian-developed technique of digging down a foot or so outside, and running insulation down the wall and horizontally the same distance you would ordinarily go vertically.
Depending on your situation, I might consider raising the planned floor level, adding enough rubble to support that, and filling around it with earth sloped up to protect the final depth from frost.