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Cob on top of stem wall  RSS feed

 
Kris Johnson
Posts: 80
Location: Pahrump NV
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I'm currently building the foundation to a tiny cob home. Once the foundation is complete I'll build a 2.5' tall stem wall. I was planning on building a bond beam on top of the stem wall and then starting my cob walls on top of the bond beam. But I am now wondering if the bond beam in between the stem wall and cob, will be necessary. The structure is quite small, 200 sqft and I live in earthquake zone D0. The cob walls will be around 3' thick. Even though the structure is un-permitted, I want to build it to last and be structurally sound, so am not willing to cut corners.

All recommendations welcome, Thanks!
 
Jay C. White Cloud
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Hi Kris,

As a professional in traditional/natural building, I must in good faith warn against structural cobb...especially in any area the is subjected to tectonic events. As a professional, unless working on historic architecture or related, I do not build "structural cobb" of any kind, and I probably have the qualification to actually do so.

With that stated...if I did...the walls would have to be very thick like in the suggested building that is planned for.

...I was planning on building a bond beam on top of the stem wall and then starting my cob walls on top of the bond beam...


What is being defined as a "bond beam" in this application?

What is it made of?

How will it be facilitated?

Yes...they are usually a very good idea.

If building a steam wall...a very good idea with cobb...I suggest a 1 meter height minimum if possible, and/or a well protect lower wall with proper wainscoting.

Regards,

j
 
Geoffrey Levens
Posts: 54
Location: Paonia, Colorado, USA
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Jay C. White Cloud wrote:Hi Kris,

As a professional in traditional/natural building, I must in good faith warn against structural cobb...especially in any area the is subjected to tectonic events.
Jay, does this mean you would not build roof attached to/supported by cob walls Only use cob as infill to post and beam structure or some such
 
Grant Doner
Posts: 9
Location: Denver, CO
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Jay C. White Cloud wrote:...warn against structural cobb...especially in any area the is subjected to tectonic events.

I'm sure it's a can of worms, but why?

From what I've read, it seems that cob/cobb does much better during seismic events than most structures. This is the first I've heard of somebody warning against it specifically due to tectonics. Curious minds...
 
Geoffrey Levens
Posts: 54
Location: Paonia, Colorado, USA
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Yes, good add-on. I am planning a cob build in near future and had planned on using outer walls (likely the only walls) as the roof support. Alternative would be post and beam to hold roof (built first) and then infill w/ cob.
 
Jay C. White Cloud
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Hi Geoffrey,

First...let me stress that I love clay in all its many permutations and uses. I love Cobb, and related natural building forms. What concerns me...and I have written about this several times...is the number of folks "thinking" and/or sharing "how easy it is to facilitate...At best this is very subjective...at worse...it is simple unfounded

Last year alone in a random search of "work shops" I found that a minimum of 5 or 6 out of 10 had ZERO architectural background...no engineering skills...and really did not understand anything about what they were teaching than what they had been taught of "interpreted in books."

To me, and those like me, that is alarming. So much can go very wrong in designing and building a structure, especially if long term durability is part of the presumed goal set. Reading a few books, and taking a work shop or two does not make one qualified to impart "building wisdom." Many of these individuals do not have even a basic clue at all about design, architecture, nor engineering.

...does this mean you would not build roof attached to/supported by cob walls Only use cob as infill to post and beam structure or some such...


Unless it is some example of historic restoration, I typically would not ever build a structural cobb (or related) structure in most cases.

I'm sure it's a can of worms, but why? From what I've read, it seems that cob/cobb does much better during seismic events than most structures. This is the first I've heard of somebody warning against it specifically due to tectonics. Curious minds...


Hmmmm....well in some ways it is a can of worms...especially when the mix is off or the "cobb" is based on silt not an actual clay (this happens more often that some would suspect.) As for tectonic durability...??...if there is some examples in mind to discuss I would enjoy doing so. In most cases (and examples) these are not typically "structural cobb" alone, but incorporate other material elements within the matrix of the wall diaphragms.

I am planning a cob build in near future and had planned on using outer walls (likely the only walls) as the roof support. Alternative would be post and beam to hold roof (built first) and then infill w/ cob.


I could not encourage the latter strongly enough, unless one has 15 years of study and experiences with the different moralities of "clay architecture," and/or having the design thoroughly vetted and signed off on by a PE familiar with the method.

Regards,

j
 
Peter Ellis
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Location: Central New Jersey
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I will make the observation that Ianto Evans and The Cob Cottage Company are located in the Pacific northwest, where they have been building cob structures for many years. Shake table tests have been done on at least one scale model of a cob building (1/3 scale) and the performance was sufficient for the City of Vancouver to allow the full scale structure to be built in a city park. Opinions appear to vary as to the suitability of cob for structural purposes.
 
Geoffrey Levens
Posts: 54
Location: Paonia, Colorado, USA
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Jay, thank you for that response! Sounds like post and beam will be what I do.

Sort of bouncing sideways off original topic of thread I have been wondering about building the cob walls directly on top of the rubble trench without a stem wall; this would be in Western Rockies, up well out of any possible flood zone, with maybe a 3 foot overhang of roof all around. I was thinking if the trench was wider than the building footprint by a foot or so then any run off water would just drop down into that and not impact the walls at all. Is this just a nutso idea? I don't really mind if some sort of stem wall is needed, just trying to minimize the labor ( no carrying big rocks around to build the wall)
 
Terry Ruth
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Check out this 10,000 square feet COB build in the UK, it is legit! Jay is right I think too many people jump in without getting the proper soil test (atteburg limits, Sieve Analysis, mechanically and thermal properties to a PE, nor understand the build and mating of materials (clay, concrete ad mixes & OPC, wood, efflorescence, swelling-shrinking, freeze-thaw, cracks, loads (dead, live, environmental, etc) . You can see the challenges in this build where he deviated from the PEs drawing due to tolerances on the roof/wall bond beam, so they had to reevaluate. IF you don't seal them well they leak, thermally bridge so they used foam out-sulation on this build I didn't much care for. I would think a bond beam on a stem wall needs reenforcing, anchors, and a mesh tie to the cob....A PE needs to determine based on loading. Then the wood and concrete may need to be isolated with an inert filler like Mag phosphate and a cap break. Do is low earthquake zone.

Check it out tho HUGE cob home, awesome build...the guys a master and his PE gets it.

 
Jay C. White Cloud
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​Hello All,

Peter E makes a wonderful observation, which allows me to illustrate more clearly my concerns and observations, which are more with the..."culture"...of cobb architecture than the..."medium."

There are "professional" cobb builders, and have been through the ages, I support and admire their work. ​I am sure more modalities of tectonically stabilized earth architecture will arise each decade, just as we discover more information about the heritage cobb structures that still exist. Rammed earth has built the Great Wall, and many other structures of "clay" exist.

What I am trying to stress, is these methods are not any less challenging than building a dry laid stone wall or a timber frame...and...the culture today behind "cobb" is to suggest that it is..."easier." I am suggesting from years of study, designing and building...that it may appear that way, yet must emphasis that any structure that supports a roof and perhaps more than one floor while sitting upon a foundation is not something we should ever look at as...easy.

I have read and heard, way too much of such language in workshop literature and other literary examples describing..."an easy way to build an inexpensive home." For me, such language is a "red flag," and I look deeper. I would further suggest that the term "easy" is an extremely subjective appellation...please know well what a subject is before thinking it easy. This is an offering I share with any student or client thinking to build a home...

I think "timber framing" is way "easier" than cobb building and the line of folks that disagree with me on that is very long...Which only proves that "easy" is subjective...as neither cobb nor timber framing is...until you understand them thoroughly and know well there limitations and also how when combined to work in concert, they create a superior structure to either standing alone...
 
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