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A South Australian Cob House  RSS feed

 
Jeremy Krieg
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Hi Permies !!!

I have been interested in cob for a while now and I have almost completed my first house/studio. I started researching cob as part of a year 12 assignment and I started this structure late 2014 and have worked on it spasmodically.
I have tried a few things that I haven't seen on the internet before (a vaulted cob load bearing ceiling, cob arches/trusses to support the roof and cob eaves !), I resumed building the week before last and hope to get the cobbing finished by next week ! ahaha (as I have to go back to uni the following week).

I was wondering if anyone has any advice on the eaves, (at the larger cob supports come out over a meter from the wall and smaller cob supports are about 600mm), I intend to span these supports with salvaged pallet timber and then have a green rood on top (I already have some cardboard and a waterproof membrane) ?

Thankyou very much

Jeremy
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Jeremy Krieg
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Here are some more pictures showing these "eave supports" ahaha

The second picture is looking down on the wall, the cob that runs horizontally in the photo is the main wall and the vertical cob makes up the internal arches (which support the roof) (in the photo that is above the main wall) and the vertical cob bellow the horizontal main wall forms the eave supports, because the internal arches and eave supports align I can ties them into one another reducing the stress on the main wall (i.e so the weight of the eaves is not just bending of the main wall but it is also supported by the arch which reaches across to the other wall)

that's a bit confusing, I will upload a video soon
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Ben de Leiris
Posts: 61
Location: Hinesburg, Vermont
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Interesting challenge you've taken on! I have seen vaulted earth ceilings here and there but never any cob eaves! I would definitely trust a well made cob arch since it's all about compressive forces... the eaves I'm not sure about. You say you you can tie the eave supports into the arches since they are opposite each other. Did you embed some kind of reinforcement in the cob as you went? Or you are just relying on the cob itself to tie together? Also are the arches going to be built up any more? That thin top section looks questionable to me, especially with a green roof on top.

One idea would be to lay boards down flat which go along the arches and bridge over the wall to lay on top of and pin into the eave supports. Maybe a 1x6 (50mm x 150mm?), or whatever would bend to that curve easily. You could even laminate a few together to make it much stronger. That would tie everything together and help (a little) to keep the supports from falling off. One benefit it might have is to act as a backup if one of the supports did fail. If one cracks, the boards might hold it together until you notice the problem and can fix it. Otherwise, any failure would likely be sudden and catastrophic. The boards would also serve as a nice nailing surface for whatever you use to span the supports.

I think if I were building this from scratch, instead of cantilevered eave supports I would build a series of buttresses around the exterior. They could resist the thrust from the arched ceiling as well as support the eaves. You'd need fewer of them since they'd be many times stronger than your eave supports, and overall I think I'd sleep better!

I look forward to seeing what you come up with, and seeing the finished building!
 
Jeremy Krieg
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Thanks Ben ! Yeah ahaha ... there was quite a time when I couldn't sleep thinking about whether they would work or not,

I have made all the cob by hand (starting with a backhoe but then forming each ball buy hand adding straw when necessary), I didn't embed any extra reinforcement ... I spoke to an engineer once about adding timber and he said the straw is reinforcement enough (I don't think he understood the material properly). I did however do a tension test on my cob mix a few years ago and it was quite promising (I embedded two welded prongs in some cob and then hung the cob block vertically and added weights to try to separate ... I ran out of weight ! ahah pics below) , so I believe so long as the cob is mixed well and then laid intelligently (so you have long thicker strips that 'tie' it back into the structure we should be alright ahah),

Yeah the arches will be built up to about 300mm at the crown, I can hang my weight of each crown though at the moment (70kg so I am pretty happy with that already),

I think the buttresses would be a good idea ! I would not be at all comfortable with someone else building these eave supports (and I don't think I would do it readily again in the future), I was very careful with how built them and the cob I used (and I have tested their strength the whole way along).

I think the timber is a good idea (I was going to have to embed some into the top of the arches and eaves so I have something to nail the timber I will span the arches with).
Good point about failure (I plan to do many safety tests once the roof is on, adding extra weight), but It would be good to have a bit of a warning if anything was to happen.

I would like to go on after this project and scientifically collect some data on how cob performs under different loading

Any idea Ben on how far out eaves on a cob house should go ? (I was also thinking if I do put the timber on top of the arch if it was strong enough I could extend it out slightly further past the end of the cob eaves to get me a bit of extra distance)


Thankyou very much !

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Ben de Leiris
Posts: 61
Location: Hinesburg, Vermont
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I'm with you, I always trust something I've built myself more than someone else. I seen so many professional contractors do really shoddy work. I'm not a plumber, but I think I do better plumbing work than a lot of plumbers, just by virtue of caring about the quality of my work.

Check out the attached paper, in which some engineers did a bunch of structural testing of cob. It's kind of technical and most of it I don't know how to apply to real world building, but it might be of interest to you. Your engineer might find it interesting too.

As for overhangs, I am no expert. I'm working on the same issue in a small house I'll be building this summer. The main purposes of overhangs are to control solar heat gain by shading windows, and to protect the walls from rain. It also depends on the height of the building and aesthetics. I can't think of any downside to huge overhangs unless (a) you have cold winters and would like some passive solar heat, (b) you just don't like the look of them, or (c) they add too much cost to the project. I've never been to Australia so don't know the details of your climate. If you want heat from the sun, then size your northern overhang so it still allows the sun in during the cold season. I'm guessing that at times it is hot enough that you will appreciate plenty of shade. And do you ever have strong wind driven rain? If you keep extending the eaves, eventually you get to the point where it makes sense to support them with posts, like on a porch.
Filename: Cob_Paper.pdf
File size: 932 Kbytes
 
Jeremy Krieg
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Wow that paper is very handy, thankyou very much ... It will be a good reference (Here in Australia there are a few other published cob reports .... 'the Earthbuilders Handbook' published by the CSIRO),

Fortunately for me, the structure I am building is below council regulation (so I can basically do what I like !) ... so I although I have been in contact with several engineers I do not have one for this project.

There are too many variables !! ahaha I also need to consider the weight of the cob eaves (further I go out the more stress on the cob ).

Hmmm ... things to consider !
 
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