Cob

Rocket Mass Heaters, Rocket Ovens, Cold boxes, Solar collectors, etc..
Talk about your projects

Moderator: matt walker

Cob

Postby Rhett » Wed Apr 02, 2014 9:41 pm

Something I don't understand about cob (or adobe for that matter), is what happens when it gets wet? Does it just get slick on the surface? Does it penetrate and really get muddy and soft/weak. I can't see the addition of sand and straw as fundamentally changing the way dirt is when it becomes wet. So how do cob/adobe buildings survive for hundreds or thousands of years without washing away. I don't get it.

The stuff I have read online says it survives rain (which seems patently obvious since we have adobe houses that are hundreds of years old) but it doesn't explain how. Anyone know? What are your own experiences with cob like? Matt, you've got that cob bench - why isn't that thing just a big old pile of mud every time it rains?
Rhett
 
Posts: 61
Joined: Mon Mar 10, 2014 11:46 pm
Location: Central Texas

Re: Cob

Postby pa_friendly_guy » Thu Apr 03, 2014 12:05 am

Matt put flat stones on the top of his out door unit to avoid any problem with rain etc. Adobe bakes in the sun like bricks, it is not totally immune to the rain but it can last for many years with only minor repair work. Buildings built with cobb generally have a skim coat of some kind of plaster or line coating that is more resistant to the rain. When you build a cobb building I have always heard that you need a good Hat and you need good Boots. ;) In other words you need a good roof and a good foundation to help keep it dry. Keeping the cobb at least 18" off the ground so that water from the eves does not splash up onto it, and having a good over hang on the roof are both huge in making it last. But it is still just dirt, so it is easy to patch and repair. To help put this into perspective though, sections of the Great Wall of China were build with cobb, and they are still standing. :D
Never doubt that a small group of dedicated people can change the world, indeed it is the only thing that ever has.
User avatar
pa_friendly_guy
 
Posts: 1502
Joined: Sun Jan 08, 2012 9:24 pm
Location: SW Pa They changed me to zone 6a what ever that is. I still figure zone 5

Re: Cob

Postby Rhett » Thu Apr 03, 2014 12:32 am

Right! I had read the cap and boots thing, but take the Great Wall you mentioned. Does it have a concrete boot or something? How the heck is that mud wall still standing! It doesn't make sense I me.

You said adobe bakes in the sun, but so does cob right? It dries in the sun over time. Maybe I just need to make something out of cob and see how it weathers. But it sure seems like a lot of work.

I'll look into the coating thing. That makes sense to me. Some kind of water resistant plaster or something. Maybe stucco?
Rhett
 
Posts: 61
Joined: Mon Mar 10, 2014 11:46 pm
Location: Central Texas

Re: Cob

Postby hpmer » Thu Apr 03, 2014 1:07 am

I read somewhere that large parts of the Great Wall were made with rammed earth, much like some of the natural builders today beat earth into old tires.

I assume the ramming packs the soil structure into something similar to concrete.

Other parts of the Wall were made with bricks held together with a rice paste or something which was quite the advancement at the time.

Still other parts were built with stone, and some with a cob like material interwoven with vegetative material like sticks, reeds, small trees or whatever was available.

Fascinating stuff (to me at least)
User avatar
hpmer
 
Posts: 111
Joined: Sat Aug 03, 2013 10:59 am

Re: Cob

Postby pa_friendly_guy » Thu Apr 03, 2014 12:55 pm

The plasters choices people use most commonly with cobb are either made with horse manure for structure or burnt lime for hardness. Both allow the cobb to breath and help with any moisture that gets on, or into the cobb. The lime plaster hardens almost to a hardness of limestone. Stucco is a man made product that does not breath and therefor is not recommended with cobb. Other coatings like Linseed oil or other sealers are also used to help seal the cobb.
I guess the question is more about if the produce actually works, and not if you personally believe that it works. Turns out, Earthen buildings work, adobe, cobb, waddle and dob, rammed earth, earth ships, earth bag, and many others, people have built with earth for centuries because it works. They used what they had available because of necessity, they needed shelter to survive. Trial and error refined the process until we have the collective knowledge we have now.
Never doubt that a small group of dedicated people can change the world, indeed it is the only thing that ever has.
User avatar
pa_friendly_guy
 
Posts: 1502
Joined: Sun Jan 08, 2012 9:24 pm
Location: SW Pa They changed me to zone 6a what ever that is. I still figure zone 5

Re: Cob

Postby mannytheseacow » Thu Apr 03, 2014 1:31 pm

I was able to vistit the great wall back in 2010 while studying in China. Yes, all of those methods were used at different places in the wall, to different results. A couple other factors is that much of the area is in a pretty dry climate and also sections of the wall are constantly being renovated. Still one heck of an achievement. Amazing what you can do with slave labor.

There is still quite a bit of natural building throughout rural or lesser developed parts of Asia and a real learning opportunity for anyone interested.

Another interesting thing I found was the extensive use of manure. I guess when you have 1.4 billion people in your kingdom you have to look at poo as a resource and not a waste product. In many areas, even today, chamber pots are picked up daily from homes and applied to fields or building structures. There's some wisdom there. It's a whole different culture and perspective. Can you imagine Paris Hilton or P. Diddy using chamber pots. Maybe someday Hollywood will catch on. Oops, pinch me, I'm dreaming.
"Knowledge is power. Arm yourself."
User avatar
mannytheseacow
 
Posts: 942
Joined: Sun Feb 24, 2013 12:40 am

Re: Cob

Postby matt walker » Thu Apr 03, 2014 3:36 pm

So, yeah, cob outdoors is tricky, BUT.......

Everything out there that you read about techniques or concepts for outdoor cob are lacking a major factor that we have going for us. Our cob benches have internal heat sources to power dry them. That makes a HUGE difference in what you can get away with, and it's something uncommon so you don't find too many examples to learn from.

So, Rhett, you are in Texas. I strongly believe that you can build yourself an outdoor whatever out of cob, and in a year or two you will know which parts need protection and which don't.

Sitting on cob outdoors sucks, so you will want slate, or tile, or even a wood top, or cushions, or something that keeps your butt from getting a big dirt print, or beer mud, or whatever, on it. So that's a start for the protection.

I did a lot of reading about lime plasters and other techniques (linseed oil, etc.) before playing a lot with the outdoor systems, but in practice I don't think most of those conversations apply. The heating and cooling cycle I'm pretty sure would flake a lime based coating off of the cob over time, and warm linseed oil doesn't sound nice to hang out on or near.

Basically, I think you can get away with a lot of stuff that is not recommended for cob construction when you have a heat source inside of it. Boots are important, otherwise capillary action will couple your cob to the dirt and it will stay living. A roof over it is the best protection, but mine sit unused in our incredibly wet winter and in the spring they are just fine. Wet and need drying again, but they don't melt away or anything. A few consecutive nights of fires and it will stay dry through the summer season, and periods of rain don't bother it and tend to not penetrate very far when the thing is dry and in normal use.
User avatar
matt walker
Site Admin
 
Posts: 1806
Joined: Tue Jan 03, 2012 5:50 pm
Location: North Olympic Peninsula

Re: Cob

Postby Rhett » Thu Apr 03, 2014 4:14 pm

That's fascinating.

You know, being in Texas it doesn't rain much. I live in a place that historically would get about 22 inches per year, but really that's the kind of numbers that were thrown around in my childhood. With the drought, I would be surprised if we are much over 19" on average over the last ten years. It's been tough. And that summer sun is merciless, virtually impossible to pull a real garden through the summer these days.

So yeah, I don't have to worry about rain too much, and I have a stone patio that this will rest on so the boots are taken care of. If your's is surviving an Oregon winter then surely mine would be fine.

I hadn't thought about how the heat would affect a plaster layer, but aren't most kiva fireplaces basically some kind of plaster? They seem to do all right. Clearly it would have to be the right material to handle the thermal shock. That might take some research.

I think you're right about the linseed oil though. Blech!
Rhett
 
Posts: 61
Joined: Mon Mar 10, 2014 11:46 pm
Location: Central Texas

Re: Cob

Postby matt walker » Thu Apr 03, 2014 4:26 pm

Yeah, a lot of masonry heaters are plaster coated, mine included. Some plasters have lime, some don't. I do not know if lime based plasters are functional on indoor heaters or if they just use clay/sand plaster with other binders, but outdoors I think the potential to get a little moisture under the lime plaster, then have large temperature fluctuations, let alone freezing, will cause the outer coating to fail before very long. A lot of speculation here, but basically, I don't think it's worth going down that road too far. I say, build it and see, then deal with any issues.
User avatar
matt walker
Site Admin
 
Posts: 1806
Joined: Tue Jan 03, 2012 5:50 pm
Location: North Olympic Peninsula

Re: Cob

Postby GrahamB » Sat Apr 05, 2014 2:21 am

I don't think you guys need worry too much about the cob running away from you. Back where I come from there are 5-600 year old cottages made of cob with a thatched roof and they stand up to Atlantic storms without any trouble. One thing they do to them though is seal them with lime wash. Even the modern paints can't improve on it. Like as been mentioned they have good boots, usually cobble stone (the floors on some of these are cobble too).
No matter how many mistakes you make or how slow you progress, you are still way ahead of everyone who isn’t trying.
User avatar
GrahamB
 
Posts: 234
Joined: Wed Mar 21, 2012 2:48 am
Location: SW Missouri

Next

Return to Heating and Cooling

Who is online

Users browsing this forum: No registered users and 14 guests

cron