Cob Durability

I really like the idea of cob as a building material, but I've got doubts about its ability to stand up to the elements. I'm making this post because I have very little experience with cob structures (only built one thing so far), and was hoping somebody more experienced could give me a first hand impression of cob's durability. It just seems like over time it would naturally erode as everything does. So basically I'm asking for examples of why I'm wrong in assuming cob is not a good long term building material. First post, be nice!

It will erode over time if not protected, BUT....

It is easy to patch and re-plaster and maintain. Most modern building materials need to be replaced. So it is cheap to maintain cob yourself, but it does take time.

R Scott wrote:It will erode over time if not protected, BUT....

It is easy to patch and re-plaster and maintain. Most modern building materials need to be replaced. So it is cheap to maintain cob yourself, but it does take time.

Ya I guess everything no matter how durable requires some mainenance. So cob being cheap has an advantage vs modern building materials, assuming they degrade at the same rate. What do you mean by "if not protected"? what would one protect cob with?

good hat and boots--large roof overhangs and stone stemwalls and splash guards. Maybe a wattle fence to keep really strong prevailing winds from blowing rain sideways onto the house.

R Scott wrote:good hat and boots--large roof overhangs and stone stemwalls and splash guards. Maybe a wattle fence to keep really strong prevailing winds from blowing rain sideways onto the house.

Hmm so a high stemwall to stop the cob from soaking up water, and an overhanging roof to stop water from pooling on the top. As I mentioned I made a small cob structure (just a square on the ground as a test), and it recently rained. having a good "hat and boots" would have definitely helped it resist water, so thats good to know. But what about the mid sections of the wall? Rain will inevitably hit the middle part and erode it, like it did to my test square. I guess thats where maintenance comes in.

This is a random thought... Let's say you build a small house with cob walls. After it's dry, but before you put a roof on it, you light a huge bonfire inside of it. Would you get a solid brick house?

The rum I have been drinking tonight wants to know the answer.

Bill Crim wrote:This is a random thought... Let's say you build a small house with cob walls. After it's dry, but before you put a roof on it, you light a huge bonfire inside of it. Would you get a solid brick house?

The rum I have been drinking tonight wants to know the answer.

In theory, yes, but it would have to be a HUGE fire.

This is what you do to a cob pizza oven, but you only really turn the inside inch or two of adobe into brick--the outside is still cob. And that is a contained fire that gets the surface temperature over a thousand degrees (don't know how high, it went off the scale of my IR thermometer long before the fire was done).

Ya you should be able to do something like that. I've seen people make rocket stoves out of cob, so it makes sense that the inside should turn into a hard fired clay like substance. Not sure if thats good or bad though. Would be like a large 1 piece brick

Ceramic houses are buildings made of an earth mixture which is high in clay, and fired to become ceramic. The process of building and firing such houses was developed by Iranian architect, Nader Khalili, in the late 1970s.

I've got the book about it, and it's beauuuutiful. But earthen houses really don't need to be fired, they just need to be protected from the elements. Firing would use huge amounts of fuel in a less-than-ideally efficient way, and if actually doing its thing, would harden the inside of the house, where you don't want it, and not the outside, where you might want it if your climate doesn't allow earthen structures. THough waterproofing the outside of an earthen wall is usually a bad idea (read up on the plastering of Rancho de Taos adobe church)

Some climates are perhaps inappropriate for earthen buildings, but in most climates, you can protect an earth wall with a good hat and boots as mentioned above. There are a zillion great books and website and lovely photos. It's great fun reading about it, even if you never do it. If you've already gone to the trouble of building a wall to see how it lasts, you should certainly invest some time in reading up about it.

My first hit on googling "Rancho de Taos cement plaster" was this: http://www.networkearth.org/naturalbuilding/aliz.html

A big lesson was learned at the famous St. Francis de Assisi Church in Ranchos de Taos when it was coated with cement stucco in 1967. This plaster cracked and allowed moisture to penetrate deeply into the adobes, but the relatively impermeable stucco prevented the adobes from drying out again. Large sections of the buttresses had to be rebuilt, so the community has now gone back to the annual renewal of the mud plaster - which not only keeps the church building in beautiful condition, but strengthens neighborhood ties as well.

interesting

Well, I just learned something about the importance of having a good "hat" on your cob structure. My small cob test box outside has been through two heavy rains so far, and I noticed something after I inspected it.

The box was only eroded on one side. I then realised that it was the side which was built leaning into the center of the box roughly an inch. The other side, which was leaning away from the center by about an inch (creating an overhang) was barely wet after the heavy rains.

So basically having an overhang of one 1 inch dramatically improved resistance against rain. Good to know!

yeah "good hat, good boots", is the number one thing to think about in any kind of earthen construction.

another thing is lime wash, and layers of outside plaster. the outside is often plastered with a slightly different mix than the main earthen layers. inside as well, usually get a different mix for the inside top layer. the right mix, with or without lime, would serve as a protective layer, which may need to be re done every do often, as in the case of doing a lime wash.

Humanity's oldest structures are made of mud! I have personally worked on 300+ year old mud structures that are not going anywhere for at least another 300. The sacrificial covering must be renewed periodically, but don't confuse this with a compromised structure.
I live in a 120 year old adobe(same thing as cob really) home. The indoor air quality, the feeling of being held within the Earth, the beauty of hand troweled clay plaster and knowing that when this place is finally abandoned, it will go right back to the Earth again; what more could you possibly want from a structure?