New guy about to build his first cob

Hi

My name is Will MacBride. Hey Jamie McBride who moderates this.Perhaps somewhere back in some medieval Celtic village we have a common ancestor.

My scene is this: I've been doing ordinary sorts of construction and handyman stuff for years but I'm very new to natural building. I'm about to build a cob house that I will live in and the yard of the house I now live in. I'm going to have lots of questions, but right now my questions involve basics.

One initial question, just so I can visualize better is this : does the roof of a house with simple cob walls, placed on a stone foundation, have rafters that just sit in the cob, supported thereby? Or is there, or should there be, some kind of top plate of wood or perhaps stone?

Also: how do they get those strange wavy roofs I always see on cobb houses. They look like they're framed with wood but I can never see in the pictures I see of them what's underneath. Are they using branches - saplings and stuff?

Third question: can a second floor or loft in a cobb house be cob as well, the floor, or should it be wood?

thanks

Will

Hey Will,

Welcome to Permies...

One initial question, just so I can visualize better is this : does the roof of a house with simple cob walls, placed on a stone foundation, have rafters that just sit in the cob, supported thereby? Or is there, or should there be, some kind of top plate of wood or perhaps stone?

First, just to stay consistent, I do not recommend the "novice" ever building structural cob walls. (I know many do, and many are probably just fine) As one of those "alleged professionals" in traditional and natural building, I myself will not build these with out a PE of my choosing being part of the build team. So...infill method...YES...structural cobb...NO! (in most cases.)

Now with my silly disclaimer out of the way...YES, I would recommend there to be a "band beam" of some sort. Most cob builds that support the roof have them or a version there of. Are these in all the designs I have seen around the globe? No. Are they in most (or a form of them)...Yes.

Without a cross sectional schematic and other design blue prints it is hard to recommend what modality to employ.

Also: how do they get those strange wavy roofs I always see on cobb houses. They look like they're framed with wood but I can never see in the pictures I see of them what's underneath. Are they using branches - saplings and stuff?

Give me some links to look at and I can probably tell you the system they are employing...

can a second floor or loft in a cobb house be cob as well, the floor, or should it be wood?

Only if you really have a solid design, and good experience would I recommend this for any cob structure, let alone a fully structural cob wall system. Yes it can be done. It will take tons of clay and sand, and lots of really hard work...Especially when compared to other vernacular systems.

Again...YES...it can be done...

Hey there Jay C. Whitecloud:

Thanks for the reply. The third house down on this page:
http://naturalhomes.org/cobhouses.htm is one example of the kind of curvilinear roof I'm talking about.

Or even this one: http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FCob_(material)&ei=ATafVMDsLpbroASs_YKgBw&bvm=bv.82001339,d.cGU&psig=AFQjCNEWwfh92Kt926EmoBw_rS7fa4JirQ&ust=1419806319972483

By the way: the link above connects to the wikipedia page for cob and its the picture a little ways down the page - it says its an example of a modern pacific northwest style cob house.

When you say infill are you talking about building a whole wooden frame and then simply using the cob as the non-supporting wall material? I assume so. With that I would also have a couple of questions:

1. Should (or can) the cob simply sit on the wood (say on a plywood subfloor and up against studs and headers, etc.)? I've heard or read that it expands and contracts differently, moved under wind pressure differently, and so on, than wood, so sometimes after a while you run into issues with that. Any comments?

2. Would a doubled cob wall with an air space between them be considerably more insulative than just a single thick one? I live in Portland, Or, it doesn'tget all that cold in the winter except occaisonally. Any recomendations there?

3. The soil in the yard appears quite clay-y. It should work well enough but are there any sorts of tests to see if a soil is has a high enough clay content?

4. Is it usually ok to just go to an ocean or lake beach to get sand?

5. Do you usually have to buy straw or or can you usually get that free in some way too?

thanks a lot,

Will

Hello Will,

Now I probably going to be taking you..."down the rabbit hole,"....with some of my comments, so just go with it and look things up if they interest you or your not sure...otherwise feel free to ask more questions.

1. Should (or can) the cob simply sit on the wood (say on a plywood subfloor and up against studs and headers, etc.)?

Well....Hmmm....that is hard to pin down in as much that it would depend on the design and who (or how skilled) engineers the frame and structure.

I don't use plywood very often at all and only green (fresh of the mill lumber) for many projects. Again...it all depends on the design.

I've heard or read that it expands and contracts differently, moved under wind pressure differently, and so on, than wood, so sometimes after a while you run into issues with that. Any comments?

Again...design can exacerbate this yet it generally should be an issue if all is designed well. That is not to say that there isn't always some "tightening up" and maintenance to do each year, especially the first 2 to 4 years as the architecture "settles." Wind in general (other than a tornado or Tsunami) shouldn't be an issue if things are designed well.

2. Would a doubled cob wall with an air space between them be considerably more insulative than just a single thick one?

Yes

I live in Portland, Or, it doesn't get all that cold in the winter except occasionally. Any recommendations there?

I think this is the time to suggest reading about the following methods to expand your possibilities:

Straw clay slip walls

wood chip light clay walls

Kubbhus

3. The soil in the yard appears quite clay-y. It should work well enough but are there any sorts of tests to see if a soil is has a high enough clay content?

jar test for clay soil

4. Is it usually ok to just go to an ocean or lake beach to get sand?

Well, if it is legal, and you don't mind hauling several tons...it can be acceptable...Sharp sand is better than rounded...but we all have to "plow with the horses we have..."

5. Do you usually have to buy straw or or can you usually get that free in some way too?

99.9% of the time you have to buy it...

Regards,

j