Combination root cellar/chicken coop

My two big projects for the summer are to make a root cellar and to make a chicken coop. I want to someday build a straw bale house, so I decided to practice on a chicken coop. For this to be a true test of my straw bale construction technique, I need to give it a proper foundation. To get below the frost line here, one needs to go quite deep. This is why nearly all houses here have basements. Which got me thinking, I could build my chicken coop with a basement, and that could be my root cellar. So, I've been merrily toodling along trying to figure out how to do all of this. I have a root cellaring book that has building instructions, and I have straw bale construction books. For the most part, I think I've more or less got it covered. But some of the details are confusing me.

So, to sum up that bit, I'm building a straw bale chicken coop on top of a poured-concrete-walled root cellar.

The big thing right now is the roof of the root cellar/floor of the chicken coop. I'm looking at a size of 12 by 15 feet (exterior dimensions). The root cellar will probably be divided into two equally sized rooms. My root cellaring book recommends a concrete ceiling, as root cellars are supposed to be damp and wood will rot in such a space. Also, it would make an easily washed floor in the chicken coop (I would put a sealant on it). I ran this past a friend of my husband's who has construction experience, and he suggested that the walls (or maybe the ceiling?) would have to be very thick to hold up the weight of the ceiling AND the weight of the straw bale walls. He thought that it was a bad idea and we should just use wood. This isn't for my house or anything, so I don't want to hire a structural engineer. I just want to figure out if it would be feasible to use concrete for this application, because I think it's preferable functionally.

In case it's pertinent, I'll try to give a good idea of the overall plan. From the bottom up, the root cellar will be dirt-floored, with a gravel-filled sump to drain into if required. The walls will be built on a wide footing, and they will be poured into insulated concrete forms. The proposed concrete ceiling can sit on top of that? Or maybe only on top of half of the wall? The walls will stick up out of the ground at least a foot but under two feet. After addition of moisture barriers to prevent bale wetting, bales will be stacked up. They are going to bear the roof load. The roof plate will be tied down through conduits in the concrete under the bales. The bales will be stuccoed, probably with a concrete stucco. As you can see, I feel fairly confident in my root-cellar knowledge, and fairly confident in my straw-bale knowledge, but the interface between the two is kind of scaring me. The more input the better!

A vaulted ceiling in your root cellar would both allow for thinner poured walls, and distribute much of the load into the surrounding soil.

I'm not a structural engineer either, but concrete in roughly the shape depicted on the left, with a few narrow sections "webbed" to have a section like I've depicted on the right, might work.

That's an excellent idea.  I think it's about perfect from a structural perspective and from a materials perspective, but the construction of the forms required to make it would be far beyond our capabilities, I think.  Thanks for the response, and for the great idea!  I suppose they might make forms for such a ceiling that I could look into, but I imagine they're prohibitively expensive.  More ideas are very welcome!

I wonder if reinforced ferro-cement would work for the webs?  You wouldn't need forms for that.

Kathleen

We are planning something similar, but we are planning a wood/beam floor from standing dead timber.  Walls with earthbags on rubble trench foundation with a bond beam, dirt floor.  Vapour barrier under the floor/ceiling, floor beams, straw insulation, then vapour barrier on top of the straw under the top floor.  I think if I was putting a chicken coop on top instead of a screened gazebo I would use sheet metal/roofing tin for the floor and have the floor with a slope(s) to the door for cleaning. But I don't see a problem with making your form for the cement floor right on your flooring/roof beams as long as you don't make your pour too deep.  Think I might want to add a supporting post or two underneath depending on the span though.  I would call my local building inspector and ask him though, he's a great help if I don't pester him too much!  When my mom had tile put on her floor they just poured the cement right on top of the old subfloor, then the thinset, then the tile.

Monica wrote:
We are planning something similar, but we are planning a wood/beam floor from standing dead timber.  Walls with earthbags on rubble trench foundation with a bond beam, dirt floor.  Vapour barrier under the floor/ceiling, floor beams, straw insulation, then vapour barrier on top of the straw under the top floor.  I think if I was putting a chicken coop on top instead of a screened gazebo I would use sheet metal/roofing tin for the floor and have the floor with a slope(s) to the door for cleaning. But I don't see a problem with making your form for the cement floor right on your flooring/roof beams as long as you don't make your pour too deep.  Think I might want to add a supporting post or two underneath depending on the span though.  I would call my local building inspector and ask him though, he's a great help if I don't pester him too much!  When my mom had tile put on her floor they just poured the cement right on top of the old subfloor, then the thinset, then the tile.

If there is one thing I know about cob and strawbale construction, even in Cascadia....  do not use vapor barriers!!  The walls need to breathe properly in order to not decompose and collapse.

I had the pleasure this year of visiting Cob Cottage over at Mountain Homestead in Coquille, OR.  A fascinating place, great people.  Anyway, Cob Cottage tour showed me the process of making cob, cob / strawbale, and strawbale construction of many buildings, and as I said, if there is one thing I did learn that day...  do not use vapor barriers!

Normally you would not use a vapour barrier on cob, but in our case we need to use it.  A cob structure is only as good as its "hat and boots" and since we are not using cob in this instance the vapour barriers seem necessary to keep the straw insulation dry.
The humidity in the root cellar will be high and in order to keep the straw insulation dry a vapour barrier is needed from the bottom.  As the root cellar needs to have its own ventilation system, to vent the whole roof would only cause the straw to rot.  As we are putting a screened gazebo on top the second vapour barrier serves to help keep the insulation dry from the top since the screens will allow some moisture to come in from the top.
If we were making the structures separately the plans would change, and if we are on the wrong track of course feedback is always welcomed as the idea to combine the two is only a plan.  Mostly the reason for this smaller project is to test the results of the earthbags underground in our lowlying 'misty meadow'.  I would love to have a traditional root cellar but alas we have no hills.  I am calling the gazebo "the bug house" a necessary evil for my 2 and 4 year old boys to have a place to play without being eaten alive (priority one).  The whole structure will probably be turned into a play area with an underground fort once the house is built. 
We could and will be making a hill (winter sledding) from the pond construction (aka the old lakebed) but it will be incorporated into an earth sheltered greenhouse next year, as our meadow is too wet right now for any digging.

given the viruses and bacterium in chicken and there waste .. i would do some hard core sealing and wood is not a good choice IMO. concrete is not waterproof either but would be easier to seal than wood
i would hate to get salmonella from a potato

I think this is a really, really bad idea. Hate to rain on your parade but I would not want the run down from the coop going to my vegetables and the temps don't work together. I would put these structures separate.

Both should be fairly simple buildings but trying to combine them just seems to be making it all very complicated.
Build a good chicken coop in one spot and a good root cellar in another spot.