Combo root cellar/chicken coop

[Hykue]

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!

[eatyourflowers]

Hi Hykue,
Did you do your chicken coop / root cellar combo? Just interested to know how you got on as we need to make a root cellar too.
Cheers