Combination root cellar/chicken coop

hykue · August 08, 2010, 12:38:37 PM

Hi. I know this isn't exactly the intended purpose of this forum, but I am really hoping someone can give me some help anyway. I'm having a bit of a hard time, and it is fairly applicable to home building. Give a gal a break?

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. Anybody here that can help me with this? If I need to explain something more thoroughly, or I've not been clear, please let me know.

dug · August 08, 2010, 09:09:01 PM

I like it but it seems pretty ambitious for a first go. A suspended concrete floor would be doable, but for sure it would require some sort of engineering. It's not hard to imagine the potentially disastrous results if not done right.

Also a straw bale wall would require a stem wall as wide as the bale, making for an expensive basement. I would think a standard basement with a framed wood floor would work O.K.
Many homes in the midwest where I grew up had basements that doubled as root cellars.

A concrete floor for a chicken coop is best, but I just have plywood and it holds up pretty good, not too hard to clean. Just keep a bed of shavings or straw down and change it now and then.

What about a separate earth roofed root cellar and pour a slab for the coop?

You must be planning quite a flock with a coop that size!

By the way-

August 08, 2010, 10:21:18 PM

It sounds as if it could be done. The majority of the wall (straw bale) would rest on the poured basemnt(cellar) walls and there would not be that much of a weight issue. I would imagine that 4"-5" of re-enforced concrete could probably suffice. If you had access to "Q decking" it could be used for the ceiling and re-enforced with supports from the floor of the cellar until it had set. If Glenn would chime in I know of one project that he has done with suspended concrete pours. If it were anything other than what you described and it's intended use I would probably suggest an alternative plan.

Squirl · August 09, 2010, 08:55:37 AM

can think of many ways it can be done. But I think that pouring a slab over a root cellar is not necessary.

The concept of a root cellar is to use the constant heat of the earth for temperature control and the earth as insulation. The purpose of poured concrete for a roof is because it is usually covered with earth which absorbs moisture and there is no other roof so it rains right on top. In that type of environment wood would rot quickly and collapse.

If there is a chicken coop above it though, the roof of the chicken coop would push the rain to the outside and there would be no moisture on the level above the root cellar. Hence no need for concrete. You would probably do better to with a joist insulated floor. Although, in combining the two buildings, you lose many of the advantages of the physics (cold in summer, not freezing in winter) of the root cellar.

If you are interested I am sure many are willing (myself included) to help you work through your design. There are a dozen ways to shave a cat.

Grimjack · August 09, 2010, 09:10:03 AM

How about using spancrete for the ceiling/floor?

http://www.spancrete.com/products.php

Dig your root cellar, build the walls of the root cellar then lay the spancrete over the top, extending the ends past the root cellar walls so the spancrete panel sits on the ground. then build the coop on top of it....

August 09, 2010, 11:08:18 AM

Hi hykue.

This is the proper place for anything regarding any type of building no matter what you need to know. If too far out we would just move it to off topics and still discuss it, so feel free to carry on.

I am not sure of all of the issues you have to deal with. Location makes a difference - frost lines, availability of materials, drainage, etc.

Most of the floors are engineered and thickness and reinforcing could be cut down with a beam in the center. It would then be possible to have a pretty thin floor with composite deck on top. It locks into the concrete and makes it strong. I am not an engineer but work with lots of engineered projects.

The straw bale wall should not be a problem as it would be over the walls of the cellar.

Seems this is going to be very expensive done this way though.

Seems it may be cheaper to make this into two separate projects or at least consider the possibility.

Here is a start on cellar info - first link is readable offline - PDF

homepage.mac.com/rfd1/.Public/Root%20Cellar%20Basics.pdf

http://www.hobbyfarms.com/food-and-kitchen/root-cellars-14908.aspx

Making the root cellar smaller - especially narrower could greatly reduce cost.

considerations · August 09, 2010, 01:08:38 PM

And don't worry, there are lots of girls on this forum, and lots of slack is cut.... and it is not a gender specific act. Welcome!

I'll let the others opine on the structural stuff....I'd just offer that no matter what you decide to do, root cellars are for people food, and chickens...well, chickens poop a lot. Keeping the food products and the chicken 'product' from coming in to contact would be high on my list.

hykue · August 09, 2010, 04:20:03 PM

I hope you aren't all into concise, because I don't seem to be capable of it.

Dug - it is indeed ambitious for a first go, but if it were too easy, it would be boring, right? Besides, we won't need another outbuilding for a long time, so this one needs to provide the maximal learning. I think the stem wall issue is solved by the concrete floor of the coop - the bales will be mostly over the concrete wall and project slightly onto the concrete floor - assuming the floor is strong enough this should also bear on the concrete basement wall. We actually do have a cold room but it's quite dry and small, and we might actually want the area inside the house for heated area. We are planning on having a lot of chickens, but even more importantly they will be spending a lot of time indoors in the winter, as it is much too cold for them to spend significant time outside. I don't want to have those crowded, bored chickens that pick each other all to H-E-double hockey sticks. Also, thanks for the welcome!

Redoverfarm - I asked at my hardware store and they have some sort of similar product to the Q-decking. Thanks a bunch for the idea! I'm afraid to ask them how much it costs, though!

Squirl - As far as moisture goes, I know much of the concern over moisture is eliminated by the cover provided by the coop's roof, but I think condensation will still be a significant source of moisture. My root cellaring book talks about how wood shelving will only last a few years in a root cellar. I don't want that to be my ceiling! It will be very moist in there, with the dirt floor (that's the point, after all), and the tops of the walls will probably be the coldest part (would be even if it was in the soil), so the moisture will condense there. It seems to me, although I might be wrong, that this poses a significant danger of the wood rotting, unless I can somehow isolate the ceiling from the rest of the cellar. Maybe a joist insulated floor with poly to hold up the insulation would be sufficient, or some other vapor barrier on the inside. As to the compromise implicit in making anything dual-purpose, I will cover that later.

Grimjack - the spancrete is a pretty good idea, but I suspect it's also painfully expensive? The other problem with this proposal (if I understand it correctly) is that straw bale walls should really be held up off the ground by a stem wall (or in this case, a protruding root cellar) by around one foot.

Glenn - Yay! You sound like a helpful bunch, and help is what I need (hopefully just the "ideas, mental exercise, and looking for holes" kind of help, and not, you know, the other kind . . .

). Ah, location. Sorry I forgot to add that. I'm in Saskatchewan, Canada. This means a fairly deep frost line, low rainfall (average 16 inches per year), and bleeding cold winters. More about what that means later. As far as expense goes . . . yes. Building a straw bale chicken coop is going to be expensive, and building a concrete root cellar is going to be expensive. I'm not entirely clear on whether combining them will make it more or less expensive. More on that later too.

Considerations - Yeah, I didn't really mean it that way, but I thought of it even as I was typing it. I often curse our language's lack of gender-neutral pronouns. I guess I could have just said "Give me a break," but then I sounded snarkey and sarcastic (to me). And if I had just used a made-up pronoun, you would have all decided I was crazy, right? "Give a zeb a break?" Or maybe "Give a gam a break?" Bah! Anyway, thanks for the welcome! As to the whole poop thing, my husband originally suggested the entrance be a trap door in the chicken coop vestibule area, but that idea was quickly nixed when I realized the crappy implications. Thus, the chicken coop will have a ground-level entrance and the cellar will have an underground entrance reached by a stairway. As long as there are no holes in the floor, there will be no poop infiltration. Another reason I like the idea of a concrete floor - poured concrete doesn't allow much chicken poop to pass!

This post is too long, so I'm going to split it into two.

John Raabe · August 09, 2010, 04:42:58 PM

Don't want to be a wet blanket, but here's my first impression.

Just in terms of economics it appears that you are combining two small and usually inexpensive utility structures, each with different needs, into one rather expensive structure without any real benefit to either use.

Perhaps there are space issues? Something else I'm missing?

August 09, 2010, 05:06:55 PM

So, compromise. It seems to me that's the big question: where are compromises being made and are they acceptable? If they're not, I'll scrap the idea of "stacking" the two buildings and just build them separate. So, I was thinking to make a list of the advantages and disadvantages of stacking, and see what you all thought. Maybe I'm brilliant for coming up with the idea or maybe I'll get so bogged down in the extra details that I won't finish either project . . .

Disadvantages of stacking:
1. The big one in my mind is that this is going to be one enormous root cellar. Really. I could probably store enough potatoes in a 12x15 root cellar to last the whole winter for everyone in the RM (rural municipality, our equivalent to a county). It's probably twice as big as we could ever possibly need, even in the most extreme scenarios. I was considering splitting it in two, a front (cold) half and a back (warmer) half, with the cold half acting as the airlock for that frigid winter air. It would be good for keeping things that wouldn't suffer too much from a light freeze, like hams and such. Assuming we have hams. Regardless, it would be enormous. This means more materials than would otherwise be required. One way to deal with this is to have the root cellar be half of the foundation for the coop, and build a rubble trench as the other half.

2. The earth is not insulating the cellar. This is a pretty good point, but I'm not sure it's significant. Here's why: last year, we went away from the start of January (our coldest month) until March. I left a max/min thermometer in our cold room, which has two uninsulated exterior walls. The walls project above ground about 1.5 feet. We use wood heat and have a drafty-as-heck old house, so it got cold fast, but the minimum thermometer read -4 (celsius, which is +25 farenheit) when we returned. The one I left in my garden read -40 (same in either system). I'm thinking this is some indication that only a little boost to insulation is needed. This boost would be provided in my design by the insulated concrete forms, and to a lesser extent by the chicken coop on top, acting like a relatively warm hat. The chickens should be able to keep their hyper-insulated straw bale coop reasonably warm, or at least warmer than our unoccupied house was.

3. The ideal location for these two buildings is not the same - the root cellar should be very close to the house and the chicken coop should not. I think this isn't too major, as there will be a good path through the snow to the buildings in the winter, as chickens need checking regularly. Also, we don't really have a great location for a root cellar that's much closer to the house.

4. Someone mentioned elsewhere that vermin will be more concentrated around such a delectable source of food - both chicken food AND root veggies! This is a pretty good point. I might have to train my chickens to catch mice . . . or get a cat to live in the root cellar. Or both. But I don't think the problem will be significantly worse than if the buildings were separate. Also, it should be hard for mice to get in, as they would have to make it through either concrete or plastered bale walls. The weak points will definitely be the doors.

5. If the floor cracks (even just little, non-structural-damage type cracks) or is otherwise not sealed, then we will have to stop using one of the buildings, because there will be poop on our food. Yuck.

Advantages of stacking:
1. We only have to dig one hole. I'm planning to get an excavator or something, but it really seems simpler (and cheaper) to me to just have one dig site. Maybe this is really a minor thing, but it seems like a big bonus to me.

2. We don't have to dig as deep - to bury a root cellar for earth insulation, you need to dig quite deeply, of course. If it's going to be sticking up, we don't have to dig as deeply, and the top will be insulated by warmed air and straw bales. This makes ICF's make sense (insulated concrete forms), because we need some way to insulate those sticky-uppy parts. ICF's are kind of their own advantage, because we will be spending less time on building and tearing down concrete forms.

3. The roof of the root cellar and the floor of the chicken coop are shared. This means we will be using less material, probably, as these two components are both combined. I'm not sure this balances out the amount of concrete required for the enormous root cellar, though.

4. The large roof overhang on the straw bale building (to protect the stuccoed bales) will transport water a little ways away, which will reduce the chances of cellar cave-ins. Don't worry, I'm not planning for that to be the only protection, I'm thinking also a gravel-filled sump and drainage gravel/pipe around the edges. But it can't hurt to have a bit of extra insurance.

5. This will be more like a house than any other way we could build a straw bale outbuilding, so it will give us more opportunity to learn. We want to make all of our mistakes on this building, so that when we finally build a straw bale house for HUMANS to live in, we'll have a clue.

I'm sure there are more on both counts, that I'm not thinking of. I'm good at taking an idea and running with it, MAKING it work. My husband (who is away at work and not reachable by phone or email or ANYTHING most of the summer) is good at what I call "looking for the holes". We complement each other well, but without him here I tend to latch onto harebrained schemes and then strive for them until I get exhausted. That's where you come in!

I guess I'm looking for two things right now. I need people to look at those lists of advantages and disadvantages, add to them, dispute them, or what-have-you, and help me see the options clearly. I'm also hoping someone can tell me how to figure out (I don't mind doing the math, but I don't even know where to start) how much concrete (and rebar, and decking, and such) I will need, so I can compare different options price-wise. In other words, I need to do the structural engineering thing, figure out my loads, how thick the walls need to be, how thick and how reinforced the floor needs to be, etc, etc. But I don't know where to look for the information that will guide me in how to do that.

So far, this has been the most helpful forum on this project. Thanks tons (of concrete!) for all the help already. I hope I'm not asking too much! If you need any further information, or have anything to add, whether it's directly pertinent to my requests or not, I'm more than happy to hear about it.

hykue · August 09, 2010, 05:15:13 PM

John: I think I might have covered it in the above post, but I'll answer specifically here. As I see it, what's causing the added expense is the "learning" aspect of this. We want the coop to act as our school for straw-bale design and building. This makes it WAY more expensive than most chicken coops, and necessitates a foundation - it would be a poor test of straw bale and our ability to apply it if the concrete plaster cracked with the first frost heave that came along. It would be cheaper for the chicken coop to just use a rubble trench foundation, and probably just as good of a test, but then we would need to dig two holes, and we would still be pouring concrete for the root cellar, now with the addition of the foundation for the chicken coop. Does that make sense? (I mean both, do you understand what I'm getting at AND am I missing something essential?) Probably the cheapest option would be to dig a hole, build a concrete root cellar with wood forms, bury it in earth, build a minorly insulated shed on the ground for the chickens, and call it done. But it's worth quite a bit to us to have the experience of building with straw bale, and being able to monitor it in the future. It's not just about the economics, I guess I mean. I dunno, maybe I'm a nutbar!

glenn kangiser · August 09, 2010, 05:44:38 PM

hykue, look this thread over and see what you thing about it - as to the possibility of using some of Drews ideas for your strawbale part.

http://countryplans.com/smf/index.php?topic=3681.0

Also lets think about alternate ceilings besides concrete - maybe still using ICF's for the walls then spanning them with wood the 12' wide way and building a Drew chickenhouse on top. Maybe a galvanized sheeting ceiling with insulation sandwiched to the bottom to the joists above to prevent some of the sweat and more insulation between the joists - some venting is necessary in a root cellar no matter what.

Possibly a t&g plywood floor in the coop with a plastic vapor barrier then 1 1/2 inches of lightweight concrete on it?

(Considerations is one of our favorite lady superheroes).

dug · August 09, 2010, 07:03:16 PM

QuoteI might have to train my chickens to catch mice . . .

You won't have to, they're naturally pretty good mousers. Not as sneaky as a good barn cat, but surprisingly quick!

I like Glenn's idea of at least considering alternative ceiling/floor ideas. It would seem to simplify things and not offer much of a disadvantage. Maybe thats because I'm not a concrete guy though.

I don't know your site but if you had a little hill handy I still think you should consider a dug in or bermed root cellar with an earthen roof. Inexpensive, excellent insulation, done carefully the roof can last 100 years plus. Looks cool also. You could pour a slab next to it and have a spiffy straw bale coop.

I like your idea a lot too, just throwing that out there. Looking to seeing your project get started!

Squirl · August 10, 2010, 10:45:24 AM

Actually there are "cheaper" options, as far as foundations go.

You can build straw bale on a post an pier foundation like drew did. Also you could pour a Shallow Frost Protected Slab.

Also ICF can be expensive to me. I look at them more for when labor is considered a premium. I see a lot of DIY owner builders go for concrete block.

I have been racking my brain with your dilemma. One of the problems I keep coming across is that I don't know how far concrete can span unsupported? Anyone?

August 10, 2010, 02:41:27 PM

Maybe steel roof decking with light weight concrete on top,
and some rigid insulation stuck to the bottom followed by some
latex paint to seal the bottom of the foam. Makes keeping it clean easier. Some 6" weld wire type reinforcement in the light weight concrete to
minimize cracking. It is like screen only uses thick wire and the grid spacing is 6".
Mix in concrete sealer when you pour the concrete to help keep the water from reaching the steel.
If this is going to be independant and you are concerned with water getting thru to the steel, there are two more things you can add.
one is to use 3M VHB tape to seal the steel flooring where it overlaps.
and two, put a membrane over the concrete. Follow that by some straw or something to protect the membrane from abraision.
The steel roofing panels can span a reasonable distance depending on what kind of load is on them. If you had it set up so that it was fenced off on top then all you would be concerned with is snow load.
Hope this helps.

Others, please feel free to jump in and point out any mistakes I may have made.
Bruce

hykue · August 11, 2010, 04:41:42 PM

Quote from: glenn kangiser on August 09, 2010, 05:44:38 PM
hykue, look this thread over and see what you thing about it - as to the possibility of using some of Drews ideas for your strawbale part.

http://countryplans.com/smf/index.php?topic=3681.0

Also lets think about alternate ceilings besides concrete - maybe still using ICF's for the walls then spanning them with wood the 12' wide way and building a Drew chickenhouse on top. Maybe a galvanized sheeting ceiling with insulation sandwiched to the bottom to the joists above to prevent some of the sweat and more insulation between the joists - some venting is necessary in a root cellar no matter what.

Possibly a t&g plywood floor in the coop with a plastic vapor barrier then 1 1/2 inches of lightweight concrete on it?

Sorry about the delay, I was dealing with a chicken-fox emergency last night when I intended to be posting here.

Ok, I looked at Drew's strawbale lodge thread, and it was very nice. It's always good to get a look at what other people have done. It was a long thread, though! Whew! Fun, so it was well worth reading the whole thing, but I'm starting to think I should have started this stage of the planning process in earnest MUCH longer ago than I did.

I'm not understanding 100% what you mean about this alternate ceiling/floor - is this one floor possibility or two? If I understood you correctly, you were suggesting one other possibility. From the chicken coop down it would be:

1.5 inches lightweight concrete
plastic vapor barrier
tongue and groove plywood flooring
joists with insulation between them
more insulation
galvanized sheeting.

Is that right?

If so, it sounds like it would also work. In fact, that last layer of concrete might not be necessary, if the vapor barrier could be put under the plywood flooring and trusted to keep all chicken dust out of the root cellar. I would trust it more with the concrete, though. And concrete would be easier to clean.

Quote from: dug on August 09, 2010, 07:03:16 PM
I don't know your site but if you had a little hill handy I still think you should consider a dug in or bermed root cellar with an earthen roof. Inexpensive, excellent insulation, done carefully the roof can last 100 years plus. Looks cool also. You could pour a slab next to it and have a spiffy straw bale coop.

All of the conveniently-located hills at our house are south-facing. That is to say, we're on a hill, near the southern edge of it, and the other edge is too far away/too forested/too snowed in in the winter. And our soil is not something I would want to trust my life to - it's glacial till (I think that's what it's called), so in some places it's very rocky, in some sandy, in some clay. I wouldn't count on it for structural stability. But thanks for mentioning that, it would be a fantastic idea if the site were right for it.

Quote from: Squirl on August 10, 2010, 10:45:24 AM
You can build straw bale on a post an pier foundation like drew did. Also you could pour a Shallow Frost Protected Slab.

Also ICF can be expensive to me. I look at them more for when labor is considered a premium. I see a lot of DIY owner builders go for concrete block.

I have been racking my brain with your dilemma. One of the problems I keep coming across is that I don't know how far concrete can span unsupported? Anyone?

It's true that we could use post and pier, but I was of the understanding that the uprights (I don't know any building terms . . .) would have to be sunk down past the frostline still. Am I crazy? It would definitely still be cheaper, but for some reason it scares me an inordinate amount. Maybe because I have bad balance, so the thought of trying to perch a building on top of a bunch of little uprights is scary? Hmm. That's a stupid reason, I should look into it more. I do know that that type of construction is virtually NEVER used here. That might be for a good reason, or it might be just some weird coincidence. (Tangent - ignore if you wish - my dad told a story about his cousin who always cut the corners off of her steaks before she cooked them. No visible reason. He asked her why, and she said, "Because my mom always did it that way." So he asked her mom why, and she said, "Because otherwise they wouldn't fit in my little frying pan.") I realize the ICFs are a little bit expensive ($24 for each 48" by 16.5" at my local building supply store). That adds up pretty quick to a super-expensive root cellar. I might have to reconsider combining them just for that reason - without ICFs it will get a lot harder to sufficiently seal and insulate the root cellar when it's partly above ground. I guess the question of how far concrete can span is what's holding me up. If I could figure that out - how thick, how much reinforcement, how thick of walls were required to hold it up, then I could do a more complete comparison of the concrete ceiling system to a different ceiling system, and of the stacked-building plan as compared to the separate building plan. I wouldn't want to build it without figuring out those specs, but I can't even make a decision until I figure them out.

Quote from: firefox on August 10, 2010, 02:41:27 PM
Maybe steel roof decking with light weight concrete on top,
and some rigid insulation stuck to the bottom followed by some
latex paint to seal the bottom of the foam. Makes keeping it clean easier. Some 6" weld wire type reinforcement in the light weight concrete to
minimize cracking. It is like screen only uses thick wire and the grid spacing is 6".
Mix in concrete sealer when you pour the concrete to help keep the water from reaching the steel.
If this is going to be independant and you are concerned with water getting thru to the steel, there are two more things you can add.
one is to use 3M VHB tape to seal the steel flooring where it overlaps.
and two, put a membrane over the concrete. Follow that by some straw or something to protect the membrane from abraision.
The steel roofing panels can span a reasonable distance depending on what kind of load is on them. If you had it set up so that it was fenced off on top then all you would be concerned with is snow load.
Hope this helps.

Steel roof decking with lightweight concrete on top seems to be the best way to go if I'm going with a plain concrete ceiling - it can be supported from below and act as the form until the concrete sets, then be my ceiling once the supports are removed. The hardware store guy that I talked to suggested that would be the way to pour concrete for a ceiling. The weld wire reinforcement would help with cracking, for sure. I'm not sure I understand what you mean when you say, "If this is going to be independant and you are concerned with water getting thru to the steel, there are two more things you can add." Sorry, I'm just not getting something.

So, nobody has any online resources (or even names of books I could get from the library) that I could figure out engineering specs for concrete from? Or just get a rough idea? I've already got a few calculations to do (like the cost of just the ICF's - yikes) but I'd really like to make an informed decision about whether or not this stacking-with-concrete-ceiling thing even COULD work.

firefox · August 12, 2010, 02:49:45 PM

"If this is independant" refers to the root celler being
on its own with no chicken house over it.
I would use spray foam on the underside of the steel roof decking
to fill in the valeys of the steel and act as glue to put maybe 1 inch
foam sheet insulation up there. This will eliminate any condensate from dripping down.

so idealy this is how I would layer it:

dirt or something lite like straw
roofing membrain
lightweight concrete with weldwire and sealer
galvanized steel roofing deck
spray foam in the valeys of the steel
1 inch insulation sheets
laytex paint

You will need to rig something to hold the insulation sheets
against the wet foam until it dries. Maybe some sheets of plywood
proped in place with some 2 x4s.

If you go with thicker foam sheets, you may need to hold it
in place with something more than the spray foam. I'm not sure how strong the spray foam is. Ofcourse, the thicker the foam
the more temperature control you have, so it might be worth the effort to add some supports for it.
Bruce

glenn kangiser · August 12, 2010, 04:59:57 PM

Quote from: Squirl on August 10, 2010, 10:45:24 AM
Actually there are "cheaper" options, as far as foundations go.

You can build straw bale on a post an pier foundation like drew did. Also you could pour a Shallow Frost Protected Slab.

Also ICF can be expensive to me. I look at them more for when labor is considered a premium. I see a lot of DIY owner builders go for concrete block.

I have been racking my brain with your dilemma. One of the problems I keep coming across is that I don't know how far concrete can span unsupported? Anyone?

Depends on how you do it - no reinforcement - concrete could crack and fall in, so you get to various reinforcement methods of all types usually always requiring an engineer. Composite deck can help but there are a tone of various decks and engineered configurations. With no deck requires rebar. Expensive nearly anyway you do it.

glenn kangiser · August 12, 2010, 05:46:55 PM

Composite deck is pretty expensive.

Looks like page 25 in this catalog has some that will span 12 feet.

http://www.docstoc.com/docs/25705445/Vulcraft-Steel-Roof-and-Floor-Deck-Catalog

A free download - an engineer would likely need to instruct you on proper use - attachment - edge connections etc.

You need to register to download - just skip the adsense thing.