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kebwi 01-29-2010 12:12 PM

Terraced enclosure
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Here's a really crazy design. I have, from the beginning, wondered if I could cover the thing in dirt and let moss, grass, and other weak-rooted (nondamaging) plants grow on it. I especially like the idea of terraces, as shown.

There are two major issues: preventing dirt, water, and roots from damaging the structure and building such a self-supporting structure in the first place.

On the first issue, I would put down several layers of heavy plastic and only make the dirt an inch or two deep so only small plants could grow in it. I would also provide good drainage off the sides, and maybe even slope the terraces a little so they run off the outer edge.

On the second issue however, I am less certain. This is basically a corbeled arch, which is notoriously weak. I would need to build annuli (flat rings) of rebar-enforced concrete.

The question is, do you think these concrete annuli can be built in such a way that they will support the load of the upper terraces (and a few inches of dirt)? How thick should the terraces be?

In addition, how the heck would I do it? If I form and pour an annular terrace in place, I won't be able to get the plywood out from under it! Could I make the concrete form out of that hardi-backer stuff (is this just sheet rock?) and then just leave it in place under the poured concrete? Could such a material be cut to this kind of shape?

On a related note, could I simply build the terraces out of hardi-backer (perhaps several layers) and not bother to pour concrete terraces at all? On major problem with that approach is there won't be any metal (rebar) to support it.

There must be a way to build this structure. It doesn't have to support much weight after all (just itself).

Is this totally crazy?

nissanneill 01-30-2010 03:52 AM

Re: Terraced enclosure
Hi kebwi
I don't see a problem at all provided that you have adhered to the proper shape and proportions of the Pompeii and insulated then sealed it carefully. This would be critical to ensure that no moisture will get into your oven and no heat get out to steam your plants/grass/moss etc.
I would get some chicken wire or fine weld mesh, fold it over once and then 'put a halo over your oven' and then ferro cement it in place. This is then repeated at the next level and so on until you have the dome 'terraced'


kebwi 01-30-2010 05:27 AM

Re: Terraced enclosure
I think I understand your idea. One thing I want to clarify though. Are you suggesting that the ferro structure ultimately rest against the oven such that *if* it sank even the small amount it would immediately become load-bearing upon the oven?...are you are suggesting that the ferro structure be free standing from its lowest-outerest footprint against the hearth?

I don't want it to rest against the oven for two reasons:
  1. There will be about 3" of "fluff" insulation and I want additional loose vermiculite between that and the outer shell.
  2. The fluff insulation layer will obviously not be load-bearing. Any significant pressure will compact it down onto the dome, thus not bearing a load anyway while simultaneously reducing the insulation thickness.

But ultimately I like the idea of a ferro structure. I had imagined some things like that (for other designs) but didn't know the lingo for it. Thanks for the head's up on that.

Do you have a recommendation on the kind of cement or the basic recipe for cement to use in ferro construction?

Tscarborough 01-30-2010 08:34 AM

Re: Terraced enclosure
That would be a great place for the herb garden.

Archena 01-30-2010 09:05 AM

Re: Terraced enclosure
I think it's a neat idea but would probably be better as a free standing structure surround. You can add insulation in the void between the two. Use stringers to tie the rings together and to support each terrace. As long as everything is connected firmly it should eliminate the problem of stressing the arch and would probably support the terraces much better.

I agree with Tscar (who needs a shorter, easier to spell user name) - it would be wonderful for herbs (non-invasive, naturally)! Actually, if your climate will support it thyme would make a good ground cover and you could use annuals which wouldn't endanger the structure. Basil, sage, dill, - I'm getting hungry!

kebwi 01-30-2010 10:44 AM

Re: Terraced enclosure
Help me understand your idea Archena. Are you suggesting a skeletal framework *above* the above, from which terraces are suspended, like a suspension bridge?

stonejohn 01-30-2010 11:02 AM

Re: Terraced enclosure
Hi All, I will Only speak to the issue of moisture into the oven and surrounding masonry. I worked for a builder of water features, ponds, fountains, and pool type structures. Any time we had loss, leaking of water from the structures themselves we would use a product called " Pond Armor" It was a two part epoxy coating, worked very well. It would make sense to me if it keeped the water in the fountain it should keep the water out of the oven. I'm also making the assumtion the areas to seal are the outer shell of the wfo, are insulated and do not get hot. i just googled it, It still around.

kebwi 01-30-2010 11:09 AM

Re: Terraced enclosure
Yes, thanks. I am quite concerned about the aquatic and botanical threats my idea poses. Aquatic, as you said, in the form of seepage from my dirt, and also from Seattle's weather in general, which had strongly warded me off any "stucco"-style (of which this design counts, i.e., no true roof). Botanical in that I am worried about stronger plants breaking the outer shell with their roots...but I guess plants don't generally break pots that are too large for them, so perhaps that concern is unwarranted. For both reasons, water and root, I would put down a layer of plastic under the dirt.

Pond Armor, noted. Thanks.

kebwi 01-30-2010 11:54 AM

Re: Terraced enclosure
How about this idea, assuming you guys are following this thread. I had been hoping to fill loose vermiculite inside a walled cavity, much like a house enclosure, but what if I simply mold the terraces out of vermicrete? To get nice sharp edges I could basically form each terrace with plywood and fill it like pouring normal concrete (I might use large 2" concrete blocks for the vertical walls of the terrace).

Would that be a much easier way to achieve my intended result than various enclosure ideas (concrete-wall-and-floor or a ferrocement)?

Related questions: If this structure must support the weight of perhaps four to six inches of dirt, should I use a stronger vermicrete mix than is often used for domes to make it stronger? 5:1 to 12:1 seems to be the FB range. FBers often go stronger at the hearth to support the weight of the oven and weaker over the dome for maximum insulation. Should I go a littler stronger (sacrificing insulation for strength) if the structure will have half a foot of dirt, or is that so inconsequential a weight that the 10:1 to 12:1 mixes would be sufficient?

Thanks again.

jmhepworth 01-30-2010 12:04 PM

Re: Terraced enclosure
I believe you will need a 100% certain moisture barrier between the oven insulation and the dirt. While I didn't use vermicucrete, my understanding of it is that it will likely absorb moisture if there isn't a moisture barrier around it. I like your idea to make the terraces. But I would figure out how to do it by building an enclosure that is impervious to water and put the terraces on top of that.

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