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Chris’s Scoria Dome in Crestone

Chris’s reciprocally framed scoria bag dome in Crestone

Chris’s reciprocally framed scoria bag dome in Crestone


“This is Chris who’s building the 24′ reciprocally framed scoria bag dome in Crestone. It has a 12′ gothic arch out the front southside; a vaulted airlock entrance; a 100 square foot cool pantry; and loft. I have a few months into this, 90% solo.

Why I did a reciprocal roof on the dome:
Once I started corbelling the rows in a good bit above the loft level, it became apparent that I was building at the limits of size for a self-supporting scoria filled earthbag dome. This 24′ dome would become quite tall if it was to be built with the typical lancet arch side profile, and I would have to wrangle with the errant bags wanting to roll inward where my circle was bulging. Seeing how the loft completely solidified the bagwork on the lower dome walls, and wanting to keep the build solid, aesthetic and cheap, I excitedly chose to try out reciprocal framework for the top of the dome. Even including the expensive domed yurt skylight, I figure that the reciprocal framing route came out rather equal in terms of time and money, considering the roughly 7′ of dome height, scoria fill, bagwork time, and plastering in the skylight that I avoided. It took a few nights to peel the timbers, and a friend helped on two very cold half days to spiral it up together, bolted up top, and rebar pegged through the feet. The reciprocal framing strengthened the bagwork so much that I pulled the rebar and bottle-jacked out the bulges into a nice aesthetic domed loft space. This held shape perfectly before I pounded rebar back through the timbers and bagwork.”

Anyways, been busy working a lot of overtime out of town so I haven’t gotten to do any work on the dome in a while. But after watching my build, there are half a dozen people now interested in building with earthbags in the San Luis Valley. My coworker is interested by my talk of rebuilding the walls of his destroyed 20′ yurt with scoria filled hyperadobe, utilizing a single billboard tarp and 2 dozen small timber posts, like VelaCreations/Abe Connally built off-grid in Mexico. Should be quick and cheap.”

Chris’s Picassa
Note: Scoria is a type of lightweight volcanic rock that’s ideal for earthbag building. It is fireproof, rot proof, highly insulating, light and easy to work with. This is what Kelly used to build his dome home. Scoria requires extra steps to stabilize the walls, because the material is loose and can shift. Search our blog for more details.

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13 Responses to “Chris’s Scoria Dome in Crestone”

  1. Chris says:

    I have to get to work soon, so I’ll try to keep this short.
    After reading what there is on reciprocal frames and building a few, I want to stress on the ground full size mockups. There are an interesting combination of factors in reciprocal forces and building with a precision inner opening. Throw out the spreadsheet math, formulas and trigonometry long before you even touch a ladder.

    Member diameter can decrease as their quantity increases. Unless one wants a pyramidal hip roof with cupola or something, I think more smaller members are much easier to cut, carry, position, etc. And then you don’t require shoring between, which is present in most pictures online as either secondary members or thin decking/sticks. Hopefully they framed well, because I would be anxious to inhabit if their reciprocal rafters were depending on decking strength to hold up their shape under the weight of earthen roofs. But this isn’t the case. Unlike models, these roofs can be built secured into wall: strapping, joinery, rebar, cobbed in, etc. I cut ‘feet’ angles to match my pitch and hammered 5’lengths of rebar through the bagwork, extending down to the loft floor which is also rebarred into place. At the top intersections people have lashed, bolted, notched. This is helpful so that the rafters don’t roll outward before the last one its in place.

    The more members, the more likely the need to notch. this requires some thought, but its important if you want a precise skylight that doesn’t spiral up in intersection height or lose its circularity. I dished out my notches with an angle grinder and sanding pad to accommodate the overlaying round timber. It was easy to lift, notch, check, repeat, until it was perfect. Once all was set, we dropped the Charlie-crutch and let the weight settle down, it moved maybe an inch. I’m surprised at the lack of quality notching on round timber on documented builds. Our inner circle and intersection heights were perfect in another 30 minutes of leveraging timbers up to fine tune notching.

    At this point, my friend and I walked and jumped on it trying to get it to settle further and see how the corbelled dome wall, which had previously been a little funky at two bulge points, would react to this simulated tramping. Nothing. Rock solid. I took a reciprocal saw and cut the tops level, and then countersunk lag screws into each intersection, leaving room to plane. I pulled the rebar out of one foot at a time and bottle jacked out the bulges back to an even compass circle and doming in the loft. Re-pegged the rebar. Slammed two bags between each timber foot that just barely pounded flat to create a compression ring, to much chagrin of my mason bond beam friends, but the corbelling bags would not deflect upon a strong body check after the compression ring and framing.

    Next step is to wrap and fence staple two layers of stucco netting (for overhanging plaster) and some salvaged field fencing around timbers and culvert windows. bag up and over, exposing the timbers on the inside.
    So much for keeping that short.

    • Owen Geiger says:

      Excellent comments, Chris. You’ve made many good points that aren’t available elsewhere. Like they say, there’s nothing like actual experience.

      Again folks, please send us your project info so we can make this blog as useful as possible. We know there are thousands of similar projects out there, but most people don’t take the time to document their project and show others. You can keep your name and site location confidential if you want.

    • Jay says:

      Driving your rebar all the way down through the bags to the second floor makes a lot of sense.

      It “almost” makes the second floor “sort of” act like collar ties for the roof.

      Well done.

      The stucco netting you are referring to, can you be more specific? Are you simply talking about using the stucco netting to help shape and form the plaster around the logs, or are you talking about spanning it between logs to become a ferrocement roof?

      On a different topic, I would think that doing exterior plastering on the bags up under the roof eaves would be drastically easier before you lay sheathing or stucco mesh for a ferrocement roof. Once the roof surface is in place, getting up in the nooks and cranny’s could become far more difficult, but you’ve probably already considered this fact.

      • Owen Geiger says:

        The rebar in this case would distribute the forces down the curvature of the dome. Very clever indeed. This eliminates needing a bond beam to prevent spreading forces.

        Is he going to have a roof overhang? I got the impression he is was going to lay remesh or strong fencing with stucco mesh on top. Then continue the bagwork on top of this. This method would work well in a cold, dry climate like Crestone.

        • Chris says:

          No eaves for this reciprocally framed dome. there is a good picture of the feet on my picasa link. Two layers of 2″ stucco netting offset creates 1/2″ oPenings to retain 40° overhanging ceiling plaster (probably a sandy slaked lime). Run electrical for fixtures next to timbers. Slap on the plaster, and pull the shrink wrap off the already peeled and oiled timbers. Done.

          • Owen Geiger says:

            Great. Do you have a rough idea of your cost so far? How many total square feet interior?

          • Chris says:

            Interior square footages:
            450′ 1st floor
            175′ loft (125′ standing height)
            100′ cool pantry
            50′ vaulted entrance
            About 750′ total

            Shell price$$:
            320 misprinted bags, shipped
            2740 for Scoria fill, foundation insulation, delivery
            450 excavation to sink house 3-4′, well trench, septic
            220 barb wire
            370 scrap yard: grain bin arch panels, culvert windows rebar, pipe, cool pantry roof, rmh material, scrap lumber, misc
            100 timber
            100 misc hardware
            200 billboard tarps and cool pantry roof insulation (used sips, poly iso, xps…r50 or so)
            Total $4300

            I have about the same into electrical and plumbing rough in, fixtures, yurt skylight, half of my windows, doors, scratch coat, permits, pump, misc, cistern

          • Owen Geiger says:

            Great info, thanks.

  2. Jay says:

    It is worthwhile to note that the primary large diameter members of a reciprocal frame BY THEMSELVES will not stabilize walls much at all. It might be easy for someone to get that misinterpretation.

    However, once the reciprocal frame receives cross bracing or sheathing between the reciprocal frame members, then the entire roof frame becomes much more rigid and significantly stronger, and can help stabilize walls very dramatically.

    This phenomenon is very easy to see if one builds a scale model. The reciprocal frame is very unstable and shifts easily until the frame members are tied together to form triangles.

    Anyone wanting to shift bags or do any significant work on the walls would be well advised to firmly attach the reciprocal frame to the walls and sheath the roof frame, or at least put up cross lattice boards, before assuming that the roof frame is performing any stabilization of the walls. (Even if it initially appears to be doing so after lifting the primary members.)

    You don’t have to take my word for it.

    This is where a structural engineer can be your best friend, perhaps even keeping you from getting seriously hurt depending upon how your particular structure is designed.

    Cool structure. I like it. I just thought that this clarification could potentially save someone some big headaches.

    • Owen Geiger says:

      I wonder how he connected the poles at the top. And I was wondering what the shell of his dome cost (including cost per square foot). Maybe Chris can answer a few questions. Building with scoria like this is significantly faster and more insulating than earthbags and so I would imagine quite a few readers are interested in the details. As Kelly points out in his dome guide in the link above, scoria bags can be handled by one person whereas typical earthbags usually take a minimum of two workers.

      I’m also interested in a less costly skylight, possibly made of plexiglass. An operable skylight that can vent the dome would be ideal.

      • Jay says:

        I’ve never tried it, but if the skylight openings are designed small enough, I’ve always thought a salvaged windshield from a junkyard would be an amazing skylight. Just look around for the cheapest junker windshield that the junk dealer doesn’t think he will be able to sell. Or… look around for a junker car that someone wants hauled away. Take the windshield and sell the rest of the junker to the junkyard for scrap.

        The trick is to make your own window frame and appropriate flashing. Ziggy’s idea of a used car/truck/tractor tire might be useful.

        Alternatively, just building a wooden frame and scribing the wood to fit the windshield shouldn’t be too difficult or beyond the abilities of most builders. It might be tricky to get it scribed accurately, but it’s doable.

        Finally, creating a masonry or ferrocement window frame should be a snap. Just lay some mortar, put a temporary layer of plastic as a release layer, and press the window into the mortar. Once the mortar is cured, flash the mortar appropriately and apply a flexible caulking to seal the windshield to the masonry window frame.

        I’m leaving out the details of how to correctly flash a skylight and interweave that flashing with the roofing. It’s critical to get those steps correct or you WILL get leaks.

        For higher R Value, just use a sheet of regular glass on the inside of the skylight, or if it is very large, plexiglass to avoid broken glass becoming falling guillotines.

  3. Owen Geiger says:

    We’re always looking for reader contributed content of actual low cost/dirt cheap, sustainably built projects. Please document your project so you can share with others. Send submissions to me at naturalhouses [at] g mail [dot] com

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