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Old 11-08-2008, 08:47 PM
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Join Date: Oct 2008
Location: Altadena, CA
Posts: 23
Default roof pitch and dome thickness

Hey everyone. I tried to make this post reasonably short, but I failed.

I'm using self-leveling cement to bring the existing cement slab to level, and will drill a few Red Head concrete anchors as pegs to try and prevent lateral shifting. The self leveling cement is not so much for structural purposes, but rather because the existing outside cement slab is slightly inclined to drain. If anyone finds this to be a horrible idea, please let me know, ideally before I pour it ;-) since it seems unlikely that a building inspector will be sent for this project. I'd rather not raze

One idea that I was thinking of stealing from Kiko Denzer's book is to use a bunch of empty wine bottles (~3" thick + 1/2" filler/binder) in clay slip or vermiculite cement for one insulating sub-layer, and then plop a couple of sheets of 2" insulating fiber board on top of those. I found a local supplier for the insulating boards, 3 sheets to a box:

($17.54) -- 3-SHEETS/Case 2" CERAMIC BOARD, Density 17.5LB/FT3 or 280KG/M3 at McGills Warehouse

If I form a 40" x 48" x 5" void box and pour around it, I figure that would give me a "drawer" in which to seat the bottles and the fiber insulation board, and then the landing is all "real" concrete. I guess we could just skip the bottles, but I've grown somewhat sentimentally attached to the idea. Plus it is perfectly good airspace.
I am curious what the mimimum thickness for the hearth slab can be if we pour it on a rebar mesh like we did with our kitchen floor. I am guessing that the oven will weigh around 1200 pounds, which doesn't really seem like too much for a 3"x46"x61" slab holding a 39" diameter object (I calculate approximately 1 psi?). The overall thickness of the hearth slab plus bottles and 3.5' x 4' x 2" of fiber board would then be 8.5". 3 full courses of masonry blocks plus a half thickness course would put the hearth at 36.5", we have some bullnose saltillo tiles for the landing which can be underlaid to sit at 39" (even with the oven floor). Once that is in place, I planned to build a 32" (16" vault) Tuscan-style oven, using 2" of ceramic fiber blanket all around, plus loose Perlite in the enclosure "box" wherever space remains. I want to construct the top of the dome from firebrick "thirds", so a 16" vault + 3" thick dome + 2" of fiber blanket would bring me up to 60", about where I'd like the roof to peak. Is that a sensible plan of action? I figure I can make the top of the dome thinner than the bottom courses, since it won't be bearing significant load. I assume the sides should be 4.5". The same local supplier of the fiber board offers a good price for ceramic fiber blanket, 2" x 25 sq ft:

$26.62 -- CERAMIC SPUN BLANKET 2" - 8LB/FT3 or 128KG/M3 at McGills Warehouse

My wife wants to put a cement board enclosure around the top, leaving 12-15" of counter space in front. This seems simpler to build than a decent igloo, now that I am mocking up the pieces. Plus I can fill up the remaining airspace with Perlite. The thickness of the insulation is dependent upon the thickness of the oven, which brings me to the question of whether a 3" thick dome (firebrick cut in thirds) is structurally sound. While I am not planning on jumping up and down on top of the dome, I would like for it not to collapse. At the same time, more insulation and quicker heating, along with a consistent dome/hearth thickness, appeal to me. Is it a poor trade-off to use a 4.5" thick walls, a 3" thick dome (top few courses) and more insulation all around, for someone who bakes 2-3 times a week (often roasting an additional 1-2 days per week)? Up to this point, we've just been heating up saltillo tiles in our BBQ grill, and using that for pizza (takes about 4 minutes per pie).

Finally, for a boxy cement board enclosure, does a 2-in-12 roof with 4" of overhang on each side sound about right? I have been interested in a standing seam metal roof for our house, which has a low rise roof, and this sort of project would give me a chance to play with the materials and weld the peak together myself. Sensible?

This all seems reasonable on paper, and one of my neighbors is a civil engineer, so he might be willing to look at the statics of the dome for me. But from a practical standpoint, do these dimensions, materials, and ideas look reasonable, for people who've built a Pompeii oven before?

Thanks much to anyone who has the time to reply. Sorry it's so long-winded...

--t
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