36" in Seattle, second oven

[kebwi]

Would the AR/SS combo you said was an option consist of 500 of both (250 each), or 500 of each (1000 total)? This probably doesn't matter too much, but nevertheless, asking...

[kebwi]

Fibermesh 150 is sold by weight. How many 1lb bags do I buy to obtain a target volume? Or put differently, what is the volume of the 1lb bag it's sold in?

[david s]

A 1lb bag will be plenty.
Regarding the casting of the gallery, it all depends how thick you make it. Because I didn’t want it to act like a heat sink I made mine really light, (with reinforcing buttresses) only 12 kg. But the thinner you go the weaker it becomes.

[kebwi]

Thanks.

AR fibers: 1/2", 3/4", 1" (I'm seeing options as I shop around). Does length matter?

[david s]

I’m used to using the 3/4” ones and find them really good to work with. Presumably the longer ones would work better, but you may find some will stick out and you’d have to relocate them.

[Giovanni Rossi]

Unrelatedly, I wish anyone other than david would participate in this thread (thanks for a few others I admit, but my point stands). This question isn't about cast oven building at all. It's about design and personal project goals. I wish I was allowed to start new threads for questions that fit into very different topics, with a relevant subject line for those question threads, so each thread would be categorized (and searchable and attention-grabbing in the thread-list) by its relevant subject line. I don't approve of the new way the forum has been organized frankly.

I hope I'm not speaking out of turn. I understand your frustration with finding the most up-to-date and "best" practices on the Forum. That is the nature of a free, open platform. I spent well over a year researching the Forum before I even started developing my final plan. I have read every post dating back to Jan 2020 and hundreds more before that through searching. I believe users are reading your posts and questions and will reply as they can. david s is really on the ball and contributes quality info before most have had a chance to catch up. And, you are really operating in his wheelhouse right now.

There's an old (and of questionable origin and accuracy) phase in the U.S.: "there's gold in them thar hills" that pertains here. There isn't a perfect solution for all ovens. Each builder should digest the info and apply it to their individual circumstances. New ideas come and go and some "stick". If you want a quick fix and don't want to strictly copy another build...then you get what you pay for. Yes, searching within the Forum can be challenging. Another option is to use a Google search including Forno Bravo Forum in your key words. This can save some time.

Every now and again someone (as you have) pushes against putting all questions in a single thread thinking that each topic will get more focused attention. I don't think that is an issue because I believe those who are going to respond are routinely looking at the posts. One thing you could do is start your questions with good key words. I have a short cut icon on my browser that points to the Forno Bravo Forum page. On my view there is a Newest Posts section. There is a 36 character "preview" from each post. You could use those characters to focus the users attention to the specific topic of your post. Others may have other tips.

Sorry to drone on. I am enjoying following your posts and questions as you work through the process.

[kebwi]

This is mostly just me thinking out loud, but also inviting commentary if anyone has any thoughts on the matter.

After deciding that instead of building my second oven from brick, I would cast it, and then learning as much about cast ovens as possible, I have now been focusing primarily on the stand recently. I have multiple, somewhat incompatible goals for this second oven. I want it to be transportable, both on a micro-scale (around the patio in my yard to reposition as needed for various other uses of the patio) and on the macro-scale (to take it with me if and when I move). I lost my first oven because I built a 5-ton structure that, without a major crane-job, wasn't going anywhere. I hope to hell the buyers of that house are using it, considering how much time and effort I put into building it. Sigh.

The way I see it, there are three major categories of stand structure: masonry, metal, and wood. I'm not a fan of masonry for this second oven for reasons of mobility, both micro- and macro-scale. Here are my thoughts on these three approaches.

MASONRY
I have come up with a variety of masonry designs that would alleviate, if not fully obviate, my concerns:

1. Use 6" concrete blocks instead of 8". Since I have settled on a cast oven, and decreased the oven size from my first oven's 36" to 30" for the second oven, I am confident that 6" blocks would more than suffice to support the hearth and the oven.
2. Instead of a full U-shaped wall, build multiple disconnected masonry supports, either three separated walls (no less weight overall, but easier to deconstruct later), or better yet, multiple masonry "columns", say from a single stack of concrete blocks. Obviously these structures would be bolted together (discussed below). I would still build using the Pompeii design of dry stacking and then filling one core with rebar and concrete. There's no particular reason to fill the other core since a single filled core would utterly join the column (or short wall) into a single monolithic structure.
3. Place plastic on the patio underneath the dry-stacked blocks before filling them, so the poured concrete doesn't bond to the patio. The three-separated-walls option could conceivably be dismantled (with effort) at a later time, and independent column stacks could even be moved to new locations (rarely), say with a hand-truck (after removing the hearth and the oven of course). A four-6"-block column with a single filled core should weigh about 150lbs.
4. Don't extend rebar up into hearth; leave the hearth physically unjoined from the walls or columns. Instead, leave the tops of the columns flat, and then cover them with plastic while pouring the hearth so the hearth doesn't bond to the columns. This will facilitate deconstruction at a later time.
5. Cross-brace the columns to one another with metal strips or 2x4s bolted to the blocks via holes drilled through the blocks. Obviously, single columns will otherwise be intrinsically (and disastrously) unstable.
6. To attach the hearth to the columns, position plugs during pouring (just like weep holes under the oven insulation layer) that align with sturdy corner braces to be placed beneath the hearth. These corner braces will attach to the columns via the same bolts or threaded rod used for the cross-braces and then will attach to the hearth via bolts dropped through the hearth from the top. If the hearth holes are shaped with a wider section at the top, the bolt head can drop inside, leaving the hearth's top surface flush. The bolts could be stainless and the hole could either be left exposed (what difference does it make if rain drips through so long as they are stainless) or filled with something removable, like silicon, to assist in occasional bolt maintenance (tightening/replacement) and eventual dismantling for moving the oven.
7. To further prevent the hearth from any lateral movement, the cores of the columns would be left with depressions: the filled core might be left an inch shallow and the open core is, of course, empty to the bottom, so I would build a wooden structure (a 2"x3" with a plywood top that spans the core), dropped in such that the plywood top rests an inch or so below the top of the column. The plastic across the top preventing bonding between the hearth and the column would then by pushed down into these depressions of both cores, forming shallow wells, and the poured hearth would them obtain "feet" that fit into the cores, preventing lateral movement across the tops of the columns (in addition to the corner braces of course). I might put some corrugated cardboard or thin styrofoam around the perimeter of the wells too, so the concrete doesn't fit so tightly that it can't later be lifted off the columns.

This design, horrendously complicated as it is, would in theory enable the oven to be deconstructed or macro-moved (but not micro-moved around the patio). A palette-jack, forklift, or crane would be able to lift the unbolted hearth off the columns. Since palette-jacks only have a lift range of 1"-2", it would be critical that the "hearth feet" not descend into the columns deeper than that range, an inch or two. The columns could then be un-cross-braced from one another, and the whole conglomeration could be moved (or the columns could be discarded and the hearth-with-oven could be placed in a new location on a new stand). I have calculated that the hearth/brick-floor/cast-oven should weigh about 1500lbs at 54"x58" for a 4" hearth and 1200lbs for a 3" hearth (a possibility since a cast oven may not call for a 4" hearth, perhaps 3" is sufficient; yes I know slab strength follows the square of the depth, so 3" is about half the strength of 4": 9 vs. 16). These weight calculations don't include either (1) igloo rendering or (2) enclosure framing/walling/cladding/roofing, so it would weigh notably more than these estimates, but they are reasonable starting points. For example, there are heavy-duty palette jacks that can handle these amounts -- barely.

I'm leaning toward an enclosure incidentally. As much as I like the look of igloos, I'm not sure how to weather-proof it in Seattle's climate, so a roofed enclosure (later to be pergola-covered too) filled with loose perlite seems a better option -- and of course it'll more readily dry out and steam out with nothing over the blanket but loose perlite fill; no need for an integrated purge valve as has become popular with igloo designs. All that said, my first oven, also in Seattle, was functionally an igloo anyway. It didn't look like an igloo because it had planter terraces on top of it, but it was still just an outer covering of surface bonding cement, essentially stucco, and for all I know it wasn't particularly waterproof (especially considering it had literal planter beds on top of it).

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[kebwi]

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METAL
I have been arduously feature- and price-comparing steel square-tube table legs that have apparently become all the rage lately. Multiple companies online, a few Etsy shops, and Amazon, all offer a menu of nearly identical designs across a range of styles and sizes. Clearly, there's a lot of copy-catting going on out there. The most common configuration is 14-gauge 2" square tube, offered with a variety of paint-like finishes or with an oil/wax finish to mitigate rusting while preserving the metal appearance. Some places offer the put no coating on so that you can coat, protect, or paint the legs on your own. From this starting point, multiple designs are commonly presented: four single legs with no cross-bracing for example. Or end-pairs (you must buy two to build a table) consisting of an A or H shape with flat metal across the top and a horizontal tube connecting the pair toward the bottom. A few other shapes crop up too, such as X designs, etc. I like the H designs best because they support the weight above on perfectly vertical legs, the strongest structure of course. Generally, both height and width (for the paired designs) are highly configurable, often to within single-inch variation. Some places offer 3"x2" tube or even 3"x3" tube, and sometimes the crossbar is 1"x2", but 2"x2" is the norm, as is 14-gauge. A few places have offered to custom-build with 11- or 12- gauge for me, but I'm unsure on pricing yet. In most cases, the maximum width offered for pair legs tops out around 30", or sometimes 36", so they are insufficient to support an oven. Some places have offered to make custom legs with the pairs at a greater width, but the price goes up of course.

I have multiple concerns with metal legs. The official weight ratings advertised with these products usually fall far short of our needs with these ovens, even the cast designs. That's why I have looked into lower-gauge and larger tube options. I'm curious if I could access the tubes from the bottom (the feet) and fill them with concrete, perhaps even with a stick of rebar tossed in, but this isn't certain since the photos rarely show the bottom of the legs.

Another concern is lifespan. The argument against wood, discussed next, is that it won't last very long, but anyone claiming steel will last any longer is fooling themselves. I've seen a typical backyard grill rust away to pure dust in five to eight years in Seattle. Coatings on steel eventually flake away and the steel very rapidly deteriorates from that point on. It only takes the smallest weakness in the coating and then water pushes the coating off from the inside in a feedback loop. Steel legs will require just as much routine recoating as wood with its related need for recoating with protectants and sealants. I don't see metal as either (1) outlasting wood, or (2) requiring less maintenance to stave off degradation. I see both of those variables as being essentially identical between metal and wood oven stands.

As I push the default configurations into oven-supporting territory (wider leg pairs, heavier gauges, larger tubes) the price goes up, to say nothing of add-on costs like adding cross-braces *between* pairs of legs to alleviate sheer along that axis (or cross-bracing four individual legs for that matter, which at least alleviates the limited width issue, but only by putting the onus on me to cross-brace the legs, and since I don't weld, this is not a simple matter). And of course they require several bolts, washers and nuts, arguably in stainless steel, which isn't even included in the original purchase price. In the end, metal leg stands aren't price-competitive with masonry or wood as far as I can tell. The advantage they offer is relatively lighter weight and mobility. The legs can even take casters in some cases (some venders even offer this as an option, although I might put my own on so I can buy heavy duty casters).

WOOD
Which brings me to wood. I am well aware that there are those on this forum who will never recommend, and barely even acknowledge, wood stands, but this is where the reasoning above leads me. I'm very comfortable working with wood, while I have no skills or tools for welding to make my own steel frame (or event to cross-brace legs that I otherwise buy whole). It seems to me that a heavy-timbered frame, sealed as best as possible (and admittedly calling for maintenance over the years) ought to last at least as long as metal. I am eyeing huge beams. Not just 6x6, but 8x8, 8x12, and even 12x12. Depending on how the winds are blowing on Craigslist and Facebook Marketplace any given day, these materials can be obtained remarkably cheap and would be strong enough to hold up a truck full of pizza ovens. I admit, I am worried about deterioration. I don't want the stand to either catastrophically fail (fall over, which is unlikely) or just slowly degrade (soften at the joints and become rickety or wobbly, or have the bottoms at the ground steadily deteriorate away). I don't want the ends, standing post-like on the patio, to rot of course. I would take all the precautions I could, such as putting the stand on shallow masonry raised above the patio (say 2"x12"x12" pavers) so any pooling on the patio doesn't immediately impact the legs. I could borrow a FB trick vis-a-vis the hearth/insulation boundary by putting the legs (or the pavers) on a thin layer of nonporous tile to reduce water seeping up into the legs.Or, as described with metal legs, I could put heavy duty casters on the bottoms of the legs, which would both add mobility and isolate the wood from the ground by nearly half a foot. I could go so far as to clad or skirt the stand with plywood or sheet metal to keep lateral rain (to say nothing of deep snow and ongoing sun exposure) away from the legs (a crucial feature if I store firewood under the oven anyway, I suppose). These are all ideas that have crossed my mind, but I recognize that they merely reduce, but don't eliminate, the problems with building with wood. But I still don't think metal would necessarily outlast wood, and I am wary of a masonry stand for its essentially permanent and exceedingly heavy features.

So, that's where I'm at right now. I've been hemming and hawing over this issue for a few weeks now, madly bouncing ideas around trying to figure out the best solution.

[david s]

Like you have mentioned I use 6” wide masonry blocks which provide sufficient footprint for stability and with only the corner cores filled require only 2 litres rather than 3 for the 8” blocks, for each core. I use two piers pinned to the foundation slab. This is far easier than building a U shape and has the added advantage of allowing breeze through, which keeps dampness out of the wood storage area. A U shape, while increasing stability, holds in any moisture in the space.
I’ve sent you a PM re stand materials.

[kebwi]

Yes, I've seen your photos of ovens standing on two parallel walls or piers with no back wall. There has also been discussion in the forums of putting a few holes in the back wall at the very least for airflow (blocks on their side for example). But I have seen your photos with no back wall at all, and I agree that seems quite feasible. Thank you.

[kebwi]

david s It looks in many of your gallery photos like you're using a very thin hearth slab, maybe two inches. Is that two inches of poured concrete or some other material? I'm also encouraged by your ovens on rather flimsy looking metal frames and carts. Perhaps I'm overestimating the amount of support I need.

[david s]

david s It looks in many of your gallery photos like you're using a very thin hearth slab, maybe two inches. Is that two inches of poured concrete or some other material? I'm also encouraged by your ovens on rather flimsy looking metal frames and carts. Perhaps I'm overestimating the amount of support I need.

By cantilevering the supporting slab and reducing the span of the piers that support it, the thickness of the supporting slab can therefore be reduced. A stand for my ovens only require a load of 250kg because my ovens are only 21" internal diameter. The precast supporting slab is reinforced, 40mpa concrete, 2.5" thick where it needs the strength, but thinner elsewhere to reduce weight.
Obviously a larger and heavier oven requires a stronger stand.

[kebwi]

That makes sense. Of course, cantilevering reduces the storage area as well. I'll weigh my options. Heh heh.

[kebwi]

david s When you said your piers were "pinned" to the slab, what did you mean by pinned? You explained that the slab is pre-fabricated, so obviously you aren't bending rebar up into the slab from the core as the slab is poured. Is your slab actually disconnected from the piers such that, perhaps with some bolt removal, whatever your "pins" are, you could lift the slab off and take it somewhere else (with the oven), leaving the piers behind? That's essentially what I want to do, somehow.

Also, I see that in some of your photos, the piers are three cores wide. The implication of only filling the corners is that each pier actually consists of two unjoined pieces that merely slot together around the square-wave shape of the central core. Is that right, each pier is actually two pieces that aren't bonded together? Of course, filling the central core wouldn't work either because the two side cores have single blocks in them. When I made my first oven, I followed the Pompeii design of only filling every other core, but at the corners I would fill all three blocks around a corner for this reason.

[kebwi]

How far can I cantilever concrete beyond the stand to create a shelf in front, or on the sides or back, of the hearth (to tuck the support wall/piers under the hearth and more directly under the oven)? Once the hearth already contains rebar and perhaps other metal meshwork, do fibers make any difference at that point? What is the right kind of fiber to strength nonrefractory concrete like the hearth? Is AR glass or poly more appropriate in that installation -- or do I not need neither any fibers if I'm incorporating rebar anyway into the hearth anyway?

How thin can the extended shelf be, as opposed to the area of the hearth bearing down on the walls/piers and supporting the oven above? Can the shelf be 2" thick? That's probably a function of how far the shelf extends I suppose, with a farther suspended shelf calling for a deeper shelf (I suppose the bottom of the shelf could be sloped instead of flat, deeper at the back where it joins the main hearth and thin at the outer edge). What I don't know is the various relationships involved here. What specific shelf suspension width calls for what specific shelf depth? I don't know that relationship.

Thoughts?