36" in Seattle, second oven

[david s]

You need to know the the weight of the oven and the supporting slab that sits atop the piers to know how much you can cantilever.
A structural engineer will be able to answer those questions, or you can just wing it. The strength of a concrete slab is proportional to its thickness, so a 2" thick slab is only 1/4 the strength of a 4" one.
If the load on the slab is directly over the supporting piers there is no load on it.
I precast mine and reducing weight where it's not needed, have got it down to a two man lift.
The top and bottom of the slab is tapered toward the outside, where there's no load on the slab, reducing weight and volume.This has the added advantage of shedding rain coming off the dome so water does not accumulate around the base of the dome, which is a common entry point for water to get in under the floor insulation.
I have four 50 x 50 mm holes in the slab which locate into the four corners of the piers essentially making it like a tied in lintel, with rebar tying the supporting slab to the foundation slab.
Starter bars cast in the foundation slab into the four corner cores of the stand pin the piers to the foundation slab.
Because my supporting slab is thin I also use steel reo, pp strengthening fibres as well as nano graphene or carbon nano tube fibres.

[kebwi]

Sorry, I think I misrepresented my question. I wasn't thinking about having the oven over any cantilevered section, such as the oven hovering out over a cliff or something. The oven would still be essentially centered on the hearth of course and the stand would of course surround the oven. I haven't decided exactly how far I would tuck the walls under the oven so as to position them almost directly under the dome/floor perimeter, but for the most part the stand would surround the oven of course, as with almost all Pompeii designs.

My question was for a completely unloaded shelf extending outward beyond the hearth. It probably wouldn't be more than few inches, but I was curious how much I could get away with. I mean, would 2" of concrete suspend as far as 12" off a supporting edge, or is 3" deep and 8" wide the safest margin (I'm just making numbers up here)? That's my question. How far can you suspend a non-load-bearing shelf (no part of the oven would rest on this outer perimeter, the only "load" is the mass of the suspected hearth itself)?

Does that make sense?

Thanks for explaining how the pier/slab pinning works.

[david s]

As I posted before, your questions could be answered by a structural engineer, if you know the loads involved, or you could just wing it.
For my ovens the cantilever is 160mm on the sides and 150mm front and back. The table, with its 60mm thick reinforced slab, cantilevers 190mm at the front and 330mm at the sides and is pinned to the wall.

[kebwi]

I'm trying to figure out how to pour a hearth slab for a set of metal legs such as those shown in the attached photo. The idea would be to bolt the legs to the bottom of the hearth via holes that project all the way to the top of the hearth. I would either drill the holes (less preferable) or attempt to position placeholder dowels or PVC or whatever, in the same fashion as weep holes under the oven floor, and then drop bolts down from the top with nuts attaching from underneath. If the holes in the concrete are a little bit larger at the top than the rest of the hole, then the head of the bolt could fall inside such that the hearth remains flat and flush.

But how would I pour a slab in this fashion? I could attempt to precariously balance the legs, pinning them upright between vertical pieces of plywood that are themselves braced in position with bricks and concrete blocks or something of that fashion. Then I would carefully form a plywood base with rectangular holes matching the top table leg strip, perfectly flush with the top of the table legs, pour the whole thing, let it set, attach the bolts, and then somehow transfer the weight from the forms to the legs. If everything was relatively precise, it might be a matter of knocking the form supports out and having the weight settle into the legs, hopefully with essentially no harsh drop as the forms are knocked out.

This seems insane. I can't believe this would work.

Or one could pour the entire slab upside down on the ground, without the legs involved, then attach the legs to the "top", and then flip the whole thing over. But I'm not sure how I would flip it over without some serious machinery involved.

Or should I pour the entire thing as a simple slab but right side up at approximately the correct height, but without the legs actually involved in the matter. The differs from the first scenario by not trying to delicately shape the forms around the base of the legs and perfectly flush with the legs. I would simply poor a flat piece of concrete against a piece of plywood, then remove the plywood afterwards, and then attach the legs, and finally remove the frame to transfer the weight to the legs. Again in all these scenarios placeholders would be used for the bolt holes if at all possible, so as to avoid having to drill later.

None of these three ideas seem very good yet. How would you go about this?

[david s]

Do't use plywood, even marine ply will turn to mush in a horizontal position in the weather, use a cement sheet. I used this method with some holes made through the 4.5mm sheet so it would key into the cores of the supporting piers. Also not sure if your legs are galvanised, but corrosion will be a problem if they're not.
Check my solution here.

https://community.fornobravo.com/for...ble#post452729

[kebwi]

I was only talking about temporary forms. I wasn't suggesting using plywood for any permanent aspect of the installation. The notion of using hardibacker or cement board for a left-in-place surface on which to pour the hearth had occurred to me. I hadn't ruled it out necessarily, I just wasn't sure if it was the right way to go.

[kebwi]

These steel table legs are quite popular apparently. Several places are producing nearly identical products. They aren't galvanized. The are generally sold either painted or powder-coated, or simply bowling-alley waxed to preserve the metal appearance, but it is acknowledged that the wax method won't work outside. They are also offered with no coating at all so that you can seal them yourself. Of course steel will rust badly outside. I'm aware of that -- although I've always been enamored with the look of intentionally rusted steel, that uniform orange-brown patina that is done deliberately. I'm unclear how that look is achieved with only surface rust and no deeper long term degradation. At any rate, I was thinking I would just paint them. I would love to preserve the metal appearance, ala the wax coating or some other clear coat, but ultimately, I had already decided that painting them would be the only practical way forward.

But my question was simply how to pour a hearth over a structure that doesn't stand in place on its own until it already has a hearth to bolt to. Each piece seems to require the other as a prerequisite.

[david s]

Make sure the steel is at least 14 gauge (2mm) thick. Some of those pre-built legs are made from very thin gauge but made to appear way more solid than they are.

[kebwi]

Yeah, I've been in direct contact with the various places that I might look into. Of course I can't prevent them from overtly lying to me, but they have told me they use 14 gauge in most cases. Until it arrives and I actually get a view at the cross section, if that's even possible considering the tube might be closed of the ends, I guess I'll never know for sure. It's looking like the metal option would definitely cost more than masonry or wood. I'm at a loss. If I end up building a permanent masonry structure, I'm going to be pretty frustrated. Losing my first oven with the first house is a huge design motivation with this second oven. At the very least, I would like a masonry design to consist of a hearth that can be lifted off of the stand somehow. I see your rebar pins from the piers to the slab. I guess it hasn't been clear yet whether those pins form any sort of bond to the slab, or whether they are just footing's that prevent the slab from sliding laterally across the top of the piers. If it's just a dry pin to prevent lateral movement, then that makes a lot of sense. Now that I think about it, that must have been what you meant all along since you said the slabs are preformed and lifted up on top of the pier by two people. I'm literally thinking through it as I transcribe this message. Okay I get it.

Thanks again.

[david s]

In Australia and I presume in the US, when a home is sold it does not include appliances like fridge and washing machine. But strangely the dishwasher nearly always is included as part of the house. Because of the difficulty in removing and transporting it, a wood fired oven nearly always stays too. Exactly the same applies with installation. Nearly all my ovens are built on site even though I have a crane and can lift a fully built oven onto my small truck, access to where it will be used nearly always means I can’t offload it. This is why oven kits are so popular. The hire of a mobile crane is prohibitively expensive. I have made some removable by using a very weak mortar and with no rebar connection from the stand to supporting slab, with mainly gravity bolding it in position. The weak mortar can be scratched away and the oven including supporting slab can be lifted with a forklift or mobile crane. Frankly it’s easier to just build a new oven.

[kebwi]

I hear you. And likewise in the last few weeks I have been all over the map on this issue. I have seriously asked myself whether I would actually take an oven with me if I moved -- or even how likely I am to move in the next several years (although I certainly didn't see the last move coming with any premonition). I suppose it is reasonable to simple bag the idea of a transportable oven.

There is a secondary interest however: local transportability around my large patio. I have, in fact, a full-size pickle-ball court spanning the width of my backyard. It even includes two basketball hoops at each end. It's pretty ridiculous, frankly. I have already taken over various areas of the court for other usages, such as storing my Sunfish sailboat and placing a conventional patio furniture set (table, umbrella, and several chairs). As yet, I have not permanently rendered the pickle-ball court unusable. The boat is at one far end, behind the line (and could, if absolutely necessary, be moved of course), and the patio furniture could be temporarily relocated to the adjacent gravel driveway in the event of an occasional pickle-ball game. I would prefer if installing the oven in one corner also did not compromise the court's potential usage, rare as that might be. As such, I am seeking either a very tight footprint for a permanent installation or an installation on a caster-wheeled cart. I believe there is sufficient room behind the court lines for a permanent oven, certainly like the ones you install (although I'm aiming a little bigger), but I am also seriously pursuing something a little transportable that could be rolled out of the way if needed.

The court has 90 inches behind the court's back line to the edge of the concrete and 55 inches to the sides. So there is a 90x55 inch area fully off-corner. Of course, the oven's structure shouldn't reach right up to the line. At any rate, that gives me 7.5 feet to work with behind the back line, disregarding whether it extends out of the off-corner area, past the sidelines, but remains behind the back line. That's quite a bit to work with, but again, actually playing on the court would require access to some of that area, not right up to the line. But I admit, none of this really matters since the court gets no use at all and may as well not be a dominating factor in my decisions. Frankly, by the time I build either a full enclosure or a roofed pergola over an igloo to protect it from Seattle rain, I don't see how I can possibly preserve the pickle-ball court anymore. Its fate will be sealed at that point.

But, truth be told, so far in five years of owning the house, I've taken the pickle-ball gear out of the basement precisely one time. I may as well just repurpose the vast patio area for other things. But I fear making a permanent decision that I regret at a later time. What if next year is the year of my life when pickle-ball becomes some unexpectedly major presence in my life for whatever inconceivable reason.

At the same time, I hate the notion of pouring a new slab elsewhere in my yard when I have such a humungous area of concrete going essentially unused already. Pouring a new slab for the oven seems positively insane under the circumstances.

Ah me.

[nuovopizzas]

One problem with confining all my questions to this thread instead of starting new threads for each question, is that I feel like this is just a conversation between you and me. I feel like no one else will find this thread or contribute to the discussion. I don't mean to lean on you so heavily.

Has anyone ever built a cast oven with a brick arch (including the inner arch, not just the decorative facade) and gallery? Just kicking ideas around my head. I'm wondering if I can make a brick arch more easily than casting it.

I found this thread and I am currently using the conversation for research to do my 48" mobile build. Don't worry we're out here

I have built a cast oven with a brick arch. I think a brick arch is much easier than casting it for the gallery. I simply built a form and used refractory mortar to glue all the bricks together. I then stuccoed the outside. You can see this in the picture I posted on my other thread.

[kebwi]

I have read in several threads that people reuse the sand from their sand castle to make the homecrew to cast the vent. Obviously they had to use other sand to cast the dome while the sand castle was in place, but they did use the same sand from the sand castle for the vent.

Two questions. (1) Presumably they aren't using high silica sand for the sand castle, just "play" sand or ordinary concrete aggregate sand, but that implies they aren't using high silica sand for their vent either. I was inclined to use high silica sand for all my castings. Are all these people using high silica sand specifically to cast only the dome, which perhaps is put under greater heat stress, but then resorting to simpler sand for the vent, or are they just using plain sand for everything, dome included, and not bothering with high silica sand at all?

And (2), in so far as some people mix other things into their sand castle to alter its consistency (clay, etc.) to what extent are they factoring that mixture in when they reuse the sand castle for the vent homebrew? That seems like a tricky and messy calculation to get right. It seems like it would be easier to start over with unmixed ingredients to make homebrew for the vent. Even if the clay might be in both recipes, I would personally be disinclined to waste special fireclay on the sand castle but likewise would be disinclined to use ordinary clay to cast the dome or the vent, so I wouldn't see the clay added to the sand castle as being particularly appropriate to the clay used to later cast the vent.

Thoughts?

[david s]

As all the contents of a homebrew castable are pretty cheap, cost is hardly an issue. Regarding the type of sand used I really can’t see why it would be an issue. Graded sand can be an issue for mortar if some of the grains are a little too large. But for a castable or even a render/stucco, a range of grain size is actually a benefit, just as the range of aggregate size in concrete is desirable for enhanced strength. For the service temperature that ovens will see eutectic problems (material melting) in the presence of fluxes are non existent. Not really sure about thermal expansion issues of different sands, but again these issues generally don’t present themselves until at least 600C. As there have not been any reports of problems in homebrew cast ovens about this (at least none that I’ve seen), I’m assuming any sand is ok.
The only problem I can see is the shrinkage that a high proportion of clay imparts. If builders are concerned about this a simple small cast bar can be measured wet, then again when dried to calculate shrinkage. This would be a prudent step before commencing.

[kebwi]

Ah, ok, so utilizing high silica sand for homebrew mortar or casting really isn't a necessity at all. I think I used silica sand for my first oven and obviously it wouldn't hurt to do so, but perhaps its refractory properties are simply irrelevant at oven temperatures. Thanks.