Neapolitan oven restoration

[SableSprings]

Hey Peter, it occurred to me that if you are going to use a cement board type product underneath your vermicrete layer as a permanent (non-removable form base), simply cut the board 1-2 cm larger on the sides you intend to put tile or protection. That little extension would help you put the vertical form boards in place and after the vermicrete pour sets and the form sides are removed, you'd have a nice little shelf on which your tiles would get the base support they need to stay put without stressing the thinset/vermicrete bond layer.

I think I'd still put the folded metal lath in place on the sides between the vermicrete and tile as extra strength and adhesion for the thinset. I hope that this makes sense to you...I can see it in my head, but...

As a side note, David S recommends putting a few holes in the base material holding the vermicrete. In this case, 4-5 drill holes in the cement board. He does this on his ovens to allow any moisture that accumulates (or that gets trapped) to escape more easily.

[Odin]

Thanks!
That all looks clear. To confirm I’ve created a sketch.
Vermicrete should be poured on cement board or laid on it after curing in my mold? What is desired cements board thickness?
I have another obstacle on my way – I have measured my dome and it is slightly bigger than my table.
Will my vermiculite and cement board underneath stand the weight?

[SableSprings]

Yes, the sketch looks like what I was thinking. (My only concern is that you make the vermicrete pad large enough so the oven does not come down directly on the edges of the pad...remember, compression strength only. I'd want to have at least 1-2 cm of pad outside the oven footprint.) I would simply set form side boards directly on the cement board while on the stand and then pour the vermicrete right in. You've got several nice support pieces across the stand mid-section now so why do this in two steps (pour & move) when you can just set the cement board on top of the frame, set your form sides in place and pour the vermicrete directly into its new home.

I don't know what "slightly bigger" means here, but I would think if the oven is only a couple cm wider on each side than the stand it shouldn't be a problem.If it's a concern to you, there's a couple of options I can suggest. 1) The best option (but heaviest) would be to put down the cement board and pour a pad of 8-10 cm thick reinforced concrete larger than the oven footprint and then form up again and pour on the 10 cm vermicrete layer. 2) You could put in some angle iron either bolted to the frame along the sides under the cement board or cut and embedded in the vermicrete pour to give it extra strength on the sides without pouring a reinforced cement pad base on the stand.

In either case, I'd use the thicker variety of cement board that's available...here it's 1/2" so you'd probably be looking at 12-14 mm. Normally we have two thickness sizes of Hardibacker available here, 1/4" (6-7 mm) and the 1/2" (12-14 mm) so I'm assuming it's similar for you. The thicker board does not flex while the thinner board does.

Remember that the weight of the oven is going to be distributed on the base...kinda that elephant foot concept. Also remember that you are going to be adding insulation and some type of enclosure over the dome, so allow for the extra layers. I feel like I keep throwing wrenches into this project, but you're doing great. Better to think about your options now and make a decision that YOU feel comfortable with.

[SableSprings]

Peter, are you planning on putting your cooking floor sections directly down on the vermicrete (of course with a thin clay/sand layer for leveling) and then setting the oven on top of them? If you do it that way, the weight distribution on the vermicrete is very different than if the dome itself rests directly on the insulation. If you plan on setting the oven on top of the cooking floor then I would see no problem with pouring the vermicrete directly into the concrete board based form (that is sitting on the steel cart). Again that elephant foot weight distribution thing... Also doing it that way takes care of the issue of the dome dimensions being larger than the metal cart top...the cooking floor is distributing weight more evenly for you.

[Odin]

I have measured my dome again and it has 120 cm external diameter with 5 cm walls. Table is 110 x 110 cm. After your advice I will lay the cement board on the table. It will be wider by 15cm on each side. That will give me 140 x 140cm floor to work on -120cm for dome 10 cm (5 on each side) of insulation and 10 for render.
I don’t want to lay another concrete layer as subfloor as that will make oven way too heavy to move. I am already struggling how many people will be required to move my table on wheels.

Just like you said I want to put cement board, on top of that pour cement-vermiculite mix, then clay (recovered from oven), hearth slabs and mortared dome.
I am not giving up yetJ

[SableSprings]

I have measured my dome again and it has 120 cm external diameter with 5 cm walls. Table is 110 x 110 cm. After your advice I will lay the cement board on the table. It will be wider by 15cm on each side. That will give me 140 x 140cm floor to work on -120cm for dome 10 cm (5 on each side) of insulation and 10 for render.
I don’t want to lay another concrete layer as subfloor as that will make oven way too heavy to move. I am already struggling how many people will be required to move my table on wheels.

Just like you said I want to put cement board, on top of that pour cement-vermiculite mix, then clay (recovered from oven), hearth slabs and mortared dome.
I am not giving up yetJ

I think that's a very workable plan. I agree on avoiding a concrete subfloor...I just wanted to make sure you had several options available. I don't know if you saw the thread where an oven was lifted as one piece together with its base (by crane to a flatbed truck) and was moved about 5 miles...it turned out to weigh 13,500 lbs (over 6,100 KG) with the cement foundation and block stand...wow! I suspect that your rebuilt oven (even though much lighter than that) won't get moved very far or very often