Thermal Expansion Question - Dome vs Hearth

[Unofornaio]

The post this came from on that site was in regards to arches failing in vault ovens because of design or construction. This supposed info was never verified by Alan (who posts to that site now and then) and quite honestly I don't believe it.
The root cause of this matter was that the early design of Alan's when expanded (for commercial sized ovens) did not anticipate the downward force the larger arch spans exerted on the side walls and as a result some failed. No others made sticky buns by the way

Even if this "sweet theory" was true the domes weight in a Pompeii oven is distributed evenly and this expansion scenario would not bring it down. In a barrel oven the side walls are under strain exerted from holding up the arch and the load of cladding and insulation on top of it creating an outward pressure on the sidewalls. Repeated expansion and contraction in an area under force (as in the side walls) will eventually fail. This early design proved that.

In regards to the gap for the Pompeii if you are going to do it on the outside in my opinion there is really NO question, you should leave a gap. Not because the oven is going to fall down but because having something expand in a confined space (confined by the dome bricks) is just asking for it, it needs to go somewhere.

As to the on or around choice I believe the impact difference is so insignificant its not worth the added trouble to put them on the outside.
I really think its just a personal design choice.

[Dutchoven]

Having built an oven that was loosely based on an AS I also discount the sweet theory. Many of his ovens are built with the walls on top of the floor bricks and they are usually not mortared down...at least from what I have seen. The reason those ovens fail is for just the reason Uno mentioned. Some of these larger ovens are reinforced with a harness created using allthread and angle iron in the top one third of the wall height. Another solution is to load the hauches of the oven by making the lower courses wider than that of the arch and not springing the dome quite so aggresively.
The difference of on or around is IMHO the same as Uno, personal design preference.
Best
Dutch

[Carl]

Thanks for the feedback on the story above - being a newbie I had no idea how realistic it was. I've got a lot of respect for Alan, and I wasn't aware that he hadn't verified the story . His book has been a big help to me so far, and was the inspiration that started me off building an oven.

[Kemo]

I've been mulling this over for a few days. Thanks for all the feedback guys. In the end, I have concluded that it doesnt matter if you build 'on top' or 'around' the hearth. Since the hearth will never expand faster than the dome, they are basically independent of one another. As long as they are not mortared together, there should be no interaction. The choice to build one way or another seems a matter of style. For me, my decision will be based on cost and labor

[Kemo]

sorry Carl, I meant labour