Question about dome height on small Neapolitan wfo

[Forrest Roche]

I am starting to build a small (28" - 70cm) brick Neapolitan oven. I have read a lot about dome height but it seems such a small oven does not adhere to the ratios seen on larger ovens.

My tentative plan is to build a 31cm dome wiih a 14cm soldier course and a 19cm opening with a width of 38cm.

Any and all commments are welcome. I wan to make sure the dome height is neither too high nor too low given the small diameter of the oven.

Thanks in advance.

[david s]

Not sure what your plan is, by the numbers you’ve provided. Can you attach a rough drawing with measurements to be clear?
The often stated door height to interior height ratio of 1:1.6 came about from a Canadian survey of historical ovens that arrived at an average figure and folk have assumed this to be a crucial design imperative. While it is probably reasonable for most ovens between 30-40” it is not suitable for either larger or smaller ovens. The far more important factors are easy workability and heat loss. The larger the opening the greater the heat loss, but reducing the opening reduces easy work ability. For a small oven some sacrifice of heat loss is preferable to poor workability IMO. Reducing the depth of the entry is a way of improving good workability. The opening needs to wide enough to access the oven interior, placing items in and out as well as feeding fuel. It also needs to be high enough to take the largest roast or bread loaf you intend to cook. My oven is 54 cm in diameter with a 40cm wide oven mouth that is 16cm high (arched top). That’s a large mouth area to chamber volume, but works well, consumes very little fuel and is easy to work. Down side is that a 4kg turkey or a high top bread loaf is about its limit.

[Forrest Roche]

Not sure what your plan is, by the numbers you’ve provided. Can you attach a rough drawing with measurements to be clear?

David, sorry I wasn't clear, I should have included a drawing to begin with.

below is a rough sketch, not to scale, but will clarify.

My question is about the dome height in a neapolitan oven. Since the oven is only 70cm (28") in diameter, I want to place the dome height higher than the diamter/3.4 figure I see mentioned for larger ovens. That formula would yield an interior dome height of 20cm and as a consequence a ridiculously low entrance of about 13cm. I would like to place the dome height at about 31 or 32cm, which gives me an entrance height of 19 or 20cm, which I can live with.

[UtahBeehiver]

David S is our cast and small oven expert. Since the design you are proposing a low dome and full brick soldiers you need to be aware of the outward pressure the dome will place on the top of the soldiers and some type of reinforcement or banding of the soldiers may be necessary. Maybe consider a cast oven.

[Forrest Roche]

David S is our cast and small oven expert. Since the design you are proposing a low dome and full brick soldiers you need to be aware of the outward pressure the dome will place on the top of the soldiers and some type of reinforcement or banding of the soldiers may be necessary. Maybe consider a cast oven.

Well I have all of my materials already purchased and I've read all of the major threads on Neapolitan builds

... it will have substantial reinforcement around the sailor course but the only thing that I haven't seen addressed is dome height for an oven with this diameter.

There was a good thread a few years ago at the Pizzamaking forum with an 80 cm diameter oven and the builder set his own dome at 30cm, and while he posted some nice pictures of pizzas, he never posted the bottom of the pizza.

Then he disappeared immediately after he finished the build and I don't know if the floor and dome temperatures were well balanced.

This oven will have a Sorrento biscotti floor so I'm worried if I set my dome too low I'll end up with pizzas that burn on the top before they're done on the bottom.

[david s]

Thanks for the drawing, that makes things a lot clearer. Although it is a small oven, placing a low dome over bricks laid as sailors is asking for problems IMO. Because serious cracks always start at the base of a dome, which is its weakest point, laying sailors rather than soldiers reduces the wall thickness and depth of mortar joint. This will seriously weaken the structure, especially one that experiences thermal cycling with its associated expansion and contraction characteristics.
Also because strength is proportional to the square of the thickness. (eg half the thickness gives 1/4 strength).

I’ve also never been a fan of bricks laid on ends, either soldiers or sailors, because it results in a long vertical mortar joint at the base of the dome, inviting vertical cracks. Brick walls for homes are never laid on end for the same reason). A stronger method is to lay two courses flat and on bond. Flat (large radius) domes, laid over soldiers have proved to fail unless buttressed or braced, hence the steel band idea. Cast ovens, because they don't rely on separate brick units, get away with no bracing or buttressing and can therefore employ thinner walls, usually 50mm (75mm for large ovens) Because your oven is small you should have less problems, but at least stay with soldiers rather than sailors and use a steel band near the top of the soldier course where sideways thrust is at its maximum.

Regarding height of oven mouth I think you are over worrying this point, just make the height whatever you are comfortable with, finding a compromise between workability and heat loss.
With my design I built one for a friend who wanted extra oven mouth height by casting. a 50mm ring around the base to raise the dome, I was worrying how the extra height, which apart from raising the dome height, also raised the oven mouth by 50 mm, would affect how the oven performed.
There was absolutely no perceived difference in the firing or performance of the higher domed oven, which surprised me because I did expect some difference. Many builders here adhere strictly to a 63% oven mouth height to interior oven height ratio, which I now consider has little value for either small or large ovens, although is probably most suitable for average sized ovens.

[Forrest Roche]

Thanks for the drawing, that makes things a lot clearer. Although it is a small oven, placing a low dome over bricks laid as sailors is asking for problems IMO. Because serious cracks always start at the base of a dome, which is its weakest point, laying sailors rather than soldiers reduces the wall thickness and depth of mortar joint.

David,

I would rather not use sailors, but I am building in a preexisting niche in a conrete retaining wall and space is critical. I am following the build thread of Tscarborough on pizzamaking - he used sailors and then a girdle of steel reinforced refractory concrete (using broken up refractory bricks as an aggregate) tied into two pieces of verticle rebar embedded in the concrete slab and insulating perlcrete slab flanking either side of the entrance. This will give me more clearance on the sides and allow enough room for hands and trowel to coat the perlcrete insulation with a waterproof mortar.

The space the oven is going into is problematic - it is built into a shale ledge behind the house and the owners before me built it with a niche to house two hunting dogs. There's a whole set of reasons thiis is my best choice for location and so it is going to be a very weird enclosure and I have to adapt fo the circumstances.

As I advance I will post pictures and it will be clearer, but below are pictures of the space. I had to remove the floor and jackhammer through ledge and then pour a new floor so it wouldn't end up at shoulder height.

The sailors are already in place because my question was just about dome height, so I am committed now anyway, but I promoise it will be more than sufficiently butressed. There are a lot of peculiarities to the build but I won't bore you. I will start a build thread at some point.

Thanks for your time and for putting me at ease about the dome height. I have settled on 32cm.

Forrest

[david s]

Hi Forrest,

Ok I can see now that you are well down the track with your plan. The steel bracing requirement to support the sailor/soldier vertical course introduces the problem of going across the entry. I see you are using the method of pinning it to the supporting slab. This results in difficulty placing the band high on the soldier course where the side thrust is the greatest. Although you are too far into your design, there is a solution that others reading this thread may consider. It is to brace the soldier course at the top and go across the entry using angle and threaded rod as shown. The threaded rod needs to be fairly hefty, say 10mm diam. It is also possible to have them exposed (not covered by vermicrete) so adjustments can be made.

But for a 28" internal diameter oven, all this may be overkill.

Dave