My first design

[dilby00]

Hi all -

I'm planning to build my own wood fired oven, and have come up with an initial plan which I've pretty crudely drawn up in Sketchup (screenshot attached).

In short I want to be able to cook a variety of things, not just pizzas, so I went with a box shape. I've already got the stand built which is a wooden
frame and part of an existing garden bed structure, and the door would be insulated. I plan to put a removable shelf in, plus some holes in each side
to put a rotisserie through and just fill with threaded bolts when not in use.

But I'm really not sure what material to use, and whether i should have a cavity. In my design I've shown a cavity which I planned to fill with insulation, but I've read
about people using vermiculite in their concrete mix, or using aerated concrete blocks which to my mind would be the same thing. This of course would save me some hassle!
For the base I was just planning to cast a thick concrete slab and I've got an old slate slab offcut leftover from my fireplace hearth which I could put on top for the actual cooking surface.

So does anyone have any advice or know of any info regarding insulation/heat loss properties of concrete mixes and cavities? Specifically:

1) Is there a way to make the equivalent thickness of concrete as effective as splitting it with a cavity if the correct mix is used, and what would that be?
2) Does my mix really need to be heat/fire resistant?
3) Would having slate ontop of a concrete base mean heat is lost via convection?
4) Are there any specific flaws/room for improvement anyone can spot with my initial design?

Thanks so much!

[david s]

While a cube is a fairly simple structure to build and lends itself well to loading things into, it is not particularly practical as a pizza oven. Because of the heat involved an oven will be continually expanding and contracting so it’s structural integrity becomes more and more compromised as those heat cycles increase. In particular the flat roof has a tendency to collapse. This problem can be overcome by curving the top so it becomes self supporting although this then places outward strain on the side walls. A barrel arch largely overcomes this problem although still has structural problems with the end walls. For a pizza oven the roof needs to be closer to the pizza to cook the top properly, typically half the height of the base. A hemisphere has proved to be the best suited chamber for both cooking and structural integrity. You would not be the first to think that an alternative might be simpler and superior, me included, but 2000 years of human ingenuity still puts the hemisphere with a front cross draft flue at the top of any other design. If I see your design correctly the flue exits from the top of the chamber in a classic updraft arrangement. While this fires efficiently at start up, if the chamber is empty, as in an oven, the flame will jump straight to the flue and a lot of heat will be lost. Roman kilns used this system, but the chamber was full of wares so the flame had to work through them before exiting at the top. A different story if the chamber is empty. The basic principle with any oven design that reqiuires retained heat properties, is to have a dense inner core to attain the required thermal mass and then to make sure that all that thermal mass, floor included, is well insulated so the stored heat won’t be easily lost. Have a read over some of the many builds here and download the free plans, before inventing a new design.
There is an error in the attached table. The compressive strength of 5:1 vermicrete should read 175-225

Vermicrete PHYSICAL PROPERTIES 2 copy.docx.zip

[wotavidone]

Hi all -

I'm planning to build my own wood fired oven, and have come up with an initial plan which I've pretty crudely drawn up in Sketchup (screenshot attached).

In short I want to be able to cook a variety of things, not just pizzas, so I went with a box shape.
Can't open your design for a look. What is going to hold the roof up? Hope you are not intending to rely on the mortar?

But I'm really not sure what material to use, and whether i should have a cavity. In my design I've shown a cavity which I planned to fill with insulation, but I've read
about people using vermiculite in their concrete mix, or using aerated concrete blocks which to my mind would be the same thing.
Vermiculite works as insulation. I've helped build 2 ovens over aerated concrete blocks. No one seems to know how long this will last but they have not fallen down yet.
All need a structural slab under them.

So does anyone have any advice or know of any info regarding insulation/heat loss properties of concrete mixes and cavities? Specifically:

1) Is there a way to make the equivalent thickness of concrete as effective as splitting it with a cavity if the correct mix is used, and what would that be?
I tried making the concrete slab with scoria for the aggregate after reading a paper posted on this forum that said it had a relatively low thermal conductivity. I then put some vermicrete between the slab and the brick cooking floor. The slab is accessible for temperature readings and does seem to have at least some insulating properties, but I reckon the two ovens I've seen built over aerated concrete work better.
2) Does my mix really need to be heat/fire resistant?
Yes
3) Would having slate ontop of a concrete base mean heat is lost via convection?
Yes. Well, by conduction, anyway.
4) Are there any specific flaws/room for improvement anyone can spot with my initial design?
A box won't hold itself up when the inevitable separation of bricks and mortar from repeated thermal cycling occurs.
I've always looked at the myriad "instant oven" builds where people use angle iron to hold the roof up and wondered how long before the steels warp and the whole lot falls in.

An oven with the chimney exiting from the main chamber will lose a lot of heat.

There are certain fundamentals that make a good oven:

You need a structural stand and slab that can support the weight without moving.

Then a layer of insulation.

Then a layer of dense/high thermal mass cooking surface.

Then a self supporting dense/high thermal mass oven structure on top or surrounding the cooking floor.
-This can be a simple arched barrel vault, though as Dave points out the end walls need special consideration.
-Or it can be a rotating series of arches, the classic Pompeii "igloo". Perhaps the most self supporting of all.
-Or it can be a Naples style low dome oven which puts more outward force/pressure on the walls and needs buttressing.
- Or it can be clay/cob.
- If you choose to cast an oven from refractory with some re-enforcing needles in it, you can be more creative with the shape, as it is not relying on many smaller masonry units leaning on each other and holding each other in position.

You need the chimney to exit from outside the oven entrance. The entrance should be lower than the main roof for proper circulation of the flames. 63% is the magic ratio, though anything from 60 to 68% would be OK.

You need to insulate over all of it. The insulation is of particular importance for slower cooking - roasting, baking, etc.

It must be weather proofed or built inside a shelter.

Omit/modify the fundamentals and you get lesser performance or even complete failure.