My first WFO design, any comments before I build?

[Mark1986]

Hi all,

This is my first post on this forum, so forgive me if anything is off..

I got enthusiastic about building my own WFO in order to make the best pizza one can make. As I have financial constraints, I had to look for economic WFO designs. I decided to build one, casting over an exercise ball with perlite. It was quite hard finding all the build information in one place, so I gathered it as best as I can.

I am really fond of Sketchup, so I decided to model my build in it. Find the screenshots below.

It would really help me if someone could have a look at my design, and tell me:
1. If the design is good enough to work
2. Are the insulation layers of the dome and floor big enough?
3. Am I wasting any money on materials in my design?

Dome
4" perlite cement inner dome
4" insulation blanket
2" perlite cement render

Floor
2.4" fire brick
4" perlite cement

I am really looking forward to building my first WFO! Thanks in advance!

Cheers,


Mark

[SableSprings]

Welcome Mark! We have had many builders try this method (perlite/concrete over exercise ball), but it has many problems. Primarily, perlite (and vermiculite) is normally used as an insulation and does not retain heat. It is also not even close to durable (crumbles/breaks easily). Casting an oven is a great building option, but using denser material such as refractory cement (expensive) or "homebrew" (3:1:1:1 - sand, cement, clay, & builders lime) is a much, much better option. There are many well documented builds in the forum that you should look at before continuing a build.

A cast oven (2" thick) is normally done in sections because of the weight. The hearth needs to be able to support that weight as well. Wood bases such as in your design can work but the top slab is generally recommended as 4" thick reinforced concrete. A base barrier/drain system separates it from a layer of insulation such as CaSi board or 5:1 (perlcrete or vermicrete: cement). On top of that insulation lies your oven. Either ceramic batting or 10:1 insulating cement (again either perlite or vermiculite:cement) is recommended. Most builders apply a stucco type render over that outer layer as well as trying to waterproof as much as possible since most insulation materials will rapidly absorb water (making them useless as insulation).

Also in your oven entry, you'll want to create a reveal. This allows you to seat a door so you can "seal" the oven for retained heat bakes such as bread or roasts (and many other things ). We're here to help you as much as possible based on lots of experience from the forum community... So again welcome & please invest time reading & learning through the experiences of other builders.

[Mark1986]

Hi Mike,

Thank you for your quick response!

Welcome Mark! We have had many builders try this method (perlite/concrete over exercise ball), but it has many problems. Primarily, perlite (and vermiculite) is normally used as an insulation and does not retain heat. It is also not even close to durable (crumbles/breaks easily). Casting an oven is a great building option, but using denser material such as refractory cement (expensive) or "homebrew" (3:1:1:1 - sand, cement, clay, & builders lime) is a much, much better option. There are many well documented builds in the forum that you should look at before continuing a build.

Can you tell me where I can find the well documented builds? I can't seem to find a topic or page dedicated to build plans.

Ok, so perlite and vermiculite insulate well but don't retain heat. Is the retained heat what cooks your food from above? If I would replace the inner dome material with refractory cement or homebrew, the problem in my design would be fixed? Can I still cast this material over an exercise ball?

A cast oven (2" thick) is normally done in sections because of the weight. The hearth needs to be able to support that weight as well. Wood bases such as in your design can work but the top slab is generally recommended as 4" thick reinforced concrete. A base barrier/drain system separates it from a layer of insulation such as CaSi board or 5:1 (perlcrete or vermicrete: cement). On top of that insulation lies your oven. Either ceramic batting or 10:1 insulating cement (again either perlite or vermiculite:cement) is recommended. Most builders apply a stucco type render over that outer layer as well as trying to waterproof as much as possible since most insulation materials will rapidly absorb water (making them useless as insulation).

Ok, so the 4" perlite cement layer I used is not strong enough to support the weight of the oven? I will look into using reinforced concrete then!

Also in your oven entry, you'll want to create a reveal. This allows you to seat a door so you can "seal" the oven for retained heat bakes such as bread or roasts (and many other things ). We're here to help you as much as possible based on lots of experience from the forum community... So again welcome & please invest time reading & learning through the experiences of other builders.

I still need to design the door reveal in Sketchup, will definitely include it! Thanks for your information and I am going to look into the build plans!

Cheers

[SableSprings]

I'll list a couple post threads below that will be helpful. The heat/flame from an active fire session is a large component of the top "heating element" for your pizza, however the radiated & stored heat from the refractory dome is a very significant contributor. Your basic design is good in concept...just the dome material & base structure need addressing. I'll put a link below to a nice illustration of a Pompeii oven cross section in the forum. Most casting is done with a sand inner mold that is covered with wet paper & lightly oiled to keep the homebrew from sticking. In general people find this method pretty straight forward and it works well. You can use an exercise ball, but you've got to figure out a way to support the "other half" from below...more work than it's worth (IMHO). A 4" perlite cement layer is very weak structurally. Many of us have used the 5:1 perlcrete underneath the oven floor bricks because that ratio will support the floor...but not much more...and it can be physically damaged very easily during construction. So yes, do look into a reinforced concrete hearth to support your oven.

Here's that nice, cross section illustration of a dome build:

https://community.fornobravo.com/for...901#post406901

This is a great collection of well documented builds:

https://community.fornobravo.com/for...n-the-archives

Here's a couple specific ones that deal with casting methods and often with comments from our "cast method expert" David S. (Most casting threads are in the Other Oven Types section/topics of the forum.) There are many well documented casting builds...these are just two I had in my notes and will get you started in the correct direction.

https://community.fornobravo.com/for...shelbyville-ky
https://community.fornobravo.com/for...sted-over-sand

Hope that all helps...keep researching and reading here...it's well worth your time to avoid build errors.

[Mark1986]

I'll list a couple post threads below that will be helpful. The heat/flame from an active fire session is a large component of the top "heating element" for your pizza, however the radiated & stored heat from the refractory dome is a very significant contributor. Your basic design is good in concept...just the dome material & base structure need addressing. I'll put a link below to a nice illustration of a Pompeii oven cross section in the forum. Most casting is done with a sand inner mold that is covered with wet paper & lightly oiled to keep the homebrew from sticking. In general people find this method pretty straight forward and it works well. You can use an exercise ball, but you've got to figure out a way to support the "other half" from below...more work than it's worth (IMHO). A 4" perlite cement layer is very weak structurally. Many of us have used the 5:1 perlcrete underneath the oven floor bricks because that ratio will support the floor...but not much more...and it can be physically damaged very easily during construction. So yes, do look into a reinforced concrete hearth to support your oven.

Here's that nice, cross section illustration of a dome build:

https://community.fornobravo.com/for...901#post406901

This is a great collection of well documented builds:

https://community.fornobravo.com/for...n-the-archives

Here's a couple specific ones that deal with casting methods and often with comments from our "cast method expert" David S. (Most casting threads are in the Other Oven Types section/topics of the forum.) There are many well documented casting builds...these are just two I had in my notes and will get you started in the correct direction.

https://community.fornobravo.com/for...shelbyville-ky
https://community.fornobravo.com/for...sted-over-sand

Hope that all helps...keep researching and reading here...it's well worth your time to avoid build errors.

Thank you so much! I was starting to doubt if I could pull it off within a reasonable budget, skills and time, but there are some good resources in there! Love the Shelbyville build, so simple but effective!
On of the other reasons I was put off, was that I am going to build the oven in the garden of my girlfriend. Her parents own the house and I don't think they will like me putting concrete slabs in the ground

Remaining questions:
1. Is there a way for me to avoid a concrete slab in the ground? And can I still work with wood?
2. How much refractory cement would I need to fill the inner walls of a 30 inch oven?
3. You mentioned homebrew mix: " (3:1:1:1 - sand, cement, clay, & builders lime) is a much, much better option. ". I don't need to add anything else? like stainless steel needles and such. I read that in some posts.
4. How much water do you add to the homebrew mix?

Thank you so much again, I got some of my motivation back now

Cheers!

[david s]

1. Provided the oven is small, it can be built on a stand and relocated with four blokes and a couple of 4x2’s slid under it. A timber frame is not recommended, particularly if the oven is in a garden setting out in the weather. A welded steel frame, preferably galvanised, with its base no smaller than than the oven diameter is a better solution. My own 21” internal diameter oven weighs 250 kg.
2. Work out the volume of a sphere (outer radius of internal dome) then deduct the volume of a sphere (internal radius), divide the result by 2 because it’s a hemisphere. The volume lost for the oven mouth equals that which surrounds the mout so just work on the volume of the hemisphere.
3. You don’t need the stainless needles for reinforcing. They are expensive and hard to access in small quantities as well as making placement difficult and painful if not experienced with them. Their correct name is “melt extract fibres” The polypropylene fibres are more important however, as they provide a measure of steam spalling protection.
4. Make the mix up with water addition until you reach a “ball up” consistency. The difficulty is getting the mix to stand up vertically without slumping at the base. The mix has a high clay content and is quite workable so this is not that hard to achieve. At least the reports from builders using this method have not reported too many problems. Too sloppy and it will slump and shrink. Too dry and you’ll end up with voids on the inside.

[Mark1986]

david s Thank you for the information, that helped a lot! I am working on an updated plan. I will share it here somewhere this week!

1. Provided the oven is small, it can be built on a stand and relocated with four blokes and a couple of 4x2’s slid under it. A timber frame is not recommended, particularly if the oven is in a garden setting out in the weather. A welded steel frame, preferably galvanised, with its base no smaller than than the oven diameter is a better solution. My own 21” internal diameter oven weighs 250 kg.

I didn't opt for steel as I thought this would be an expensive option and I have financial constraints. Am I right for thinking a steel frame is more expensive? If it is cheap enough I might try to make one myself, will be the first in my life though!

2. Work out the volume of a sphere (outer radius of internal dome) then deduct the volume of a sphere (internal radius), divide the result by 2 because it’s a hemisphere. The volume lost for the oven mouth equals that which surrounds the mout so just work on the volume of the hemisphere.

Wow my geometry classes are starting to get back to me! I found a way in Sketchup to get the volumes of the dome pieces.

One more question that has been going around in my head about how to know how much materials you should buy for a certain volume of a substance. For instance, if you put 1 liter of a non-water absorbent material in with 1 liter of water, you get 2 liters of substance. But what happens if you mix 1 liter of water-absorbent material, like cement, to 1 liter of water? Do you get 1.8 liter of substance and is this depended on how much water cement can absorb? Or do you still have 2 liters of substance?

3. You don’t need the stainless needles for reinforcing. They are expensive and hard to access in small quantities as well as making placement difficult and painful if not experienced with them. Their correct name is “melt extract fibres” The polypropylene fibres are more important however, as they provide a measure of steam spalling protection.

I ordered the polypropylene fibers, thanks for the tip!

4. Make the mix up with water addition until you reach a “ball up” consistency. The difficulty is getting the mix to stand up vertically without slumping at the base. The mix has a high clay content and is quite workable so this is not that hard to achieve. At least the reports from builders using this method have not reported too many problems. Too sloppy and it will slump and shrink. Too dry and you’ll end up with voids on the inside.

I am thinking about adding the first 8 to 10 inches on the first pass, and then the rest. When I finish the dome, should I cover it with wet towels for a week? How long should the whole drying process take, before curing?

[david s]

"how much materials you should buy for a certain volume of a substance." This is a bit of a difficult question because if you take for instance a standard concrete mix of say 3:2:1 litres of heavy aggregate, sand and cement, and then add a couple of litres of water, you don't end up with 8 litres of concrete because the sand fills the spaces between the aggregate, the cement fills the spaces between the sand and the water fills any spaces left. You end up with, in practice, around 4 litres of concrete. Attached is a spreadsheet with required materials, but I'm not sure how accurate it is.
Oops, sorry I can't upload an XL spreadsheet file, forum won't accept it.

Once the mix has set the sand should be removed as their is some shrinkage and it needs to be allowed to do so. Also the voids on the inside should be filled after 24 hrs, there will be some, so remove the paper, inspect the voids and fill them with the same mix, forcing it in hard. Then cover with plastic to retain the moisture for a week, Then you can proceed to form the mould, in front of the oven mouth, with sand again to cast the flue gallery.

[david s]

If casting a vermicrete underfloor insulating slab, 4" thick 5:1 is the usual required thickness. It is a good idea to allow this to dry thoroughly before building over it as the under floor moisture is the hardest to eliminate.

[Mark1986]

If casting a vermicrete underfloor insulating slab, 4" thick 5:1 is the usual required thickness. It is a good idea to allow this to dry thoroughly before building over it as the under floor moisture is the hardest to eliminate.

How long will this take? I'm using perlite concrete. Will a week be sufficient?

[UtahBeehiver]

David S uses a cheap soil moisture meter to check for moisture. A week will not be enough time for 4" of pcrete to dry. Could be as long as 3-4 weeks.unless you are in a very arid area. There is deceivingly large amount of water in Pcrete and once covered with bricks will take a long time with curing fires to drive out.

[david s]

How long will this take? I'm using perlite concrete. Will a week be sufficient?

You can drive out the moisture by fire, but it will take a long time, particularly if you don't have weep holes in the supporting slab. Here's some info on drying a vermicrete slab that may help you.

Vermicrete insulating slab PDF.pdf

[Mark1986]

You can drive out the moisture by fire, but it will take a long time, particularly if you don't have weep holes in the supporting slab. Here's some info on drying a vermicrete slab that may help you.

[ATTACH]n437285[/ATTACH]

Thanks for the document that helps! I read that a normal concrete slan also dries in a couple of weeks, but that it can support weight after 7 days. Can the same be said about a vermicrete slab?

[david s]

The strength of a vermicrete slab is way lower than that of standard concrete, but the same principle applies. As the bar graph in the link shows the strength can be doubled if damp cured for 28 days rather than no curing. Usually a week is considered sufficient,

https://theconstructor.org/concrete/...uration/11119/

[Tallyrc]

polypropylene fibres... Do you use the long or short ones.. and is perlite or vermiculite preferred? Piggybacking on this thread instead of starting another...