My 30" Homebrew Castable WFO Build

[teddyvikings]

Its time to share the process of my Homebrew Castable WFO build. Just yesterday we had our first pizza cook and it went so well. The oven still needs the finish insulation layer and a final stucco render, I feel like its time to share what I did and what I learned in the process.

Firstly, it will become apparent that I am eternally grateful for the wisdom and experience of this forum. I have checked and double checked my questions with the communal knowledge of this community and have charted my path from it. So in a word "thanks" for everyone who shares their pizza oven journeys with the rest of the world.

Our family loves making pizza and gathering groups of people together to share in the eating. Early in June 2019 a youtube video professing that I could build a woodfire pizza oven for $130 USD caught my attention and it led me to this point. So here is what my thinking was as I began to prepare to do my very own build.

My Design

- construct an igloo shaped castable WFO using the Homebrew Castable recipe (3:1:1:1 Sand: Portland Cement: Hydrated Lime: Fireclay)

- the base would be constructed out of Pressure treated lumber and integrated directly into my present Deck.

- the Oven itself would sit on top of 6" Vermicrete Insulated Base (2.5:2.5: 1 Perlite: Vermiculite: Portland Cement) which was resting on a 1" Hardie Backerboard.

- the thermal layer of the oven would consist of a 2.5" thick Firebrick floor with a 2" castable Dome over top.

- the Dome would be insulated with 3" of a Ceramic Insulation Blanket, plus a 1.5" Vermicrete render ( 5:5:1 Perlite: Vermiculite: Portland)

- finally it would be finished with a waterproof stucco render.


Well that was the plan and in my upcoming posts I will walk through each step of the process. For now I will share some of the final product ie. the pizza.

Ted

[teddyvikings]

The Pizza Oven Base

When I began my pizza oven build journey, I was thrust into a world of many decisions to make. My initial intent was to integrate the oven base right into our preexisting wood deck. But from this forum I heard many people advising to go with either a metal base or a cinder block base. I was almost convinced to go a different route but thanks to the base design of Jumbalaya I felt confident that I could produce a sturdy and safe base for my oven. The biggest factors for me was the increased cost of the other alternatives and the fact that I wanted it to be fit into our existing structure.

The front 4x4 posts sit on posts that originally supported a bench. The central and rear posts sit in deck blocks dug into the ground. For safety purposes I covered the wood that will support the cement board with metal so that any heat that comes through the cement and insulation layers is further protected.

Although not shown in these pictures I have plans to put further cross braces in and construct a floor to hold firewood.

Ted

[teddyvikings]

The Vermicrete Insulating Base

On top of the wooden base, I fastened 2 sheets of Hardie Backer Board (1/2" thickness). I then created a form for the insulated Vermicrete layer. The idea was to make it 6" in thickness with an indentation for the Firebrick to lay in.

My Vermicrete insulating base was to be in a 2.5: 2.5: 1 ratio (Vermiculite: Perlite: Portland) to volume. I was able to source the Perlite from a large local greenhouse and the Vermiculite from a local hardware store. I mixed the Vermicrete mixture in fairly small batches about 30 litres of mixture plus water per batch. In the end I used roughly 4 full batches of the mix. I used david s advice on water amount and it helped create a very workable mix that was easy to use and also had enough firmness to create the Firebrick indentation without another form. Simply used pressure from the trowel.

Understanding that is takes a long time for the water to come out of the Vermicrete I let it rest for 2 weeks before the next step, exposing it to the wind and sun (a very dry summer this year) and covering when there was threat of rain.

Next Step laying the FireBrick.

Ted

[teddyvikings]

The Firebrick Oven Floor

I was able to find used Firebrick in a slightly smaller size of 7.25" x 3.5" x 2.25" on Kijiji locally for a price of $4 CDN per brick. I used about 46 bricks altogether.

I wasn't super clean with the cuts I made using a cheap Angle Grinder with a masonry bit, but it did the job and once the Castable meets the sides of the floor it will not be seen.

I used Playground sand for leveling the bricks.

Next Step build a Sand Form and Entryway for the Homebrew Casting.

Ted

[david s]

Are you planning on the dome casting sitting on the floor bricks or the outer vermicrete ring?
Also are you adding diagonals on that timber stand as timber weathers and moves. The weight you are asking it to carry will be more than you'd think.

[teddyvikings]

Hi Dave,

Thanks for all the help you provide and the questions you ask.

1. Yes, the Dome casting is sitting directly on the edge of the Firebrick with a little overlap onto the Vermicrete. Is that a problem?

2. Yes, I have already added bracing to the stand and I also intend to add planks around 3 sides that will also provide extra stability.

Ted

[fox]

Looking good although I would consider replacing the sand around the perimeter of the bricks with something that allows the bricks to expand.
That might not be an issue as the vermiculite might absorb the expansion but some dry vermiculite might work better than sand. I use ceramic fibre for that job myself.

[teddyvikings]

HI Fox.

As I am sharing much of my oven journey after the fact, I can't do exactly what you said, but the little sand on the side was quite loose, so hopefully there is enough space for expansion. So far I have finished fire curing and have had 3 pizza cooks without any evidence of problems.

Ted

[david s]

1 yes I think that will work ok. If the dome casting sits outside the floor completely I think there may be a problem, but if at least half of it is sitting on the bricks I think you”ll be fine. If you’ve laid the floor bricks loose over the sand there will be enough room for expansion not to cause a problem. The only downside to having the dome sitting on top of floor bricks is that it will be pretty much impossible to remove a brick once the dome is sitting on it. I don’t think this is a problem as any brick deterioration wil only happen to those in the middle.
How thick are you planning on making the casting?
Have you considered adding polypropylene fibres to assist moisture removal? The thicker you make the casting the more imperative an issue this becomes.

[teddyvikings]

Thanks David,

As I mentioned above, I am reporting on the stages of my build up until completion of the fire curing process but prior to the final layer of Vermicrete insulation and the stucco finish. In otherwords, I will have to live with the decisions I have made up to that point.

That being said, I would say that the casting is sitting mostly on the edge of the brick floor as to create an integrated Thermal mass for the oven. I will share the casting process next.

Ted

[teddyvikings]

Preparing the Sand Form and the Homebrew Casting

Just to reiterate that I am doing retrospective of my build through the fire curing process. My strategy was to make a 2" Homebrew Cast Dome using the Forum standard of 3:1:1:1 (Sand:Portland:Hydrated Lime: FireClay). I decided to do it with out needles or polypropelene fibres. Hopefully I will not be sorry in the long run.

1. Used playground sand, water and some fireclay to create the Sand form. For the most part it worked well but I found it difficult to get the full arc of the sphere. Maybe I should have used more fireclay to make stickier.

2. My next step was to lay the wet newspaper strips over the sand. This was surprisingly the most difficult task of the whole project. It was super windy and I could not get the paper to adhere to the dome. I finally called an audible and used plastic cling wrap.

3. The next morning I began the casting process. I made the Homebrew in small batches by hand and began to work the casting against the form. I was concerned about the wetness of the mixture but it felt like it was quite workable. As I began to work up the form it was starting to sag somewhat, so I was continuously drawing the casting up to maintain the 2" thickness. As I got near the end I noticed that I was running out of Sand. I had what seemed like enough product to finish the casting.

4. I covered the casting with a wet blanket and watered it a number of times over the next 24 hours.

5. The next day I noticed a hairline crack running through the casting. I removed the sand form and it was clear that I had skimped on the thickness at the top.

Next step inspecting and repairing the cast.

Ted

.

[teddyvikings]

Repairing the cast.

I chalked up the crack in the dome to possibly 2 reasons. 1) The cast was too thin at the top of the dome and in the gallery roof. and 2) that it was very dry and I should have watered the casting even more than I had.

I decided to use david s repair advice. I created a slurry of the Homebrew and brushed it into the cracks. Then I created a peanut butter consistency of the Homebrew and pasted it over the cracks both on the inside and outside. Then I added an additional layer of Homebrew over the top of the dome and the gallery, taking the opportunity to cast the flue permanently in place.

I then let the casting wet cure for 7days, Constantly wetting the blanket to keep the dome wet while it cured.

Ted

[Carbivore]

teddyvikings What did you use for your flue? It looks inexpensive, which is what I'm aiming for :-)

[teddyvikings]

Hey Carbivore.

I used a 6" Black Stove Pipe joiner from HomeHardware. Worked great. Ted

[david s]

Make sure it’s stainless or you will probably have to replace it in a couple of years.