My first WFO design, any comments before I build?

[Tallyrc]

Another, if casting, should my dome be monolithic or cast or cut into sections? Is there any benefit (I would think so)

[david s]

A one piece casting is far easier to build but is more likely to develop a crack because of the uneven heating which sets up uneven thermal expansion, The crack will usually start at the base of the dome opposite the oven mouth. If this happens the cast will not fall apart and the crack usually never gets ant worse. For this reason most manufacturers of cast ovens provide multi sectioned casts which just introduces designed cracks where you want them to be. They are also far easier to handle for assembly rather than dealing with a very heavy single cast. Some manufacturers offer a one piece dome specifically for mobile ovens to reduce the tendency for them to rattle to bits during road travel. If you cast in situ the problem of moving a heavy dome is eliminated.

[david s]

The polypropylene fibres are the really fine ones, finer than human hair, usually 19mm long. The longer and thicker fibres, as part of a two pack concrete fibre addition can be used for the outer shell, but not as burnout fibres for the inner casting.

Perlite and vermiculite are interchangeable, I use a mix of half each, medium grade.

[Tallyrc]

I've actually got access to a concrete saw if cutting it into sections is beneficial. Casting in sections doesn't seem out of my scope of abilities either though likely far more involved. I'm looking forward to getting underway. Been lurking quite a while and just bought a new home that is begging for backyard improvements..

[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

Its been a while since I updated on my progress. I decided to change my design with regards to the inner dome and other advice that david s gave me:

Dome
2" homebrew inner dome
3" insulation blanket
1.2" perlite cement render

Floor
2.4" fire brick
2" perlite concrete
4.5" reinforced concrete slab

Stand
0.5" cement fiber board
4" wooden beams

The homebrew will consist of 3:1:1:1 - sand, cement, clay, & builders lime. I will also add some polypropylene fibers to the mixture.

david s: What ratio should I use for the perlite concrete under the pizza oven?

I will upload photos and videos of my build coming week

Cheers Mark

[fox]

So if your budget would stretch, I would recommend 4”” of ceramic blanket over the dome and a fibre cement top coat rather that a perlite top coat.
fibre reinforced cement is mixed at a 2-1 ratio and is extremely crack resistant and water resistant too.
Also I would highly recommend a 3” ceramic fibre board base under the bricks.

[Mark1986]

So if your budget would stretch, I would recommend 4”” of ceramic blanket over the dome and a fibre cement top coat rather that a perlite top coat.
fibre reinforced cement is mixed at a 2-1 ratio and is extremely crack resistant and water resistant too.
Also I would highly recommend a 3” ceramic fibre board base under the bricks.

Hi Fox,

Thanks for your advice! I will see if I can use more ceramic blanket!

What does a fibre cement mixture consist of? It is mixed 2 parts portland cement to.. ?

Thanks in advance!

Cheers

[david s]

Be careful, too rich a mix in cement can cause shrinkage and cracking. Fibres in the mix can also be overdone. Follow the product dosage recommendations. There are many different types of fibres available for use with concrete, mortars and renders. I use AR (alkaline resistant) fibreglass fibres, which I find easy to use and work well. Get them from the concrete countertop suppliers.
https://www.google.com.au/amp/s/alld...mix-ratio/amp/

[fox]

Fibres reinforcing for cement products has been around for decades and it comes in many mixes and has many uses.
Where I live it is used extensively in the building trade, ranging from long thick fibres for concrete to ultra find fibres for finish plaster.

You would obviously follow the manufactures recommendations but 2 parts fine sand to one part Portland cement mixed carefully with the correct amount of fibres will give you a very hard a crack resistant casement. It sets like sheet metal and water wont easily penetrate through the high cement content.

My own method is to lay several thin coats of 4-1 find sand mix directly over the ceramic fibre matting, you do this so the weight of a single thick coat will not squash down the matting. Then the next day add your top coat of fibre mix cement, you can lay up to 4” thick without sagging so it is easy to shape your dome how you want it.

The main disadvantage with strong mix, fibre cement, is getting a very smooth finish as ordinary plaster tools leave the fibres standing up.
There are a few ways to get a good finish, one is to manipulate the mix as best you can with a trowel and use a mini paint roller to get a neat finish.
Different rollers with give different textures from smooth to heavily embossed and it is very easy to do.

However for a truly professional finish, you can add a finale top coat of 4-1 cement (no fibres) as soon as your main coat is firm.
So .... day one two thin coats of plain 4-1 ... day two one ‘thick as you wish’ coat of fibre cement and one final top coat or final top coat within 24 hours.

[Mark1986]

Fibres reinforcing for cement products has been around for decades and it comes in many mixes and has many uses.
Where I live it is used extensively in the building trade, ranging from long thick fibres for concrete to ultra find fibres for finish plaster.

You would obviously follow the manufactures recommendations but 2 parts fine sand to one part Portland cement mixed carefully with the correct amount of fibres will give you a very hard a crack resistant casement. It sets like sheet metal and water wont easily penetrate through the high cement content.

My own method is to lay several thin coats of 4-1 find sand mix directly over the ceramic fibre matting, you do this so the weight of a single thick coat will not squash down the matting. Then the next day add your top coat of fibre mix cement, you can lay up to 4” thick without sagging so it is easy to shape your dome how you want it.

The main disadvantage with strong mix, fibre cement, is getting a very smooth finish as ordinary plaster tools leave the fibres standing up.
There are a few ways to get a good finish, one is to manipulate the mix as best you can with a trowel and use a mini paint roller to get a neat finish.
Different rollers with give different textures from smooth to heavily embossed and it is very easy to do.

However for a truly professional finish, you can add a finale top coat of 4-1 cement (no fibres) as soon as your main coat is firm.
So .... day one two thin coats of plain 4-1 ... day two one ‘thick as you wish’ coat of fibre cement and one final top coat or final top coat within 24 hours.

Ok, so this layer would completely replace the perlite render? I have polypropylene fibers, would those work? Is it also an option to make the layer from perlite concrete and finish with reinforced cement finish? Or does this defeat the whole purpose of using the reinforced cement?

Sorry for all the questions, this is all really new to me!

[Mark1986]

Be careful, too rich a mix in cement can cause shrinkage and cracking. Fibres in the mix can also be overdone. Follow the product dosage recommendations. There are many different types of fibres available for use with concrete, mortars and renders. I use AR (alkaline resistant) fibreglass fibres, which I find easy to use and work well. Get them from the concrete countertop suppliers.
https://www.google.com.au/amp/s/alld...mix-ratio/amp/

Ok! Would the polypropylene fibers also work? I read somewhere that 1.5% of the volume is the right amount. Would you also go for a reinforced concrete with fibers as a render? Instead of the perlite concrete?

Cheers!

[Mark1986]

I am a bit worried about the floor insulation looking at it now. Will the perlite concrete and firebricks do enough for insulation and thermal mass? Or should I adjust things? Maybe I can use homebrew under the oven floor?

[fox]

You must decide what you actually want from your oven, you don’t have to go all out and spend loads of extra money if you just intend to use it for a few friends and family.
If you add all the bells and whistles you will get slightly faster warmup time and longer heat retention but is that really necessary for what you need ?
There are hundreds of wood oven builds on this forum that just use a basic build format because it works well.
However if your budget extends to using ceramic fibre products in place of perlite or vermiculite cement mixes, then it will speed up the build and improve performance … it is just not essential.

[david s]

Ok! Would the polypropylene fibers also work? I read somewhere that 1.5% of the volume is the right amount. Would you also go for a reinforced concrete with fibers as a render? Instead of the perlite concrete?

Cheers!

You can render directly on to the blanket as Fox describes, but because the surface is somewhat lumpy you require a lot of thick render coatings in order to get a smooth hemispherical form. I find it preferable to make a layer of 10:1 vermicrete around an inch and a half thick to restore a perfect dome shape as well as providing a firm substrate to work against. Also, providing the layer is pretty lean (10:1) it also acts as additional insulation reducing the cost/need for extra blanket layers.This then allows me to use a thinner (around 12mm) fibre reinforced cement render layer which I apply in one coat to save time. You can also use chicken wire as reinforcement, but I found that way too time consuming being applied over a compound curve. The fibres are way quicker. The really fine polypropylene fibres used for concrete do not impart strength, they are for shrinkage crack reduction, but can also be used as burnout fibres for the inner dome if casting or in the mortar if brick building. The longer thicker polypropylene fibres can be used for strength enhancement but for a thin rendered layer I prefer AR fibreglass fibres as used by the concrete countertop builders.
The addition of hydrated lime in the outer render is, I believe, quite important as it imparts some elasticity and self crack repair qualities. (see link I posted in previous post) The outer shell is subjected to higher temperatures than renders in other applications as well as steam expansion pressure, so it needs to be pretty strong. I use a 4:1:1 sand, cement and lime mix. There are plenty of alternative methods to achieve the outcome you’re after.

[Mark1986]

david s
@fox

Thanks for the advice! I think I will go with the perlite concrete layer with reinforced cement render over it. Is it water resistant when I do it like that?

Last minute question. I'm going to get the wood for the legs now, so I hope I read your answer before I buy them as it affects the height :P. I'm thinking that the perlite concrete layer of 2 inches may be too thin. Should I make it thicker?

Cheers!