Newbie Build - Seeking Criticism ! :)

[PizzaTini]

Hello,

I've really enjoyed this forum. I've taken bits of pieces of advice and am down the road of putting together a fairly budget but hopefully sturdy useable outdoor pizza oven.

I'm going with a wood stand (all cedar except for the plywood which is pressure treated). I know this is unconventional which is making me a bit nervous. Likewise, my inner dome is percrete (see picture - already done), which I know isn't the greatest.

My planned next steps are:

Pour a 5 inch percrete floor
A thin layer of (dry) refractory mortar
Lay 2.5 inch fire brick (I'll have a 1/2" expansion gap around the perimeter). I'm not planning to remove any wood after.
Drill some holes through the upper edge of the plywood to allow any water to drain (I'll then add small piece of painted trim around here with a small gap between the trim and the frame)
Put the percrete dome on the brick
Add 2" of ceramic blanket with chicken wire
Finish with a final coat - I'm not sure what exactly but likely just regular portland cement with sand (not planning on having this layer connect to the fire brick - so that the entire dome is unattached from the floor)
Add a chimney and a door

Inner diameter is 25"
Door is 22"
Chimney opening is 5"

Take a look at the picture and I'd love to know what my weakest points are, I'm sure there are many! Would really appreciate any advice on how I might best approach things as I move forward from here!

[david s]

Timber stands, especially if in the weather swell and contract which heavy masonry doesn't like. The lack of any diagonal bracing makes them even more vulnerable.
A concrete block stand is cheaper and far more permanent.
Using plywood for the supporting slab is also not a permanent solution. Although it may be pressure treated the constant moisture combined with heat will quickly test its longevity.
A 5" perlcrete insulating slab contains a lot of water, in fact third of its volume is water and most of that needs to be eliminated.
If you are leaving a 1/2" expansion gap between the floor bricks and the dome then the dome sits on the insulating slab not the floor bricks.
A 22" wide door for a 25" diameter oven is really wide and may create too much cooling for decent use. Probably too late to change now seeing you've already cast your dome.

[deejayoh]

A 22" wide door for a 25" diameter oven is really wide and may create too much cooling for decent use. Probably too late to change now seeing you've already cast your dome.

You could use bricks to reduce the door size. Just mortar in a smaller arch
However, you don't want to mortar your firebrick to the floor. Unless I misunderstand your approach. Set them in sand + fireclay mix

[PizzaTini]

Thanks to you both! In hindsight I should have built with blocks. I chose this because I wanted something less permanent in case I didn’t like it or parts of it, or wanted to move it. The way it’s built I could easily separate the dome, bricks, slab, and table. This will mainly be used for backyard pizza parties a few times a year.

I’ll add some diagonal bracing. My expansion gap is planned to be between the brick and wood frame - the dome will sit directly on the brick (not mortared). I should have a few bricks left over so the brick arch to reduce the entrance could be a great idea if I’m having issues keeping it warm. Thanks again, truly appreciated!

[PizzaTini]

(My plan is to set the bricks on actual refractory mortar [dry powder...not mixed with water], which I believe should be a fine alternative to a sand / fireclay mix. I already have some refractory mortar on hand)

[MarkJerling]

Hello,

I've really enjoyed this forum. I've taken bits of pieces of advice and am down the road of putting together a fairly budget but hopefully sturdy useable outdoor pizza oven.

I'm going with a wood stand (all cedar except for the plywood which is pressure treated). I know this is unconventional which is making me a bit nervous. Likewise, my inner dome is percrete (see picture - already done), which I know isn't the greatest.

My planned next steps are:

Pour a 5 inch percrete floor
A thin layer of (dry) refractory mortar
Lay 2.5 inch fire brick (I'll have a 1/2" expansion gap around the perimeter). I'm not planning to remove any wood after.
Drill some holes through the upper edge of the plywood to allow any water to drain (I'll then add small piece of painted trim around here with a small gap between the trim and the frame)
Put the percrete dome on the brick
Add 2" of ceramic blanket with chicken wire
Finish with a final coat - I'm not sure what exactly but likely just regular portland cement with sand (not planning on having this layer connect to the fire brick - so that the entire dome is unattached from the floor)
Add a chimney and a door

Inner diameter is 25"
Door is 22"
Chimney opening is 5"

Take a look at the picture and I'd love to know what my weakest points are, I'm sure there are many! Would really appreciate any advice on how I might best approach things as I move forward from here!

Two friends of mine have a similar setup to yours. In the one case, it's worked quite well as they have a roof over the oven which keeps rain off the timber. In the other, the timber frame has weathered.

[PizzaTini]

That's somewhat encouraging thanks Mark. I was initially planning to put the structure in an existing covered area, but that exterior roof is directly attached to my 100 year old barn and I was scared that a single spark could quickly lead to a huge fire. Probably unlikely if I put a spark arrestor in the chimney but the anxiety was taking the fun out of it. It's not too late to move it - Do you know if errant sparks or hot exhausted ambers can be a thing? I also thought it might get really smoky since the structure is covered with a roof, but also entirely on two sides (it's like a car-port that you can drive the tractor through).

[MarkJerling]

That's somewhat encouraging thanks Mark. I was initially planning to put the structure in an existing covered area, but that exterior roof is directly attached to my 100 year old barn and I was scared that a single spark could quickly lead to a huge fire. Probably unlikely if I put a spark arrestor in the chimney but the anxiety was taking the fun out of it. It's not too late to move it - Do you know if errant sparks or hot exhausted ambers can be a thing? I also thought it might get really smoky since the structure is covered with a roof, but also entirely on two sides (it's like a car-port that you can drive the tractor through).

I suppose it comes down to what your barn is made from and how the flue pipe will pass through the carport structure. While there would be a (very minor) risk of sparks in my view, if you have a decent height flue, you're more likely to see sparks exiting the mouth / door of the oven than all the way from the end of the flue. Smoke may be an issue if you build a big fire and the flue does not draw correctly. It will also depend on the wood you burn. Some wood is more likely to create sparks than others. Your highest risk is likely heat transfer where the hot flue pipe passes through the structure. If you have a double flue pipe with an air gap, that risk will be much lower. The trick is to make sure you do not have any combustible building materials in close proximity or touching the hot flue pipe. Another point to consider is how high the flue terminates adjacent to other structures. In my case, I made sure that the flue terminates well higher than the eaves of the house so as to ensure that any sparks, however unlikely, can only land on a corrugated iron roof. If you have a shingle roof or thatch, then this would be of greater concern.

[mefornaio]

My planned next steps are:

Pour a 5 inch percrete floor

• IF THAT IS REBAR IN THE PHOTO THIS WILL DO ABSOLUTELY NOTHING FOR YOU BESIDES ADD WEIGHT ALL BE IT MINIMAL.
• ASIDE FROM SOME COMPRESSIVE STRENGTH THE PERLITE MIX HAS ABSOLUTELY NO STRENGTH TO IT AND WILL NOT EVEN GRAB ONTO THIS BAR.
• IT WILL NOT ADD ANY STRUCTURAL VALUE TO THIS SLAB IN ANY SHAPE OR FORM.
• THE OVEN IS GOING TO HAVE ABSOLUTELY NO THERMAL MASS TO IT AND A 5" INSULATED SLAB UNDER THE OVEN ADDS NO VALUE AT ALL.
• IF YOU ARE MAKING IT THIS THICK OUT OF CONCERN FOR HEAT TRANSFER TO THE WOOD BASE CONSIDER THIS. WE RUN OUR MOBILE AT 950° THE HEARTH BRIKS SIT ON 4" ON INS-BOARD THAT RESTS ON A 1/8" STEEL PLATE THAT HAS 2X2 SUPPORT TUBES UNDER IT. AFTER 6 HOURS OF SERVICE THE BOTTOM TUBES ARE NOT EVEN CLOSE ENOUGH TO IGNITE ANYTHING EVEN SOAKED IN PURE JET FUEL.

A thin layer of (dry) refractory mortar

• YOU SEEM SET ON USING THE MORTAR INSTEAD OF SAND WHICH HAS ADDS ABSOLUTELY NOT VALUE TO THE STRUCTURE OR PERFORMANCE AND MERELY ADDS ADDITIONAL COST BUT IF YOU HAVE NO OTHER USE FOR IT THEN I GUESS WHY NOT. PURE SAND IS MORE THAN SUFFICIENT.

Lay 2.5 inch fire brick (I'll have a 1/2" expansion gap around the perimeter). I'm not planning to remove any wood after.

• 1/2" IS EXCESSIVE THOSE BRICKS WILL NEVER EXPAND THAT MUCH. ALTHOUGH THIS AREA WILL EVENTUALLY FILL WITH ASH YOU CAN REDUCE THIS BY HALF 1/4" IS MORE THAT ENOUGH
• A BETTER METHOD WOULD BE TO PLACE THE OVEN ON TOP THE BRICKS RATHER THAN AROUND THEM.

Drill some holes through the upper edge of the plywood to allow any water to drain (I'll then add small piece of painted trim around here with a small gap between the trim and the frame)

• EVEN EXPOSED THAT INSULATING SLAB THIS TIME OF THE YEAR (NO MATTER WHAT CLIMATE YOU ARE IN) WILL TAKE A GOOD WEEK TO DRY OUT. WITH THE PLASTIC UNDER IT AND THE WOOD ACTING LIKE A HUGE SPONGE GOD KNOWS HOW LONG IT WILL TAKE TO SET AND DRY. ALTHOUGH THE MIX CONTAINS A LOT OF WATER YOU SHOULD NOT HAVE ANY ACCUMULATION OF WATER TO "DRAIN OFF" IF THE MIX IS THAT WET ITS RATIOS ARE OFF AND WILL BE EVEN WEAKER THAN IF MIXED PROPERLY. IT SHOULD BE JUST MOIST ENOUGH TO WET THE CEMENT AND COAT THE PERLIGHT.

Put the percrete dome on the brick

• YOU DO REALIZE THE PERLITE DOME IS REALLY JUST A TEMPORARY STRUCTURE RIGHT?
• EVEN WITH THE MOST DELICATE FIRING (TO MINIMIZE STRUCTURAL CRACKS) IT WILL NOT LAST
• THE MIX MAKE UP WILL EVENTUALLY DETERIORATE AND FAIL FROM THE DIRECT FLAME.
• PERHAPS FOR OCCASIONAL LOWER TEMP COOKING BUT CERTAINLY NOT FOR VERY LONG WITH REGULAR USE.

Add 2" of ceramic blanket with chicken wire

• THIS WILL CERTAINLY HELP WITH HEAT TRANSFER TO THE OUTSIDE (FOR PEOPLE TOUCHING THE OVEN) BUT ITS REALLY NOT GOING TO ADD MUCH MORE PERFORMANCE FOR YOUR SITUATION IN RELATION TO THE ALTERNATIVE BELOW.
• YOU WOULD BENEFIT MORE BE BETTER OFF ADDING THAT THICKNESS OR MORE OF OF MORE OF THE MIX YOU USED FOR THE OVEN ITSELF. THIS WILL GIVE YOU MORE STRUCTURE AND ALSO INSULATINING PROPERTY.
• YOU CAN THEN PUT THE CHICKEN WIRE GO OVER IT WITH A STANDARD HOMEMADE STUCCO MIX AND FINISH IT OUT.

Finish with a final coat - I'm not sure what exactly but likely just regular portland cement with sand (not planning on having this layer connect to the fire brick - so that the entire dome is unattached from the floor)

• NOT UNDERSTANDING HOW YOU COULD AVOID HAVING THIS COME INTO CONTACT WITH THE FIRE BRICK?
• IF THE DOME IS GOING TO SIT ON THE INS-SLAB DID YOU CALCULATE FOR THE LOSS IN HEIGHT OF THE FLOOR?
• HOW ARE YOU GOING TO FINISH OUT THE TOP OF THE INS-SLAB IT MUST BE COVERED WITH SOME TYPE OF CEMENT SAND MIX.
• A BETTER CHOICE WOULD BE TO PLACE THE OVEN ON TOP OF THE FIRE BRICKS TRIM AROUND THEM AND RENDER OVER THE WHOLE THING
• YOUR GEOMETRY IS ALREADY ALL OVER THE PLACE SO PLACING THE OVEN ON TOP OF THE BRICKS IS NOT GOING TO CHANGE PERFORMANCE AT ALL PLUS THIS WILL GIVE YOU A BETTER OPTION TO REDUCE THE SIZE OF YOUR DOOR WHICH IS WAY TO BIG

Chimney opening is 5"

• THE FLUE HOLE IS ALREADY CAST INTO THE OVEN SO THERE IS NOT MUCH YOU CAN DO HERE BUT THIS IS WAY TO BIG
• BETWEEN THE DOOR AND THE FLUE YOU ARE GOING TO SEE MASSIVE HEAT LOSS
• FOR AN OVEN THIS SIZE 3-4" WOULD BE SUFFICIENT FOR THE FLUE
• THE DOOR OPENING SHOULD BE IN THE 16-18" WIDE X ABOUT 9" HIGH RANGE

THERE ARE OTHER COST EFFECTIVE WAYS TO CAST THAT DOME OUT OF BETTER MATERIAL. THE HIGHER INITIAL COST WILL JUSTIFY ITSELF WITH THE LONGEVITY OF THE OVEN. AGAIN EVEN WITH MODERATE USE THE CLOCK IS TICKING ON ITS FAILURE. I KNOW ALL THIS IS PROBABLY NOT WHAT YOU WANT TO HEAR BUT. IF YOU REALLY LOOK OVER THIS SITE MANY HAVE TRIED TO RE INVENT THE WHEEL AND MOST FAIL RELATIVELY QUICKLY AND ALL HAVE EVENTUALLY.

[mefornaio]

That's somewhat encouraging thanks Mark. I was initially planning to put the structure in an existing covered area, but that exterior roof is directly attached to my 100 year old barn and I was scared that a single spark could quickly lead to a huge fire. Probably unlikely if I put a spark arrestor in the chimney but the anxiety was taking the fun out of it. It's not too late to move it - Do you know if errant sparks or hot exhausted ambers can be a thing? I also thought it might get really smoky since the structure is covered with a roof, but also entirely on two sides (it's like a car-port that you can drive the tractor through).

You can make a spark arrestor fairly easy by cutting out a portion of the cap adding galvanized mesh and even adding s.s. brillo pads to ensure no sparks of size will survive migration to the adjacent structure.

[PizzaTini]

Thanks mefornaio!

The rebar is recessed into the 2x8's which I thought would give the frame some tensile strength (that was my intention rather than providing any value to the slab).

I'm currently building a roof structure over it, and also planning to let the slab dry out all the way until March. The 3mm liner / tuck tape was intended to stop the wood from acting like a sponge. You're right the holes to drain water are unnecessary, especially now that it will have a roof. I have extra roof metal laying around, and I'm getting my 4x4 cedar for about a buck a foot (Canadian)!

At a 5:1:1 mix it actually isn't that heavy - about 40 lbs of perlite plus 100 lbs or so of concrete. As you said the water will mostly evaporate.

The oven is going to sit on top of the brick, I'm not sure how I confused you there. I thought this would provide it with thermal mass. The 5" slab will be 45" by 49.5". So too will the layer of fire brick. The oven then sits on the brick - but not mortared to the brick. I was going to do this by adding the ceramic blanket and final layer on another table, and then lifting the dome (with the help of some friends and / or my tractor forks), onto the brick.

I'll save the mortar for creating a brick archway to reduce the entrance, and use sand beneath the fire brick as you suggested.

My percrete dome is actually already ~5" thick around the base (the entry way is more like 2"). I'm thinking I can wrap a 3" chimney with an inch of ceramic blanket (which will yield 5" total diameter, fitting into my existing hole. I can then stucco / cement around that. That was a great tip as was the commentary on entrance size previously noted by David.

All in all, your comments were very helpful and exactly what I wanted to hear! I knew my perlite dome was the worst part of my plan for effectiveness and longevity. I was hoping that if my base stands up and the oven deteriorates over time, AND I use it and like it enough, then I could fairly easily drop a proper oven onto the same base in the future. This all might just be some practice for the future if / when I decide to build a proper oven at our cottage. Most importantly, I'm having fun right now.

I'm most surprised to hear that my slab under brick set up is so terrible - I thought that was the standard approach based on what I read around here...perhaps you misunderstood that part and if not I'd love to hear the explanation as to why - or what would have been better.

I'm encouraged to hear that I should have no heat to the exterior of the dome if I add the ceramic blanket. Do you know whether a 'home-made' stucco would stick well to the blanket as a final layer? I'd like to keep that lightweight if possible. My current dome has been drying for about 4 weeks already and feels rock solid, so it doesn't seem to need any added 'strength' by way of the final layer.

Again, really appreciated.

[Gulf]

I'm sorry that I haven't been following completely, but I would like to add this about spark arrestors. They are a good thing to add to any WFO build that is near any other structures or in amy area that is subject to open burn bans. They are a must for some building codes, and at the very least, satisfy some building inspector's qualms about a design that they know very little about. They can help when someone fires their oven with materials that should not be used in the oven, IE paper. 1/2" mesh is just fine for that. Lower the mesh size too much and it will creosote up blocking the flue. I definately wouldn't occlude the flue with steel wool.

That said, I have never seen a spark exit my flue which is a Pompeii design. I can't say that about my outdoor fireplace which sits nearby. There are totally different draft systems in each.

[david s]

I agree Gulf. I have never seen a spark from mine either. I think ,because we build our ovens with the flue at the front, it's a cross draft system and the flame doesn't really enter the flue pipe unless you overcharge the chamber, unlike an updraft system with the flue pipe in the chamber, or like a pot bellied stove which is also an updraft system, they definitely require spark arrestors. The fitting of one can do no harm though, although it probably needs regular de-sooting.

Regarding flue size for your 25" oven, I believe a 5" flue is what is required. My ovens are 21" diameter internal and the 5" flue works well.

[mefornaio]

Hey guys,

About the spark arrestor you are absolutely correct in over 20 years of oven operation the only sparks I have seen come out are from using "other" materials. He seems pretty freaked out by the possibility so that was my suggestion.

I had purchased a arrestor cap for my mobile at the "strong suggestion" of the local FD..they act like these oven are nuclear reactors..lol I use it for park and residential jobs just to appease the homeowner or park officials but that's it.

What I meant by the steel wool was the one I have has, I believe it's a 1/4" hardware cloth or some expanded metal just below the cap and I meant just adding a bit of the steel wool around that. He could even wrap another layer of the same size cloth around it for insurance but like you both said and I agree it's really not necessary.

Pizza Tini,

Plus 100 lbs or so of concrete
* Wait is your slab going to be a mix of concrete and perlite? Or are you calling pure cement powder concrete?

The oven is going to sit on top of the brick, I'm not sure how I confused you there.
* I might be confused by the 1/2" gap you mentioned I took it to mean setting the brick inside the perimeter of the oven and leaving a gap between the oven walls and the hearth bricks. Re reading it I see you meant the wood perimeter.

I was going to do this by adding the ceramic blanket and final layer on another table, and then lifting the dome (with the help of some friends and / or my tractor forks), onto the brick.
* There's no reason why you can't do this with the done in place. Just put some plastic down over the bricks and you will be fine. Unless I'm missing something of concern?

I'm most surprised to hear that my slab under brick set up is so terrible
* It's not that it's terrible just that given the overall thermal mass of your design the perlite doesn't have much density to it like say a refactory or brick build (which still isn't much) and realistically your not going to have that much thermal build up to retain. That coupled with the occasional use I just don't see a need for much insulation.

I guess I look at a home use oven from the perspective that in my experience, realistically most people use the oven occasionally for a party or family night and that's kind of it. So the oven is fired from cold used that night maybe the next day for cookies or something lower temp and then sits out of use for periods of time so minimal insulation serves this useage and super insulating or even using the blankets or boards, which don't get me wrong are amazing insulators but for home use are really not necessary.

Not that 5" is super insulating but from a practical standpoint the retention achieved by an inexpensive vermiculite mix serves a home use oven just fine. I see lots of home ovens that folks get a little crazy on expensive insulation materials that are overkill given the usage.

You are correct it is pretty standard and it certianly will not hurt anything

Do you know whether a 'home-made' stucco would stick well to the blanket as a final layer?
* I used a standard Portland, sand and lime mix with some fiber and a little acrilic for the mobile and it's seen almost 5 years of HEAVY usage and travel without one crack.