Over the past few weeks I've been working on a pizza oven outside.  Most people think the idea is pretty cool but some of them question my fanaticism about good pizza.  In reality, a good pizza oven could be used for baking and roasting all kinds of things.  Because it's outside, I don't have engage my oven and my air conditioner in a battle.  It's just too hot here half the year to be baking and roasting stuff indoors.  The outdoor kitchen is all the rage now so there's a growing pile of information on your possibilities.  I thought I'd collect what I've researched together here.  This post might get updated over time.

Generally, a good outdoor oven is of a round shape with some kind of rafractory material inside and some kind of insulation on the outside.  The refractory material (fire bricks, cobb, refractory cement) absorbs heat and gives it back out over time.  It is this that does the basic cooking, not the fire itself.  A classic shape is something like an igloo with a chimney near the front entrance.  Some people also do half barrel shapes, but this is generally regarded as a more inconsistent cooking surface.

Good planning generally involves figuring out what you really want to accomplish, and then picking the method you think you will most be comfortable doing.

Consider how big you might want the oven to be.  You will likely need room for wood and wood embers to pile up while you bake things at the same time.  You'll need room to do at least one pizza at a size you think comfortable.  Do you even care about pizza?

Size

Assuming you're making the igloo shape here, then we're talking a circular interior, a semicircle for a roof, and a squarish region at the front door.  A 36-inch diameter can make four 16" pizzas with room for a fire to still be going on; two pizza would be a more comfortable number.  That should comfortably feed a family of four.  If that is your main goal, than going larger means having a larger oven in the yard, having to wait longer for it to reach a decent temperature, and then having a lot of leftover heat that you may or may not care about.  A good practice or individual oven has a diameter in the range of 26 to 30 inches.

Generally if we're talking pizza here, the maximum height of the oven should be 18 inches.  The door should be as wide as two thirds the diameter, and as high as two thirds the maximum height.  So for a 36-inch oven with an 18-inch maximum height, the oven opening should be 18 inches wide and 9 inches high.  If you intend to do more things than pizza and bread then you might want to up the height.

Some people will go all the way up to an oven with a 48-inch diameter.  At this point you're getting into commercial pizza territory.  That can fit something like 6 16-inch pizzas at the same time.  It might seem attractive to do that, but consider the average bake time on a pizza is around 90 seconds.  Will you even have enough time to put the sixth pizza in before the first is done?  An average adult generally has to strain to touch the back of a 36-inch oven, so imagine with 48 inches how much effort is needed to manuveuer things around in there, let alone to clean it out.

The chimney diameter also varies with the overall size of the oven.  You want something that will handle the fire you're going to be starting underneath it.  For a 36-inch oven, a general recommendation is 8 inches, but there is no hard rule here.  A soft rule then is to make the flue diameter a little over a fourth the diameter of the oven.  A good height recommendation is to reach the overall height of the oven, but you must go higher if you intend to enclose the oven in a structure.

Somebody on the fornobravo forums came across some general guidelines in Italian.

60-80 cm (24-32 in)

20 cm (8 in)

 

80-120 cm (32-48 in)

25 cm (10 in)

 

+120 cm (+48 in)

30 cm (12 in)

 

Thickness

Oven thickness doesn't get much attention but it can dramatically matter in how well an oven cooks for what you want it to do.  Let us assume that regardless of material, you wanted to use fire bricks on the floor of the oven.  This isn't too far-fetched; the oven floor is what actually contacts the food so it's generally wise to pick the right tool for the job there, even if you're planning to make your oven out of basically mud (see materials).

If the oven is too thick it will soak up a lot of heat.  And though it will give it back, it might take a forest worth of wood to hit a pizza temperature.  However, it might crank a lot of bread in one firing; I have heard the number "100" bandied around when talking about how much bread you can make with a 4 inch think oven floor.  You can hit 4 inches by laying fire brick on their sides.  Generally if you want to get up to 900F then you want to lay the bricks on their normal wide side. 

What I ended up doing was increasing my oven height so I could put in some extra bricks if I needed to do a lot of baking.

Oven Interior Materials

If you haven't looked up anything about this yet, you will be surprised by how many different ways one can build one of these ovens.  I recommend you wait on what material you want to use until you've constructed the stand.  I say this because you might be scared of brick right now, but less intimidated by it after having worked with masonry a little.  There are a few general groups under which these all fall:

Bricks: The Conventional Method

The most recommended method is to lay fire brick around the dome, using a refractory cement mortar to bind them together.  Each brick is cut to something resembling a trapezoid.  And that right there is where people freak out.  You probably want to buy or rent a wet saw/brick saw/something to cut bricks.  Although you could go with a chisel and clang away, you'll waste a few bricks.  Of course you might waste less bricks moneywise than what you'd have to pay for the equipment. 

You must get actual fire bricks.  The big box stores do not tend to deal in fire bricks.  What you need is something like a "medium duty" firebrick.  It must be able to withstand temperatures in excess of 1,000F degrees of repeated uses.  You'll probably have to find a mom and pop place to acquire these.  I warn you though that you should arrive with time to spare; when you tell them what you're doing they might have a ton of good advice.  You might find out the person on the other side of the counter has built at least one.  In my case, it turned out the main running the register had been the one to reply to me about something online!

It is recommended to lay the bricks on their flat sides.  This makes the walls of the oven much thicker than the floor, which means the walls will not get as hot as the floor, but will continue to give off heat for much longer.  You can try to use the side of the brick, but apparently it requires much more skill to keep the bricks together on their sides. 

Generally one builds a form out of styrofoam sheets on which the bricks can rest as their mortar dries.  There is also a tool people will contruct that anchors to the center of the dome at the dome's radius that you can move around and up to the spot where you need to lay the next brick. 

As the Three Little Pigs show, brick is the superior building material.  A brick oven lasts forever, so long as you don't screw it up.  Because then you might have to knock the whole thing apart.  If individual sections are failing after the fact, you could probably break them out and put in fresh brick.  I might not make it sound so good, so I'll put it this way: If I were to do it over, I'd probably do it with brick.  Having done the base in cinder blocks and having broken up some bricks for the oven floor, I got a lot more confident I could have done it.  The being said, I did throw some leftover bricks into the cob construction I made.

Earthen Construction: cob/cobb and adobe

Cob is a mixture of clay and sand additionally blended with all kinds of natural aggregates like straw or manure.  This is stamped together with enough water that one could shape a brick out of it in one's hands and it would maintain that form.  These are then slapped together around a sand form of the oven.  This cures about a week and then the sand form is dug out.  Cob can even be used for the insulation with a lot of straw (not hay!) thrown into the mix.  Adobe is basically cob that has been dried in forms to make bricks.  You have a chance to be much more precise when working with adobe. 

If the Internet implies anything, I think cob is the unofficial oven building material of Oregon or something.  I am not so sure why.  Generally it's a sustainable material that can be dug out of ones back yard, though you'll have better luck if you're right next to a body of water.  It is also very merciful since it's easy to repair.  However it generally takes a lot of cob to handle both the interior and the insulating exterior.  And it won't insulate as well as synthetic materials.  So cob gets kind of poo-poo'd over the long term.

I decided to use a synthetic cob because I couldn't get the right clay out of my yard.  I bought clay at a pottery place and went with that.  I would recommend Hawthorne Bond.  You can ask them what is a good clay that withstands high temperature.  This was mixed 2 parts sand to 1 part clay, by volume.  The bags of sand came in 100 pound sacks, so beware. 

If you are still aiming to be Mr. Sustainable but you don't think you can get the right clay mix, I guess you could dig up your yard and supplement with a sack of clay.  You want to dig below the top soil and then down into the stuff that cuts cleanly when you shovel it.  Get a handful of it wet and roll it in your hands.  It should have some plasticity to it--like Silly Putty or Play-Doh or ... well, clay.  If it's still crumbling apart then consider supplementing with clay.

Cracks are a fact of life with cob.  It will crack even as it dries.  It's best to have some cob material left behind to fill it in, or just live with it.

Refractory Casted: For Those Who are Better Carpenters than Masons

All to themselves, the weirdos who want to build plexiglass forms and fill them up with refractory cement.  The results can be really impressive.  The walls are very smooth both inside and out.  With sufficient skill one could cast the whole thing in one hit, but it's also possible to do it in separate pieces and then attach them together with a little refractory mortar.

This all comes down to how well you can construct the curved shape of the oven out of materials that could support the cement, yet by dismantled both inside and out when the cement has cured.  I've seen these alternately made out of metal and plexiglass.  I've speculated one could probably use styrofoam blocks to form it but I don't really know if it would work. 

I don't know what kind of cement to recommend here specifically.  You'll want to triple check that before you first pour.  Conventional cement and concrete are not the way to go.  You need something that can withstand the big heat changes.  Heck, you could probably pour a wet cob mix in. 

If the cast forms a structural crack, then you're in for a lot of trouble.  You can try to glop on some fresh cement in the affected area.

If you prefer to work meticulously on a mold over days or even weeks and then have the whole thing go down in about 30 minutes, then you are probably the type build a refactory-casted oven.

Oven Exterior: Insulation

If you do not insulate the oven, some of the heat will work its way out that you want rather to stay in and cook things for you.  Not all insulation is alike.  There are composite materials that can insulate much more effectively than a cob mix of straw, but then you're working with synthetics.  An important thing to note is it's important to insulate more aggressively under the oven than around the sides.  The fire contacts the oven floor and its the floor that does most of the cooking.  You want all that energy sent up into your food.  Things to keep in mind:

• Perlcrete: A mixture of perlite and cement.  Do note I wrote "cement" and not "concrete!" Concrete is a mixture of cement, sand, and "aggregates" (rocks).  The cement is what binds, so you basically dilute the binding action by using concrete.  A typical mixture is 6 parts perlite and 1 one part cement.  You will want to do this with a respirator on because perlite goes everywhere and will make you cough compulsively and die of strange lung diseases or something--if you were to inhale the whole sack.  Perlite is very light.  It will come in a 4 cubic foot sack of the average volume of today's overweight adolescent.  For this amount of space, you will be astonished you can fling the sack right across your driveway if you felt like it.
• Thermal blankets: Not the kind you snuggle up in during the winter.  These are sheets that wrap around the oven.  They are especially useful since they can handle all the curves. 
• Thermal composite boards: Good for under the oven.  One of these can do the job of a few inches of perlcrete.  You cut it to the silhouette of your oven and then build on top of that.

 

Oven Exterior: Aesthetics

There are two common choices here.  One is to keep the dome shape, and another is to build a little house over it.  If you build a house, you will have to make sure the chimney is high enough to reach out the roof at its peak.  The benefit to building a house is you get another chance to cram in some more insulation, and you get another chance at keeping the oven waterproof.  Try as you might, after a big rain, water will try to get in there somehow.  A house is also a bold statement in the yard and will probably become the focal point wherever you build it.  If you don't want that then consider not doing it.  It is also something else that might have to be removed if you ever want to repair the oven.

If a roof is something you want but its construction too daunting, you could go with a single slanted roof, which would look oddly trendy.

Preparing the Base

If you're going the sustainability route, then you'll probably just stack up some stones you have laying around on which you'll later put the oven.  Everybody else pours a lot of cement and stacks up cinder blocks.  This might be your first foray into masonry so the whole thing is probably really scary.

At this moment you might think you could build a fireplace into your oven, and that would be correct.  These steps don't assume that.  You're on your own there.

Foundation

Here you can't go wrong with a slab 4 inches deep that supports all the cinder blocks.  Mix to a consistency of chunky peanut butter in a wheel barrow and dump it in batches.  You don't have to get a cement mixer and do it in one solid pour.  You will do best to build a form out of 2x4's.  This seems like a waste but you'll have to use it again to make the counter.

You might try an alternative material.  One is tumbled glass pebbles from recycled glass.  The City of Austin offers this for free if you can and scoop it up yourself. 

Walls

Cinder blocks are a good way to go here.  They generally measure 8x8x16, although technically they've had roughly 3/8's of an inch taken off each side.  This is to factor in the mortar you'll end up using.  You should do a mental rehearsal with the cinder blocks to see if you'll end up needing to get some square 8x8x8 pieces.  You probably will have to get some if you plan to have an opening on the front side to store wood.

You will want to have a nice leveler.  The expensive ones seem unneccesarily heavy, but what they all you do is run them across a length of uneven blocks, and beat on the level to affect the blocks into a uniform level.  The weeny ones can't do that.  As you start out you'll want to be paranoid about your levels on all sides, but the cinder blocks don't have to be perfect.  They will look like crap no matter what, and you will cover them up with something anyways.  Start from the corners and work out and around from there, going upwards from the corners and spilling down from the edges.  Mortar the cinder blocks that will join the slab too.

You generally want to mix mortar to a consistency similar to the concrete slab as mentioned before.  But the real test is how it handles with a trowel.  Run the flat side over the mortar a few passes to smooth it out, then cut into it with the edge of the trowel.  You should get a line that doesn't break apart (too dry) nor fill back in (too wet).  If you pass this test, cut two lines into the mortar and scoop out the chunk that it makes.  Hold it over the mortar, and turn your trowel 90 degrees so the edge faces down.  The mortar should stick for at least a second before it slides off in a solid mass.  This is important for mortaring the sides of the cinder blocks.

Wet down the blocks you're about the join.  Use the cutting method I described to make nice, clean chunks for laying mortar.  Be generous with the stuff.  Joke that a hurricane could come and blow away the cinder blocks, leaving a frame of mortar holding everything up.  After it has dried a little you can go back and clean it up.  You'll get the hang of it right around the time you finish--doh! 

Counter

You must build the counter in place, at the desired height.  You do this by spanning the walls with some cement backer board, laying out a frame, meshing together some rebar for extra spanning support, and then pouring concrete in.  Oh--that stuff is heavy, so you want to have some boards inside supporting the cement boards that you can knock out later. 

Repeat after me, you cannot pour the form on the ground, pick it up with a bunch of people, and expect it to hold together in transit.  Perlcrete cannot be used for this slab either.  I tried to do this and it broke apart as I picking it up.  Well, the first attempt was perlite and concrete, and that crumbled apart, but my second attempt was properly with cement.  It oddly enough held together--generally.  But don't do it since it might just bust up where you're holding it.

If you have an open face on one side of the oven, you'll need an angle iron to span the gab.  The cement backer boards won't hold up enough to the concrete.  These are hefty iron pieces like girders shaped into a right angle.If you lay one of these down you might have to do some hand waving with the levels of everything since it'll be resting on top of the cinder blocks, messing with the level on that side.  So that being said...

Now is a good time to double check your levels.  You can make up for any mistakes you made earlier, short for the whole thing sliding down into the Earth.  You can also create new ones if you don't keep an eye on the level.  Wet spare boards do a good job of scraping around the concrete.

If you are really eager to keep going, mix your concrete with some rapid drying variety.  I did a 1:1 bag:bag, though the fast drying was 50 pounds, and the maximizer concrete was 80 pounds.  It was holding up within two days, though I waited a week to knock out the legs.

Oh some people like to run a thermocouple under the oven for checking temperature.  Most people stop using them after using the oven a few months, but if you want it, you need to make a little hole for it in that concrete.  Better to drop in a straw or some tube to make sure the concrete doesn't fill in where you don't need it.

You now have an altar on which you can sacrifice babies.  Or depending on how large you went, maybe toddlers!