In Part 2 of my ‘Build a Better Outdoor Cob Oven’ series, you’ll learn how to build a cob arch doorway, install a chimney, prepare a sand form, and build the actual cob dome of the oven.

This is where the real fun starts… Check it out below!

(Need a refresher from part 1 of the outdoor pizza oven series? Check it out here!)

Building a Doorway Template

Once your fire brick hearth is complete, it’s time to build the doorway to your oven. Seems out of order, right? Building the door opening before you build the dome ensures a nice fit, however. For this oven, I am going to propose a simple cob arch opening.

Building the opening is one of the few times you need to be pretty precise about some of the measurements. That’s because the optimum door opening in a cob oven is 63% of the overall height of the interior dome. The classic dome as suggested by Kiko Denzer is 16″ tall, which means a 10″ opening is ideal. That’s what I’ve always done and it’s been solid, so I’m going to propose you do the same. This ratio of door opening to dome ensures an excellent draft while firing.

The other important ratio is door width to dome width. Ideally, the opening should be about 50% of the dome diameter. For a 27″ dome, that means a 13.5″ wide opening is in order. Now you can make a template based on these dimensions.

The simplest template I’ve designed is two pieces of wood (or plywood), cut to the same shape and size. I draw my 13.5″ width and mark the height — however, I’m going to reduce the height to 9″ (instead of 10″) here. This is because the form will be propped up on 1″ shims when building the actual arch. The shims will make up the height and allow the form to be removed much more easily.

Okay, so… using either a compass or bucket lid, I trace an arch on the wood. The arch should be wide and stout, not pointy and narrow. I start the curve an inch or two from the bottom, as opposed to the corner itself. Using a jigsaw, cut out the shape, trace onto another piece of wood, and cut again.

Now you can join these two arch templates, and for that I recommend two layers of cardboard. Set the two templates about 8″ apart, and cut two lengths of cardboard 8″ wide. Using small tack nails or tiny screws, attach the cardboard to the top of the arch template pieces to create what looks like a tunnel. The point of all this is to have something rigid to use as a backing to build your cob arch against.

Make the Cob Arch and Install Chimney

With your form complete, you can set it on the protruding firebricks of your hearth. Center the form, and don’t forget to prop it up on 1″ shim sticks. Now you’re ready to mix up some cob and build the arch. Make a nice sticky cob mix with a good amount of straw, and build up the arch evenly and in layers. Knead the layers very well together — you want the arch to be very solid and strong, of course. As you approach the top of the arch, you should make some especially straw-heavy cob and span the peak with long cob loaves loaded with straw that is parallel to the curve of the arch.

As you are placing these final straw-y cob loaves, you’ll want to position your stove pipe. It helps to have extra hands here. You’ll want a single piece of galvanized stove pipe, single wall, 6″ in diameter and 24″ in length. The stovepipe should be positioned in the center of the arch, at least 1-2″ from the dome side of the doorway opening. (You want to be able to close off the dome from the chimney with a simple wooden door when you are baking later.) Use a short level and make sure the pipe is totally plumb.

Do your final cob shaping — it’s easy to make the arch a bit top-heavy. Try to avoid that. You can build up something of a “cob collar” around the stove pipe to give it extra support, too.

Making a Sand Form

Now it’s time for one of my favorite parts of building an outdoor oven: preparing to build the cob dome. Sand is a perfect medium for forming up the dome. Think of the sand as the void of the oven. You want to shape the sand to the correct size and proportions for your actual cob dome. The sand will support the cob while it firms up and it later gets removed to encourage faster drying of the cob.

Start off by drawing a 27″ diameter circle on the fire brick hearth. Use a pencil on a length of string and trace your circle. It should take up most of the brick area. Get a wheelbarrow (or two) of sand and start loading it by the shovelful onto the bricks. As you continue to add sand, you should periodically wet down the sand with a watering can so it sticks to itself. If you don’t wet it, the sand will slide off itself. If it’s too wet, the sand will slump.

I like to shape the sand form with a roughly plumb face at the bottom 2-3″. (This makes the edges of the oven interior easier to clean out later.) The taper should be slow, gradual and rounded, and the overall shape of the oven should be something like an egg cut in half. I’m personally not a fan of the hershey kiss-shaped cob ovens out there. Putting the peak of the dome smack in the middle isn’t as great for heat flow, either. Think of the peak being somewhere about 1/3 in from the rear of the dome.

Mark a wood stake at 16″ and insert it into the sand form at a point when the stake will stay upright. This 16″ mark is the peak of your oven. When you reach the mark and you’re done adding sand, be sure to take a little bit of time to do some final shaping with your hands or a smooth piece of wood. Your dome should be consistent, smooth, and of pleasing proportions. Don’t spend all day on this thing, either… just saying. It’s a simple step.

Finally, tear up some sheets of newspaper, dip them in water, and lay them across the dome in one or two layers. This will help keep the sticky cob from messing up your freshly shaped sand form.

Building a Cob Dome

Once your sand form is complete, you should continue straight away to building the cob dome. You can use your leftover cob from building the doorway arch, plus probably one more batch. The cob dome should be 4″ thick all around the form — about the same width as your hand. You can use your hand as a measuring device as you continue to build.

I use straw in my cob for the dome. I see no harm in that — cob is much stronger with straw, and it really won’t burn out anyway. Build up the dome evenly all around the perimeter, and avoid stitching the cob in the direction of the sand. Meaning, when you use your fingers to stitch the subsequent layers of cob around the sand form, don’t push the cob into the sand itself. Be a bit gentle here.

Also, as you start to climb the dome, try to maintain your width carefully… When you’re at the peak itself, still remember to stitch the cob back into itself. Be careful. Your dome should be equally thick all around, but don’t sweat the small stuff. This is the last time you’ll ever see the dome from the outside, so little visual inconsistencies are not a big deal.

At this point in the game, I like to give the cob a day or two (or three, depending on the sun and wind) to firm up a bit. Once the cob is firm to the touch, the doorway template can be removed, and the sand scooped out. With the sand removed, the cob will dry out much faster. Try to be patient, and let the cob dome dry out most of the way before proceeding to the next step… which I’ll cover in part 3.

Continue Building Your Own Outdoor Oven

Click here to read the final part of my ‘Build a Better Outdoor Oven’ series!