Trusting the structure of a masonry dome (you’ve built...)

No matter how much you read about the strength and stability of a dome construction, it isn’t until you actually walk inside something like the Pantheon in Rome that you appreciate the incredible forces involved. Considering that this particular (and huge) Roman dome was built over 2,000 years ago and yet still keeps tons of rock “hanging” above your head without any seeming support is what I consider a miracle of construction. That said, when I cleared out the supporting materials under my WFO’s completed dome I was really cautious/fearful of putting my head into the oven. While knocking that first support out, I actually stood off to the side in case the oven collapsed (after all, I'm not Roman).

Turns out, the structure and design of a dome is something that even someone with no Roman ancestry or masonry experience (such as moi) can create successfully by following a few simple instructions. When I cleared out the sand from the oven chamber, I was struck not by hundreds of falling firebricks but simply by the high humidity of the interior. It didn’t take long for it to sink into my beady brain that the dome was not going to collapse or shift in any significant way. It was an incredible relief to finally believe my oven was really going to work as an oven and not become a final monument to my ineptitude (while doubling as my tomb with the poignant, yet simple inscription, "Here Lies a Dome Builder Who Should Have Stuck With His BBQ").

My plastic covered sand form worked well to help make pretty smooth & consistent angles on the inside of the dome. Just before clearing out the sand trash, I put some thinner concrete blocks along the oven front to raise the outer landing. I needed to raise this area so when I put bullnose bricks on top of them, the bricks would be flush with the inner landing (on the other side of the ash slot). After doing a final inspection & cleaning inside the oven, I started thinking about the building of the front arches for the chimney. In the “putting more than faith inside the oven” picture, note the piece of angle iron across the opening. The bricks forming the outer faux chimney will be laid along this angle iron to get them even & horizontal across the dome's curvature. The actual chimney flue pipe will span the gap between the inner & outer arches while resting on each arch (or so I hope).

As I stood back and collapsed the sand form, I really did
wonder if the structure would remain standing.

As I cleared most of the plastic bags and sand from the oven
chamber, I was pleasantly surprised to note that;
#1 The dome was still intact and #2 The dome inner surface
didn't look half bad...some people might just be polite and
say "Well, it is unique..." 

This is looking up at the dome roof. The bottom
right is pointing towards the front opening.
Again, this short beavertail design of the dome
is most apparent when looking from the top
or bottom of the oven's dome.

It was more than a little intimidating crawling into the oven
  chamber the first time...all the while I was wondering if I'd
applied the mortar properly. Apparently my wife was
thinking, "Well, if we can't afford cremation, I'll
know we can DIY the job here".

Closing in...

As I started laying bricks by myself, I realized that the more difficult job of putting the bricks into increasingly vertical position was all mine...oh, joy! The options I found at the time included filling the oven with sand or a large exercise ball. I compromised and made a plywood platform just below my current brick course. On the platform I put a piece of Styrofoam that was angled slightly out (hopefully to catch wayward sand or mortar from above). On top of the Styrofoam went plastic bags full of sand and then some loose damp builder’s sand. The plastic bags would make it faster to clean up when the dome was closed and the loose sand allowed me to properly shape the dome ceiling. After I added the first chain on the sand, I started worrying about the sand sticking to the mortar and making for a difficult cleanup. Based on that thought, I laid a piece of plastic over the sand form to keep the sand and mortar separate. (Yes, that is a bag from the original SPAM museum in Austin, Minnesota.)

Now almost everyone that builds a masonry oven at home uses a variation on what’s called (at least on WFO builder sites such as Forno Bravo) the Indispensable Tool (IT) to place these "hanging bricks". Either I missed the IT (probably the case) or the idea became the defacto standard after I was past this point in my build (another one of my mental outs to avoid responsibility for doing something else the hard way...).

Anyway, I cut and laid the remaining bricks to close in the dome pretty quickly. I cut several sequential bricks, numbered them and laid them on the domed, plastic covered sand form. When I had enough cut to justify opening up the bucket of ‘Sairset high-temp mortar, I gave the bricks a quick dunk in water and mortared them into place. Because my dome had the “teardrop” shape, some of the bricks were a bit challenging to cut & fit...but I figured that was part of my penance for wanting to build my own WFO.

Being spatially challenged (and running out of bricks to cut incorrectly), I was very glad a friend came over to help cut & set in the last chain and the two part keystone. I freely admit that my adult beverage that night tasted especially good and felt quite well deserved.

No, it's not the proto-type for Stonehenge.
It's my platform for the sand dome form...to help set and
support bricks in place until the mortar grabbed. 

Sacks of sand on the Styrofoam platform were covered
with a layer of damp builders sand. The sand was then
shaped and smoothed to act as a form for the dome bricks.

The sand form worked, I just had an anxiety attack about sand
sticking to the brick mortar and added a piece of plastic to
separate them. Note that the bag is from the SPAM museum.

A fair bit of challenges here for my inability to spatially visualize
how to cut the angles for the top several brick courses...but I just thought
that with enough mortar nobody would ever see how ugly it really is.

With the help of a friend (who isn't spatially challenged), I got the
final set of bricks cut that were my keystone for the dome.
Time for an adult beverage and self-congratulations.

Finally, the Oven Build Begins

I’m pretty patient, but waiting for the top slab to cure was pretty difficult. I removed the rickety form the day after the pour. The concrete was indeed smooth where it had been poured on the plastic covered plywood. Turned out I was very lucky that not only had the concrete stayed in the fragile form but removing the form in most places was pretty easy...it literally fell out of a couple of places. I tried to remove the ash slot void form, but it didn’t come out. I figured that I’d better wait for a full cure before I tried any brute strength techniques to remove it. I sprayed the concrete with water every day and kept it covered to make sure I got an adequate cure before continuing.

While waiting for the slab to cure, I had tried several brick patterns on the hearth mock-up. Although I liked the offset pattern, I’d seen several online recommendations for a herringbone design. The herringbone just looked a little intimidating to me.

After a week, I decided I could remove the ash slot void form and put the insulating pad/slab on top. From my mockup, I determined that I would need an insulation pad about 55" deep (from the oven edge of the ash slot) and about 45" wide. I found that the 2x4 ash slot void was almost impossible to remove. The wood had absorbed water and swollen considerably. I thought I could beat it out with a hammer...not! Next I tried drilling holes to weaken it enough to beat out...not! Ended up drilling, chipping, and beating the 2x4s for about an hour before finally getting it removed. Slot turned out nice, but I’ll always use foam for any future void form I may need in a concrete job.

Built another form on top of the slab for the perlite/cement insulation layer. This perlcrete would be mixed in a 5:1 ratio (by volume). The reason I used perlite is that it was pretty cheap ($12.54/bag) at the place where I got my block & firebricks and the each bag contained 4 cubic ft. I added five buckets of perlite into a wheelbarrow and one bucket of cement. I mixed the two dry ingredients together first and then slowly added water. The materials are pretty weird and you have to be careful to do more of a folding motion with the shovel. When I’d added enough water that I could take a handful of the mixture, squeeze it, and have it remain in a single lump--I was done. I used a bucket to transfer the perlcrete up into the form. I needed a little over that 4 cubic foot bag to fill the form, fortunately I had purchased a second bag...

I tapped the sides of the form and screed out the top level. After a day, the perlcrete had started to firm up, but it seemed too crumbly around the outer edges to remove the form. My brother-in-law (with actual masonry experience) stopped by and thought the perlcrete had been a bit too dry when I’d mixed it. So, we dug out about an 1"-2" along the form edges and refilled with a slightly wetter mix (still 5:1) which cured with much better “firmness”.

Mark, my brother-in-law, and his wife were willing to stay with us for a few days to help me get the hearth and some of the brickwork done for the oven...talk about relief! I spent quite a bit of time showing him pictures of WFOs and we talked quite a bit about they worked and what I wanted for this one. After he had the basic concepts down, we went out the garage and he discovered I had no masonry tools, so off to the hardware store to pick up several types of trowels, sponges, and some wooden wedges--I do enjoy shopping. Borrowed a 10" wet saw from a neighbor. Ended up getting a new diamond blade and a new belt to get it working to cut our firebricks.

Mark first set & leveled damp builders sand on the cured perlcrete pad. Next he started laying out and leveling the hearth bricks. It was easy for him to do the herringbone pattern and I’d recently read that it was best because it wouldn’t catch the edge of your pizza peel. We did a dry run for the layout of the oven chamber base. I wanted a short beavertail design so that I could easily pull out the coals and ash into the ash slot. This design also let me more easily access and use all of the hearth area. Once we’d both agreed on the layout, he started to show me how to properly set the bricks. We used ‘Sairset pre-mixed high temperature mortar and did not taper any bricks. I had read it would only take one, 5 gallon bucket for the oven...but by the time Mark & I had finished the fifth chain/course of bricks, we had emptied 3 buckets of ‘Sairset. I wish I had found the home-made high temp mortar info on the forum before I finished the build.

Mark & his wife had to leave us after finishing the fifth course, but he’d made sure that we’d talked about setting the chimney/flue placement. So, I guess I’m back on my own again...but now I actually think I know what I’m doing.