This is Part 11 of the 20 part series on building a brick oven. To see more, visit The Ultimate Backyard Brick Oven Tutorial: How to Build Your Own.
A chimney on the pizza oven is the last functional piece needed before I can make pizza. It’s kind of like the last piece of the puzzle before you can step back and say, “It’s complete!”. Except, it’s still only partially complete. But it’s functionally complete. That’s good enough for now!
In fact, a chimney isn’t even functionally required for a pizza oven. If a chimney wasn’t there, the door would serve as the chimney. Cold air would enter the oven through the bottom of the door while the smoke and hot air would escape out of the top. But, who wants smoke coming out of the front door when you are trying to make pizza? Not only would the smoke make it difficult to see and breath, but it would leave black stains all over the front of the oven.
When thinking about putting a chimney onto the pizza oven, there are again a lot of things to think about. Considering I have never built a chimney, I had to do a lot of research to understand what I was doing. Some of the questions that I had were:
- What material do I use for the flue? Stainless pipe or clay flue?
- How will I support the base of the chimney?
- How does the chimney go through the roof without leaking water?
- What prevents rain from coming into the chimney?
- Since the chimney will get hot, do I need to insulate or brick around the chimney? If so, what prevents water from seeping between the flue and the outside brick/insulation?
If you have any experience with chimneys, these questions may seem pretty dumb. But I knew nothing. So below is what I found.
Stainless or Clay Flue?
Ultimately, either a stainless pipe or clay flue works the same. They both exhaust heat out of the oven. It is the requirements other than the pipe that really make the difference.
First, let’s look at a metal pipe. The metal pipe must be stainless steel. A galvanized pipe will rust out from the exhaust fumes. Also, the metal pipe should be double-walled. A single wall pipe will get scorching hot and may have an effect on any sealants between the pipe and the roof. To connect the pipe to arch transition, an anchor plate is needed. After all is said and done, using a double walled stainless steel flue is expensive. Here is an example of this double walled stainless flue pipe and anchor plate. Once the flashing is included, you are looking at >$200 just for the chimney. Despite the expense, however, a metal pipe is very easy to implement. You mortar in the anchor plate, attach the pipe, and attach the flashing. That’s it.
The other option is a clay flue. Clay flue’s are dirt cheap. I bought a 2 ft section for less than $10. However, they require a little bit more work. A clay flue should be surrounded by brick leaving a 1″ air gap between the brick and clay flue. Still, after including the brick, the cost is ~$30.
Between these two choices, I decided to go with an 8 inch by 8 inch square clay flue.
As I mentioned above, a metal pipe will require an anchor plate. For a clay flue, you can mortar the flue directly to the brick.
In either case, a transition will have to be created between the landing arch and the chimney. To make the transition, I used my angle grinder to cut angles on bricks. Once mortaring them into place, a flat and level area is created for the chimney flue to rest on.
Insulating the chimney
If choosing a metal pipe for a chimney, you should choose an double walled pipe. For these pipes, there is no need to insulate around the chimney as they are already insulated. Clay flues require brick to be laid up around the flue with a 1 inch air gap.
How to keep water out of the chimney?
Since I haven’t actually made it to the roof yet, I can only speak to what I have read. Whenever I get to the point of putting on the roof, I’ll give an update on how I did it. But, in the meantime…
There are a lot of gaps around the chimney for water to potentially invade. First, there is the gap between the chimney and the roof. To prevent water intrusion here, metal flashing is commonly used. For steel pipe, you can buy these for the right diameter of the pipe. For a clay pipe, you put the flashing into the brick exterior. There are tutorials out there for flashing which are pretty helpful.
The other place that water can invade is between the clay flue and the brick. Since there is an air gap, this could easily allow water to seep down the flue. To prevent this, a concrete chimney cap is commonly poured in place. During this pouring process, window seal insulation (the blue foam stuff) is wrapped around the clay flue. After the concrete cures, the window seal insulation is cut down and the gap filled with high temp silicone sealant.
Alright, now that the chimney is in place, I have a working pizza oven! It may not be protected from all of the elements yet. It may not look extremely appealing (in fact, a little redneck at this point). But, it works! But before firing it up to full temperature, it will need some smaller fires to cure it. I’ll update you soon on how to cure the oven.