What is the "Dreaded Droop"

[CapePizza]

I've seen this expression used but not really sure what it is..... I just know I don't want to experience it in my build
Can anyone please give a bit of description of what it is.
Thanks

[mongota]

For the most part, each course of dome brick is made from bricks that are all the same size. And by stacking a same-sized brick, positioned using the IT, on top of another same-sized brick that was also set using the IT, everything is peaches and cream. The dome goes higher and all is well.

When you transition from mortaring dome bricks that are set on top of other "level" dome bricks, to having to cover your arch? Now you're mortaring a "level" dome brick on top of a "curved" arch brick. This is what throws some geometries off.

Most of the dome brick are half bricks, 4-1/2" long. Or third-bricks as you go higher. When I was transitioning to the arch, I'd sometimes use a longer brick to make the bridge from dome to arch. Or maybe a shorter brick. It all depends on how the infilling goes.

The hazard is that because of the dome-to-arch intersecting geometry, the end of the brick that meets the arch may drop a bit. It not corrected, the error can compound as you cover the arch.

The thing to be cognizant of is that your IT wants to keep the inside dome radius consistent. The "L" bracket at the end of the IT will also keep the face of the brick "in sphere" with the inside surface of the dome. However, the IT will simply position the brick you are setting on top of the bricks below it. The IT controls course radius, but not course elevation. So when you transition to cover the arch, YOU have to ensure that the vertical distance from the oven floor to the top edge of the brick you are setting is consistent with the other bricks in the dome chain.

I didn't have any pics of my own, so I looked for someone who had a photo showing the setting of a transition brick. I hope they don't mind me using a photo from their build.

In the attached photo, for explanatory purposes, I've drawn three the vertical red arrows. Consider all three arrows to be the same length, a vertical measurement from the oven floor, going straight-up vertical, to the top edge of the brick being set. If you have a framing square, it'd be prefect for this task.

In this photo, I've marked it up to show that the transitional brick is drooping a bit. For the transitional brick to NOT droop, the top edge of the brick should follow the blue line. Because it falls below the blue line where it meets the arch, it's now below the height of the red arrow, and as a result it can result in arch droop.

It's really just a matter of maintaining an equal chain height above the floor as you transition from dome to arch. If you can do that, plus use equal sized bricks in that chain, it makes it easy. If you do what I did, which was use for example 4-1/2" half-bricks in the dome chain, then perhaps a 6" long single brick to transition to the arch? You have to watch the geometry of how the IT places that 6" brick. The IT will position itself consistently, as well as the CENTER of the 6" brick consistently. But because the 6" brick is longer than the adjacent half-bricks, the two ends of the 6" brick may fall a bit behind the ends of half-bricks that they abut. You'll see that with your eyes. That's okay. You can accept that. Or you may choose to tweak the transition bricks slightly as you go. It's really not a big thing. We're talking fractions of an inch.

You can accept the position where the IT places the longer (or shorter) infill brick. Or you can massage it in or out a bit. However you place the brick, the top edge of the brick HAS to stay consistent with the red arrow height.

Your subsequent courses will build upon the ones already set. The IT will keep the overall inner radius of the dome consistent as you go higher. Honoring the Red Arrow height will prevent arch droop, keeping each course of brick 'level'.

If you maintain the Red Arrow height, then once the arch is covered you can just go back to using the IT to 'slap' each brick in place for your final courses, all the way to the plug.

I hope that makes sense!

[Gulf]

Great explaination Mongo. Here are a couple of pics that may help also. One is that level line drawn in real life with a laser level. The other (the most practical) is the way Colin oasiscdm used the floor as a reference point to maintain level into the arch. He clamped a level to his IT (any ridgid bar would work) to maintain the same distance from the floor. Just set it on the last brick in the course before the transition. New Build in Werribee

[CapePizza]

Okay... thank you both. And yes, Mongo, really great description, and easy to understand. Thank you! I suppose, there could also be a "dreaded rise" if one is not careful. The way I interpret all this is that each vertex on all the bricks in that particular row should (like where your red arrows point to) should align on the same plane. Great idea using a laser level. That would help. Thanks again! Appreciate it.

[UtahBeehiver]

It is easy to get the dreaded droop if you are not watching carefully. That said, it you do get some droop, correct it over several courses and as well as from the front to the back. This is where the adjustable IT really comes in handy..

[JRPizza]

Here is a different take on why droop happens. Kind of like what mongota said but a alternate "why". Either way don't let bricks adjacent to arch lay too flat.
https://community.fornobravo.com/for...405#post381405

[Alomran]

For the most part, each course of dome brick ............

I hope that makes sense!

Well said, thank you