Dome arch placement?

[brad mole]

I have been searching for days for the correct place to put the dome arch so i finally have given in to asking, i figured if i cant find the info then at least it might help others in the same position as me if i can get the answer.

My main question is where is the correct place to position the dome arch, my oven will be a 42" pompeii oven with a tapered arch, 508mm wide and 317.5mm high (20" x 12.5")

Heres a link to my build so far https://community.fornobravo.com/for...-uk-42-pompeii

I understand the basics and think i understand how the taper works etc. I also am aware that the bricks will become longer towards the top of the arch, is it a case of just starting with a full size brick at TDC and they will become shorter by the bottom?

Also is there an ideal depth that the former should be in order to carry the bricks right?

I have attached a screenshot (Image 1) of my build plan where i am figuring out the arch former position (my arch former is 508mm wide and 317.5mm high (20" x 12.5") now am i right in thinking the former will sit at the point where the inner side of the former intersects the inner circumference of the dome? if so then how do i determine where the front (or outer) of the former should stop? obviously it should stop where the front, flat face of the brick arch stops but how do i know where that should be?

I cant figure out in my head looking at my 3D model how to determine where my bricks will sit on the former (front to back) or at least where to start working things out from

Image 2 shows what i think is right, what it shows is 4 lines from the centre of the dome.

- Bottom line - When the first arch brick is laid lining its front edge up with the front of the arch former, this line is a straight line out from the centre of the dome floor to the point where the first arch brick intersects the outer circumference of the dome to give me a cut line for that brick?

- Second bottom line and top line - These two lines come out vertically and horizontally at 90 degrees to each other to show the radius of the dome bricks (the shaded arc)

- Third bottom line - Am i right in thinking this line is worked out by taking a line from the centre of the dome floor to the point where the brick intersects the outer arch?

If i am correct in thinking the above then that just leaves the length of each brick to work out, the diagram shows a full length brick (230mm) cut using its maximum length, so it could be anwhere between 230mm down to as little as around 195mm before the bottom of the arch will start to merge into the outer arch circumference, is this length just a decision left to me which makes no difference later on?

If this is the case and i left my TDC brick as a full brick, this makes my arch former around 157mm deep, just thought id write that in there incase it helps others

Sorry this is such a complicated way of asking the question but it all seemed obvious in my head but as soon as i started writing it i realised its very hard to explain what im asking in a few pictures.

If anyone could clarify that would be great, thanks!

[JRPizza]

Brad, your placement looks about right to me. There is a 1-2" (or so) fore aft movement of the TDC brick that will make an acceptable arch - I did like you show and my TDC brick was full length from the outward face to the point where the inner surface of the dome on the lower half of the brick intersects with the slope on the top where the dome brick will sit on the arch brick. The length of all the subsequent bricks will similarly be determined by the intersection with the inner surface of the dome and the slope needed for the brick sitting above it to be in proper orientation. Some builders have done beautiful jobs building their arches first, but I found building as I went helped me as I could not visualize where I wanted all the surfaces. If you are trying to keep all your inner surfaces as tight as possible (smallest mortar joints) make sure you have enough angle on the top sides of the arch so you can get some mortar in and still keep the inner faces in contact (if that makes sense the way I explained it)

[david s]

As Mike has pointed out, many builders construct the arch first and then build the dome to it, butting the dome bricks to the arch. It is far better, although more difficult to tie the arch bricks into the dome by staggering at least some, as you would with a conventional brick corner.

[brad mole]

Brad, your placement looks about right to me. There is a 1-2" (or so) fore aft movement of the TDC brick that will make an acceptable arch - I did like you show and my TDC brick was full length from the outward face to the point where the inner surface of the dome on the lower half of the brick intersects with the slope on the top where the dome brick will sit on the arch brick. The length of all the subsequent bricks will similarly be determined by the intersection with the inner surface of the dome and the slope needed for the brick sitting above it to be in proper orientation. Some builders have done beautiful jobs building their arches first, but I found building as I went helped me as I could not visualize where I wanted all the surfaces. If you are trying to keep all your inner surfaces as tight as possible (smallest mortar joints) make sure you have enough angle on the top sides of the arch so you can get some mortar in and still keep the inner faces in contact (if that makes sense the way I explained it)

Great, thanks JRPizza, good to know it looks like im on the right track, i had planned to construct the full arch beforehand but sounds like it may be more difficult than i first thought, i will see how it goes when i get the materials. yes im aiming for tight mortar joints on the width but expecting to full the wedge under each brick as im not tapering in all directions, only the sides.

As Mike has pointed out, many builders construct the arch first and then build the dome to it, butting the dome bricks to the arch. It is far better, although more difficult to tie the arch bricks into the dome by staggering at least some, as you would with a conventional brick corner.

Thanks david, so do people tie in the dome bricks to the arch on a tapered arch? i seem to always just see them butted up? is butting up going to be strong enough?

[JRPizza]

I'm not sure I have seen a build where the dome bricks interlaced with the arch bricks. I suggest finding a build or two you admire, see how they were built, and try to emulate as close as you can.

[Gulf]

I remember a few that were doing this when I was doing my research. It wasn't every course. I remember them interlocking the first couple of courses of the dome and inner arch where the two were on about the same plane (which is always a good practice when completing the arch first for stabilization). I've seen a few haphazardly interlock a course or two near the top, but it looked very difficult. If you were to google brick arches you wont see examples where the voussoirs are interlocked with the adjacent wall brick that surround them. It's just not practical.

[mongota]

Brad,

It was a bit tough for me to imagine, but once I got to the point where I could set bricks on top of the arch form, it all made sense.

Your drawing is very similar to how I built mine. I positioned the base of my arch so the base of the arch template met flush with the inside face of the dome.

I built the arch by first cutting the bricks for the bases of both legs, and built upwards towards the arch keystone. The following describes cutting the top keystone brick, since that's what I have photos of.

I placed a brick on top of my arch form with the edge of the brick aligned with the outside face of the arch template. This usually resulted in the brick overhanging the inside edge of the arch template, and hanging inside the dome.

You can use your indespensible tool, or simply a string. The photo shows a line (red) that had the inside (yellow dot) and outside (orange dot) radii of the dome marked on it. I held that up to the side of the brick so that the orange dot was on the top edge of the brick. I used a sharpie to mark the location of the yellow and orange dots on the side of the brick. I then marked a spot on the bottom edge of the brick where the brick met the arch form (blue dot).

I connected those three dots and cut the brick on those two lines.

[brad mole]

Brad,

It was a bit tough for me to imagine, but once I got to the point where I could set bricks on top of the arch form, it all made sense.

Your drawing is very similar to how I built mine. I positioned the base of my arch so the base of the arch template met flush with the inside face of the dome.

I built the arch by first cutting the bricks for the bases of both legs, and built upwards towards the arch keystone. The following describes cutting the top keystone brick, since that's what I have photos of.

I placed a brick on top of my arch form with the edge of the brick aligned with the outside face of the arch template. This usually resulted in the brick overhanging the inside edge of the arch template, and hanging inside the dome.

You can use your indespensible tool, or simply a string. The photo shows a line (red) that had the inside (yellow dot) and outside (orange dot) radii of the dome marked on it. I held that up to the side of the brick so that the orange dot was on the top edge of the brick. I used a sharpie to mark the location of the yellow and orange dots on the side of the brick. I then marked a spot on the bottom edge of the brick where the brick met the arch form (blue dot).

I connected those three dots and cut the brick on those two lines.

Hey mongota, thanks so much for the response, that actually makes perfect sense, i kind of had a similar thing in mind with the string to mark one line, then marking 114mm (4.5") back down that line which would be the point of the brick, then returning to the arch former from there. but now you have said to mark the line with 2 marks, im guessing those dots are 4.5" apart to match the rest of the dome brick depths?

Thank you so much for that response it really has cleared things up for me on the tapered arch build

About to order my materials this week but cant decide how many bricks to order, i know it wont be less than 250 or more than 300, but delivery is expensive if i need more so im tempted to just got for the 300, seems a lot though for a 42" dome and vent?

[mongota]

...but now you have said to mark the line with 2 marks, im guessing those dots are 4.5" apart to match the rest of the dome brick depths?

About to order my materials this week but cant decide how many bricks to order, i know it wont be less than 250 or more than 300, but delivery is expensive if i need more so im tempted to just got for the 300, seems a lot though for a 42" dome and vent?

Brad, the first question regarding the two dots and the 4-1/2" dome thickness, you're correct. Most of us build domes 4-1/2" thick, so those two dots would indeed be 4-1/2" apart. Using a string versus the IT, either will work. I actually used my IT to register the marks. A string will work as well. I mentioned "string" since the IT was not shown in the photo.

Second question regarding brick count?

My dome is a 42", I used square-cut half bricks set flat for the first two courses, then as they angled to shape the dome in the 3rd course I started to taper them. I think I stayed with half bricks up to about the 9th course, though as I got up towards the 9th they were smaller than half-bricks due to the tapering. From the 10th through 14th I used third-bricks. Then the plug.

I believe I bought 270 bricks to start with, my plan was to get enough to do the dome and vent and I'd make a second trip to get whatever else was needed, because I was unsure of how I would do the chimney. When I was done with the floor, the dome, and the vent arch, I had 4 bricks left over. 270 minus 4 equals 266 bricks not including the chimney transition.

I needed 12 brick total to transition from the top of the vent arch to the metal chimney anchor plate. My chimney transition is pretty simple; twelve bricks set on edge, two bricks wide by six bricks deep. The edges of all twelve bricks scribed to follow the curve on top of the vent arch.

So as best as I can tell, I used a total of 278 bricks. The dome, I'm happy with. I could have been more efficient brick-wise with my vent arch build. But I also could have used more bricks by buttressing the vent arch, which I chose not to do. Overall, I have no complaints.

Best, Mongo

[UtahBeehiver]

Each tapered arch brick is different so don't take the measurements on one brick and cut a bunch assuming they will all fit. Even the left and right side of "each" arch brick will be slightly different. The top dead center brick of the arch will be the longest brick. Half brick will not work on a tapered arch so start with a full brick. Once the TDC brick is placed you transcribe one of the angles to the adjacent left or right brick and use the IT or string to mark the far side of the left or right brick of TDC. Hope this makes sense.

Can't help you on brick count since my bricks were not even close to standard size, 6 x 9 x 3"

[mongota]

Agree with Russell 100%.

For the dome arch, I built my arch form and set it in place.

I did not taper cut my arch bricks. I considered it, but discarded that idea. So I placed full bricks on the arch form and shimmed them into place, the temporary shims taking place of the eventual mortar. I placed the bricks so the key brick at top dead center would be...centered at top dead center. lol

I also numbered my bricks, L1, L2, and R1, R2, etc, from the base of each leg up to the keystone brick.

I also marked where the edges of the bricks landed on the arch form. This prevents "brick creep" when you eventually mortar the brick in place.

With the layout lines on the arch form marked and the bricks numbered so they go back in their intended locations, I took them all down off the form. I started by setting the base brick (L1) for the left leg of the arch. Mark it, cut it, set it in place, either set it in a bed of mortar, or set it dry and shim it to account for the thickness of the eventual mortar bed while you lay out the others. I think it's easier to leave the bricks unmortared and to just shim them. That way you can mix one batch of mortar and mortar them all at once, without delay. Nothing wrong with mortaring as you go if that's what you prefer.

With L1 shimmed or mortared in place, now shim the next brick (L2) in place. Just like Russell wrote, mark the cut lines on the bottom face of L2 based off of the already cut and in place face of its neighbor, L1. Then use the string and dot method described earlier to mark the "upper" face of the L2 brick. Connect the dots, cut L2, and shim or mortar it back in place. Repeat brick by brick for both sides. As Russell wrote, the bricks will get longer and the cut angles will change slightly as you go higher and higher up the arch.

When all the bricks are cut and shimmed in place, take them all down, mixed up the mortar, and mortar them back in place. When you mortar them, tap tap tap the bricks in place so the edges of the brick align with the layout marks on the arch form. If you stray from the layout lines and allow the bricks to creep, your keystone brick may not fit, or your arch may come out asymmetrical.

In some ways it's tough to describe. But it's quite easy once you get started.

Best, Mongo

[UtahBeehiver]

Here is a pic of what Mongo is talking about. Mine is slightly different since I tapered my bricks but notice the tic marks on the form and the L and R numbering.

[brad mole]

Thanks Mongo and Russell, once again, perfect responses making things super clear. yes all makes total sense now im pretty confident on the tapered arch side now! think i will just order 300 bricks and accept i may waste a bit of money but rather that than come up short and have to pay delivery twice as its palette delivery only where im buying from.

Im in the middle of drawing up my arch former and dome base layout so i can CNC cut them from plywood so they are nice and accurate! perks of the job! keep an eye on my build for updates in the next week or two

Thanks again guys! much appreciated

[UtahBeehiver]

One thing I failed to mention and it does not show in the pic I sent. You need to shim so the arch form so it will release once the arch is done. It the pic it show only about 1/4" shim, this is not enough, factor in at least a 1/2" of shim for form release and save the form so you have a template for a door in the future. Shish, CNC cut plywood.............what a deal.

[brad mole]

Thanks russel, yeah i was going to allow 10mm for shimming, thanks for reminding me though!