Maximum mortar thickness - joint width

[Al Neer]

Hi all, new ambitious oven owner here.

I am going for a pretty standard hemisphere-on-a-row-of-soldiers design, but i have to admit I hit a bottleneck when it comes to brick sizing, in regards to the resulting joint gap.
The overall goal is to avoid multiple and angled cuts on the bricks. So, for the square-cornered bricks, on the interior of the dome i will just get them to touch at one corner and then the resulting joint gap won't be more than 3mm (1/8 in), that's the easy part.

But what grinds my gears is how big should I allow the joint gap to be on the exterior side, considering the overall joint bond strength and thermal-shift behaviour.

Naturally going upwards towards the top rings of the dome the width of the brick must shorten, as to reduce the joint gap (both on the inside and outside of the dome), but what is the absolute threshold I should aim for? Does a rule of thumb exist? I guess it is eventually mortar-brand dependant and on the very top rings beveled bricks are inevitable, but like i said the goal is to minimise brick cutting so to cut only when necessary, i.e. max joint gap reached.

Extending already on the matter, shortening the width of the brick also reduces the brick overlap - staggering margin. I know that the brick joint of the next row should ideally sit in the middle of the the brick in the row below, but we all know that this is impossible to maintain, hence the use of custom stagger-bricks here and there, but what is the minimum overlap of bricks from row to row we aim for? you get it, since making stagger-bricks means performing cuts.

I am surprised that a thread somewhere doesn't exist already, or if I didn't find it please refer me.

Thanks all.

[SableSprings]

Welcome to the forum Al! Many of us simply use or homebrew or mortar to fill those outside gaps. The dome build/structure is extremely sturdy and the gaps between the bricks on the inside are minimal...and that's all you're really aiming for. Some builders use shims or wedges to hold the angled bricks but usually you butter each brick with enough mortar to bond and hold it in place as the build progresses. Use any left over mortar from each chain to backfill. I built my dome and then filled remaining gaps with mortar before applying my outer dome insulation.

Wetting each brick (some folks just drop them into a bucket of water) prior to using them is important so the brick doesn't draw moisture from the mortar too quickly (or at all ). That way you get the best bond possible. As you have noted, the heat cycling of the normal oven use WILL crack some of the mortar joints. The important thing is to stagger joints between each chain (layer) of bricks so you don't have a crack develop going up through several chains via the "aligned" joints. Even with that visual, scary type of crack, the dome structure will be fine and the Cracks will dissappear when the oven cools down. In fact, before our modern methods of determining when our ovens had reached proper temps...old school simply noted the size of cracks indicated the oven was hot enough for the upcoming bake.

I didn't angle cut any bricks for my 39" dome and simply filled the outside gaps before insulating...finished in 2009, still standing & functional Relax, don't grind your gears!

[Al Neer]

Welcome to the forum Al! Many of us simply use or homebrew or mortar to fill those outside gaps. The dome build/structure is extremely sturdy and the gaps between the bricks on the inside are minimal...and that's all you're really aiming for. Some builders use shims or wedges to hold the angled bricks but usually you butter each brick with enough mortar to bond and hold it in place as the build progresses. Use any left over mortar from each chain to backfill. I built my dome and then filled remaining gaps with mortar before applying my outer dome insulation.

Wetting each brick (some folks just drop them into a bucket of water) prior to using them is important so the brick doesn't draw moisture from the mortar too quickly (or at all ). That way you get the best bond possible. As you have noted, the heat cycling of the normal oven use WILL crack some of the mortar joints. The important thing is to stagger joints between each chain (layer) of bricks so you don't have a crack develop going up through several chains via the "aligned" joints. Even with that visual, scary type of crack, the dome structure will be fine and the Cracks will dissappear when the oven cools down. In fact, before our modern methods of determining when our ovens had reached proper temps...old school simply noted the size of cracks indicated the oven was hot enough for the upcoming bake.

I didn't angle cut any bricks for my 39" dome and simply filled the outside gaps before insulating...finished in 2009, still standing & functional Relax, don't grind your gears!

Hey Sable, nice meeting you, thanks for the input.

So, shall I presume you used standard half-bricks for the dome? so brick face of 4.5" x 2.5" (11.4mm x 6.4 mm) and depth - wall thickness 4.5" (11.4mm) for your 39" dome? from which row of bricks and onwards you started reducing them in size?

I know, it's hard to build a dome that will collapse on you but, you know, maybe it's undiagnosed OCD, maybe artesanal psyche-leftovers from a previous life, I really love it when everything is "proper" so I am trying to negotiate with myself here before I angle-cut every brick... I should have picked the username "OverEngineer", it would have been more indicative...

Ok, I can go ahead with that (and let you all know for science's sake how it worked out), but I would strongly appreciate more input from the community on the matter, what worked for each or any (un)official standards on the practice that you are aware of

Thanks folks

[SableSprings]

Yes, I just cut standard firebricks in half and used the nicest face on the dome interior. Remember that the inside face is all that anyone will see once you're done. After the first ring is laid, I staggered joints and only needed to cut one per row that would keep joints from aligning vertically. The inside wall nearest the door is quite difficult to see once the dome is completed, so that's the best place to use a narrow piece. There are 2 schools of design thought for laying that first chain. The first (and easiest IMHO, which is why I used it) is to lay your cooking floor on the insulating base, with whole bricks (usually a herringbone pattern) and place your first row of dome bricks on top of them. This means there will be some edges of bricks that will extend outside the dome perimeter, easy to either leave in place or do a rough trim when placing. The second method is to put your outside chain of dome bricks on the insulation and cut the cooking floor bricks to fit just inside it. You don't need to be real exact, but shooting for a 1/2" gap is pretty easy. This gap will be filled with ash in pretty short order and works pretty well. People that use this method usually are thinking about the possible future need to replace a cooking floor brick. I've been using my oven for 16 years and all my cooking floor bricks still look good.

You do not mortar to the base insulation in either case when laying the cooking floor on the insulation base and do not mortar your first chain to either the insulation or cooking floor (depending on the method you choose. UtahBeehiver cut all his dome bricks exactly and his oven is truly a work of art. Sadly, in my opinion, his incredible workmanship of brick fitting for the dome (kinda like seeing the marvel of Macho Picchu) bricks is hidden by the fabulous copper sheathing he added. In talking with him, Russell noted if he did another over, he would not spend all the time exactly cutting each brick.

Hope that helps a bit.

Mike