36" WFO - Just outside Toronto, CANADA

[mrpubnight]

I'm finally getting to documenting my build. It is partially complete but wanted to contribute my experiences to this great community.

The Good:

My dome is complete and I've had my first fire. I'm mostly pleased with it. It is, by no means, as clean as other builds on this forum but for my first build I'm mostly satisfied. It took about 10 days of concentrated effort. I didn't waste too much brick and/or mortar and it ended up mostly spherical.

The Bad:

I still have so much to do...certainly a bigger project than I first anticipated. Measuring fractional angles is (at best) difficult using low resolution tools. Kudos to those out there that have such precision cuts and joints. It is truly an exceptional skill that photos make look easy. Having a warped diamond blade on my wet saw didn't help but it certainly wasn't the greatest contributing factor.

The Ugly:

My joints are extremely varied - some are good, some are great, and then others are just outright bad. There were times when patience got the best of me. I thought I would get better at mortaring...apparently not - something to work on for the next build. Angle grinders are VERY good at what they do...and they are not very forgiving !!!


Here are a couple pictures showing where I'm at but I'm also going to chronicle how I got here. Any comments are extremely welcome and desired as I still have more to do and do plan on doing another oven with / for a friend so any advice is most welcome - especially and criticism (I can take it).

Enjoy,
Stephan
Burlington, ON

P.S. If you're in the area and wanna stop in for some food and beer - company is always welcome

[mk e]

I think it looks great and you're probably the only one who will ever know if something is not to plan.

[mrpubnight]

Here are a few pictures of the base.

The patio and foundation slab was laid last summer. I wanted to get through one frost/thaw cycle with a non-trivial load. We can get some pretty weird winters (recently we've had a few freeze / thaw cycles in single months) so I wanted to ensure I didn't get any heave.

This spring I sourced the cinder blocks, dry fit them, filled every second core and added rebar. Funny thing is that I've always seen cinder blocks laying around on the side of roads and by dumpsters and half expected to get this stuff for free. Even some some local postings for free blocks but I found that they were all either different sizes (which was a surprise to me - even 8x16 - different manufacturers were each a little different. Enough to be a pain in the butt). So I ended up just purchasing the lot. It turned out to not be too bad all things considered.

I had everything framed and then found some really good bar fridges at Costco for $120 but needed to raise the hearth slab by a few inches so I tore up the framing, added an extra 1/2 block around and reframed things. It was a pain but ultimately worth it.

My foundation slab was poured by a commercial cement company and it was definitely wetter than the mix I made. I followed the water quantity on the package to the letter but it was not as wet. My understanding is that it will likely be harder as a result but only time will tell. There are some really dry gritty areas that didn't mix all that well - I think a few batches were mixed a little too short. I will be covering the slab when I finish it so I'm not too concerned but the smooth finish of a wetter concrete looks so much cleaner.

Next time

[mk e]

Here are a few pictures of the base.

My foundation slab was poured by a commercial cement company and it was definitely wetter than the mix I made. I followed the water quantity on the package to the letter but it was not as wet. My understanding is that it will likely be harder as a result but only time will tell. There are some really dry gritty areas that didn't mix all that well - I think a few batches were mixed a little too short. I will be covering the slab when I finish it so I'm not too concerned but the smooth finish of a wetter concrete looks so much cleaner.

Next time

I fell for the low water thing too. I read everything about strength vs water, make sure the surfaces are we, moisture cure it, blah, blah....then made concrete and mortar that was nearly impossible to use. Every actual mason I've watched used about double in mortar and about 50% over in concrete...and after trying it I see why!

The stucco lesson is the pros put on the finish coat so thin you ladle it out of the mixer, surface dry...it goes on fast and easy and the extra water is gone in minutes soaked into the base coat so you can finish it without waiting hours. My mason brother just showed me a trick with the base coats....put about 1/4 - 1/3 bag of midgrade ($15) thinset in the mix....it makes it stick really nice so you want less and get done faster.

I did learn though that you can say to much water in concrete.....then as you work it you end up with a puddle on top. I did that on a footer and had to get the wet-vac out....oops. It came out fine though and was basically self leveling, all I had to do was bounce it with a rake and it was done so that kind of made up for it I guess.

I've never tried the drystack block thing.....I bump things, it's just what I do so I mortar them in place....plus with mortar nothing needs to fit very well which is right up my alley

[mrpubnight]

Mortaring is certainly not my strength. My first attempt at dry-fitting was prone to bumping and jarring (especially while I was framing) so when I decided to add the 1/2 block I made sure I filled the cores before framing. I will certainly give those tips a try when I apply my stucco coats to the dome.

With the hearth slab in place I got started on templating the base and making my cuts while waiting for my insulation. I always make the mistake of assuming fabricated bricks/stones are all going to be the same but when I fit the base I started noticing the small differences in the bricks - one day I'll learn. It really only made a difference for me when I was trying to keep things straight. I noticed that my herringbone started to go a little askew - easy to fix but just an observation.

I had a heck of a time getting CaSi board. There were certainly a lot of suppliers in the area but either pricing was prohibitive or the I would have had to buy strange sizes and end up with waste. One supplier quoted a great price only to change it when I ordered it. I did get a piece of sample CaSi board and it was significantly more dense than what I ended up using, Insblok 19. One of the predominant themes I read was that you can't have too much insulation so I decided to maximize the insulation I bought (1 box = 16 sheets @ 1" x 12" x 36" ). I had to create a Frankenstein layer but thought it was better use of my insulation (under the base) than using it under my entrance. I ended up with 4" of insulation which I expect to be enough to keep the base insulated.

NOTE: I noticed that local suppliers of WFO kits include the Insblok 19 - I'm assuming because it is cheaper and thus has a better price point - so I wasn't too concerned about using it. While the PSI rating for the Insblok is a lot lower than the CaSi boards my simple math suggests it should be OK. If we assume that almost all the weight of the dome is focused on the outside 4.5", and that (for a 36" oven) the outside 4.5" represents 572 sq in - but removing the entrance lets call it 400 sq in). On the high end I used 130 bricks on the dome @ ~7.5 lbs each that is 975 lbs which means that at worst the dome is exerting a concentrated 2.4 PSI. I'd love to be corrected about this assumption.

My only real concern with the Insblok is that is feels like it may more susceptible to being damaged by water so I'm going to put a little more emphasis on getting the bottom layers sealed.

[UtahBeehiver]

There have been other builders who have used this product but not much feedback from them. You are probably right on the price point. I have seen a lot of this product on eBay and craigslist. Our host Forno Bravo provides an AiSI board rated at 0.5mPa or about 72 PSI with 10% deflection with their products. So as I tell other builders, it is their choice. The biggest problem with CaSi, AiSi or other mineral board insulation is it is very water absorbent. Once wet, it acts as a heat sink and floor will not heat properly relative to the dome and really gets mushy. But it can be dried out again with cure fires which is pain. Several mitigation items are possible. Raise insulation off the concrete hearth, drill weep holes in concrete hearth, etc.

Looks like the oven is really close to the top and left edges, when you factor in insulation and render (igloo) or studs and base plate structure it appears the be space constrained.

[mrpubnight]

UtahBeehiver : I agree Russell, there isn't much space to play with. Originally I was going to do a smaller oven but increasingly got more excited at the prospect of a larger oven - mostly because of the builds on this site. I have approximately 5.5" on the top and left to play with. With 3" of insulation and 2" of cladding u should have just enough space. Fingers crossed.

My first course was mostly uneventful. I did shave a little off the top to help reduce the first course's gap in the back. Generally speaking, I was pretty concerned with having a large gap in the back (well anywhere really). My goal at the onset was to keep the seams tight and have an almost dry fit oven. That dream was quickly squashed but I still was concerned about larger sized gaps. Aside: I want to applaud all builders that manage consistent, clean cuts and joints. You all certainly make it look easy and are the true master craftsmen/women of this community.

My original IT was just a string placed at the centre of the oven. I did like this approach but found that it required a lot of eye-balling and at the time I was still under the illusion that I was going to have a cleaner build and didn't want to have such an inaccurate foundational tool - more about the irony of this thought on subsequent posts...

[mrpubnight]

Couldn't edit the last post to add pictures inline - only as attachments.

[UtahBeehiver]

Mortar is you friend, especially on the back side. Once the oven is finished, the back side of the dome is covered never to be seen again. I was one who wanted to do joints but if doing again, the most important joints are the internal dome joints. Make you focus here and let mortar take care of the unseen parts. You do not have to taper the entire side of the bricks to tight joints, only where the conflict near the interior of the dome, say the first 1" of so. Spending time on a IT is worth the effort, make sure it is adjustable and the center point on the rod bisects the horizontal center of the brick.

[mrpubnight]

My biggest concern with large mortar joints was the shrinkage as it dried. I did not use the home brew. I found a local supplier of a high-heat mortar that was apparently modeled from HeatStop50 (Thermobond 915), however, their technical data sheet suggests joints between 1/8" -> 3/8". Now, I assumed slightly larger joints would not be a problem but I assumed too much mortar would be problematic.

That said, I do appreciate your point of focusing on the inside more than the outside.

Stephan

[mk e]

I think all the high temp mortar says it's for thin joints....all the stuff I saw anyway.

Mine is made with plain $7/80lb mortar.....I was told it just needs to keep the bricks in place until the dome is set, then it does nothing which made sense to me. Not that it's the right answer but I know of dozens of ovens built this way that have been happily cooking for many years.

The inside really should be pointed with high temp since it sees actual flame temps, but if the gaps are small it probably doesn't REALLY matter there either....if they're fit well the bricks aren't going anywhere and your pics look well fit.

They've been building ovens like this since before there was motar so pobably not worth losing any sleep over as long as you aren't putting anything toxic on the bricks I'd think your good....at least that's the motto I'm running with on my oven.

[mrpubnight]

I heard similar things but I also heard that Portland Cement breaks down with high heat, and while it may not be structural I did notice my dad's WFO is getting worse for wear after about 5+ years so I'm trying to learn from his mistakes.

Courses 2 and 3 went smoothly and made me optimistic. I had somewhat tight seams, things went well and I was generally happy. I used wedges between the 2nd and 3rd courses but didn't pay close enough attention to seams and noticed that some were lined up. This was mostly a function of not paying close enough attention but also the result of not cutting all my pieces the same size. I had clamps set up to align my 2 outside cuts (using Sir Chipster's method) but eyeballed the middle cut - my eyes must not have been properly aligned.

I also got the outside of my inner arch started and cut a groove into the top brick to support the end of my arch.

One thing you'll notice in these pictures is that my IT moved it's relative vertical position. I was a dumb, dumb when I made my first IT and so made adjustments to it and made a second one for the remainder of the dome...for those of you keeping count, at this point I've used 3 different ITs but at the very least they were all the same radius.

[mk e]

It looks great so far.

[mrpubnight]

Thanks mk e ...

I laid my 3rd course and for the most part it went OK. I did notice, however, that I started getting bricks out of alignment. That is to say that as I tapped some of the bricks to fit them I must have been putting some rotational stress on the previous brick which caused their faces to slightly rotate away from being perpendicular to the center. You can't really see it that much in these pictures (maybe a little on the 4th one - 3rd course first and second bricks on the left) but I can certainly see it when looking into the dome from the entrance. I'm sure some subsequent pictures will show it better.

I also finally got around to the inside arch. I struggled to mortar this properly and had to resort to non-traditional methods (i.e. mortaring 4 sets of 2 bricks together and once dry I mortared those together). No doubt there are better ways to approach this but it was the only way I was able to get this to work.

I also used full bricks and ground out the inside edges to align with subsequent courses. It worked pretty well with the 4th course but the 5th wasn't as successful.

Aside: Since firing my oven I have noticed a crack develop along the arch and I'm guessing it may be related to the way I went about putting it together. See my other post: Crack during early stage curing.

[mrpubnight]

Finished the courses over the inner arch. I butchered the inside of the inner arch - angle grinders are not very forgiving - while carving the back for the next course. On the plus side I didn't get much (if any slump) over the arch. I added some end-cuts behind the bricks over the arch to try to keep some consistency in the depth of the brick.

While adding this course I noticed that my previous course wasn't 100% centered and so I started making adjustments starting on this course. The pictures of the inside of the dome illustrate one of the problems I mentioned in an earlier post where I noticed that my bricks were turning a little while tapping them in place. I think the lighting is making things look worse than they really are but I was still a little disappointing.

On the 7th course I started mixing 1/2s and 1/3s and then on the 8th I moved to 1/3s only. I think at this point my 'circle' was getting a little better but still not perfect. While it is very exciting to get this close to closing off the top I found it difficult to work on the last courses because I was hunched over. My back wasn't the happiest with me.