New build started in Scotland

[jjnevis]

Hi folks, Naive Scotsman here attempting to build a wood fired oven for the first (and most likely only) time. I thought I would start a thread here of my progress for those interested and perhaps so that others can learn from my mistakes, just as I might benefit from the vast knowledge and experience available on this forum.

I have a rough plan (pic attached, hopefully readable), but hope to fill in the detail as I go along, particularly the opening and the chimney.

My main concern at the moment is casting the tabletop out of concrete. I'm concerned that my rebar is under spec'd (it's about 5mm mesh) and that I'll not use the correct recipe for the mix since it has to span quite a large gap between the supporting walls. I may just go and get some more rebar before I cast it. I'm also wondering how long I should leave the tabletop once it's been cast before removing the form and continuing with the build. Temps here at this time of year are around 10 degrees C.

I've decided to go for a relatively thin wall (I plan to cut 230x114x76 bricks into three and use the 76 dimension as the thickness) because I wanted a relatively quick heat up time, and then I plan to insulate quite a bit to keep the heat in as much as poss for retained heat cooking. I will probably increase the insulation by about 25mm beyond what's on the plan simply because I've got some loft insulation (rockwool, I think it's know as) lying around, which I will put over the ceramic blanket.

Anyway, I've started now, so no going back!

[mongota]

A grid of 12mm or equivalent rebar would be better.

Regarding the amount of weight bearing on the slab, for the center portion of the slab, the only weight bearing on it is the oven floor; the floor insulation and floor firebrick. The weight of the dome as a whole bears on the circumferential footprint of the dome, and a good portion of that footprint is directly over the block walls below the slab.

If you are using bagged concrete mix for the slab, you can increase it's strength by adding a shovel full of portland cement to the contents of each bag when mixing your slab. You could also add a couple of lengths of steel angle iron across that span. Example, use a diamond blade or abrasive wheel to slice a kerf in the top surface of the block, and slip one leg of a piece of steel angle in to the kerf. The angle will span the opening on the flat, and offer support to the hearth slab. Spanning openings with angle is fairly common in these builds, quite a few builders do it to span the opening in the front wall of the block foundation.

The build may seem confusing to you now, but once you start, it all starts to make sense. As the dome goes higher, the geometry of how everything joins together becomes more easy to see with the eye. ESPECIALLY how the vent arch joins the dome. That had me scratching my head until I got to it, then is was so very very simple to understand, and straightforward to build.

Most people's office cabinets hold books and files. My cabinets? They hold the same. But they also hold a wee bit of Scotland. lol

Welcome to the forum!

[MarkJerling]

Hi folks, Naive Scotsman here attempting to build a wood fired oven for the first (and most likely only) time. I thought I would start a thread here of my progress for those interested and perhaps so that others can learn from my mistakes, just as I might benefit from the vast knowledge and experience available on this forum.

I have a rough plan (pic attached, hopefully readable), but hope to fill in the detail as I go along, particularly the opening and the chimney.

My main concern at the moment is casting the tabletop out of concrete. I'm concerned that my rebar is under spec'd (it's about 5mm mesh) and that I'll not use the correct recipe for the mix since it has to span quite a large gap between the supporting walls. I may just go and get some more rebar before I cast it. I'm also wondering how long I should leave the tabletop once it's been cast before removing the form and continuing with the build. Temps here at this time of year are around 10 degrees C.

I've decided to go for a relatively thin wall (I plan to cut 230x114x76 bricks into three and use the 76 dimension as the thickness) because I wanted a relatively quick heat up time, and then I plan to insulate quite a bit to keep the heat in as much as poss for retained heat cooking. I will probably increase the insulation by about 25mm beyond what's on the plan simply because I've got some loft insulation (rockwool, I think it's know as) lying around, which I will put over the ceramic blanket.

Anyway, I've started now, so no going back!

Welcome to the forum JJnevis.

Like you, I used rockwool over my ceramic blanket and it's worked really well to insulate the oven. Yes, as Mongo has pointed out, your floor reinforcing is light.

I would recommend min. 10mm or 12mm reinforcing bars, in both directions, in a 150mm grid. I would also comment that 100mm as a slab thickness will work, but 150mm thick will be better! Considering your cold concrete curing conditions, I would recommend fully supporting and bracing your temporary concrete shuttering under the slab for a minimum of one week, preferably two weeks, after the concrete has set. Use min. 25mPa concrete, preferably 35mPa. You can, of course, happily build on top of the slab, providing you keep it supported below. In my case, I left the shuttering in place until nearly the end of the building process.

Kind regards,
Mark

[jjnevis]

Mongo, Thanks for the advice and words of encouragement. As far as the table top slab goes, I am going to have a brick arch at the front of the base, so that will help support and I'll get some bigger rebar to set in and I'll be sorted, and as you've pointed out, the majority of the load is at the edges over the support walls anyway, so I'm feeling more relaxed about that now.

You certainly have some nice drams there on the top shelf, I think the Macallan 12 year would be my pick of that bunch. That gives me an idea - I should invest in a bottle of something only to be opened when the first cooked pizza emerges from the oven! That will give me a good incentive. I'm thinking Balvenie Doublewood, although I usually go for whatever is on offer!

[jjnevis]

Mark, thanks for the advice. I will indeed increase my reinforcement and might increase the thickness of the slab (whilst keeping an eye on the final height of the oven floor - my wife is 5'2" so don't want to make it difficult for her to use). Keeping the shuttering in place makes good sense too. I am thinking that a 1:4 (cement : ballast) mix should be strong enough - I think that would be about 30mPa based on a minimal web search.

[mongota]

I'm thinking Balvenie Doublewood, although I usually go for whatever is on offer!

If you like Balvenie, I have both Doublewood and Carribean Cask behind that front row of bottles. C'mon over!

[MarkJerling]

Mark, thanks for the advice. I will indeed increase my reinforcement and might increase the thickness of the slab (whilst keeping an eye on the final height of the oven floor - my wife is 5'2" so don't want to make it difficult for her to use). Keeping the shuttering in place makes good sense too. I am thinking that a 1:4 (cement : ballast) mix should be strong enough - I think that would be about 30mPa based on a minimal web search.

You're most welcome. Yes, 1:4 sounds fine. What height do you plan to have the oven floor level at, above floor level?

[jjnevis]

What height do you plan to have the oven floor level at, above floor level?

It is currently about 1200mm from the floor, which is getting quite high, I think.

If you like Balvenie, I have both Doublewood and Carribean Cask behind that front row of bottles. C'mon over!

Sounds great! Where are you?

Snowing here at the moment, so progress has slowed even more than the already glacial pace. Pic of where I've got to attached hopefully.

[MarkJerling]

I have mine at 1250mm above paving level and I think that's about right, but then I am 6'2". My wife thinks it's fine and she's 5'4".

[jjnevis]

That's good to know, we are both a couple of inches shorter than you guys, so height is looking about right at the moment.

[Wickolad]

Hi jjnevis
I'm pretty sure I saw somewhere that approximately elbow height for the landing is about right.

Best wishes,
Lance.

[Baza]

jjnevis - great start! Advice to date is terrific and you have some titans helping you here!
Born in Scotland myself - many years here in Canada but do miss the homeland - mongota ... that is an impressive line-up mate! I wouldn't discriminate if a dram glass was produced! I think you've given JJ something to shoot for!

JJ - get the height right - for two reasons - workability and heat retention.
I wish I had added a layer of foam glass under my CalSi board - the extra 3" would have been a nice rise (and thermal support) for the floor. As it is, I'm ok, but not in a position to go back and fix all that now!!
So - really check the height for workability and lift it in a way that ensures thermal support for the floor ... you don't want heat bleeding out the bottom!

Looking forward to watching your build!
Barry

[modified9v]

My floor is at 48”. Perfect for us. I’ve worked ovens that were not that high and I hated the bending to look down the throat. My wife is 5’ 7” and she has no issues either.

[jjnevis]

Baza thanks for the info and support, Canada's a long way from home, but a great choice! On the question of insulation, I was wondering about boosting it a little (I currently have 50mm of ceramic fiber board and was wondering if I could put an inch of kingspan under that. I believe kingspan (also known as Celotex) is rated to 100 C. The question is - is the temp on other side of 50mm ceramic fiber board from brick floor likely to get much higher than 100 C? I could also use rockwool, but it's not structurally ridged, so I would worry about sag in middle of floor. I could use vermiculite, but it's not as good and I have the other options lying around. Any thoughts? Is 50mm ceramic fiber board enough?

[Baza]

Hey JJ! Yes, Paisley and the Buddies are a long way from here! (no jokes! Ha!)

Your insulation question is a good one! I have that amount of CalSi board under my floor and it is in keeping with FB's build directions - many here have improved on them over the years and the prevailing approach appears to be TWO layers of CalSi board or CalSi under the floor and on top of FoamGlass (so, concrete hearth, FoamGlass, CalSi, brick floor). I'm not familiar with kingspan but I wouldn't put anything under this oven that is rated any less than you CalSi Board as the floor temps get VERY hot.

The 50mm has been used by builders in the past and they are delighted with their ovens.
Those who double up are also delighted with the fact that their ovens retain heat longer - and that is great for days worth of cooks!

At this point, I would still recommend doubling up at least, but with nothing rated less than your CalSi board.
My humble insight.
Barry