Few Questions

[P3 Stoaker]

G'day,
I'm a little unsure if this post, and the opportunity to ask questions remains valid. I have looked through many many posts and I'm probably well over saturated with info overload. Obviously there are many ways to achieve satisfactory results and I'm mostly sorted with my way forward. I do however request thoughts regarding a couple of things. I'll ask the first and see if this post is active.
I have the base brickwork complete as as seen in the photo I've used double brick where the inner bricks are lower. This build means the bricks match the house and once the dome is up the remainder of the top will be finished in 40 by 20 inch slabs of 1 and half inch thick granite, followed by the insulation covering the cuts. The plan is the inner bricks will support the hearth and the outer bricks will contain the slab and vermiculite edge. I have allowed 2 bricks of height so I'm somewhat limited and hope to have enough height. The centre area is a little under 80 inch square as friends built smaller and said go bigger. I'm about to add more support for the hearth in the centre, a wall extending forward from the back wall meaning no span of hearth will exceed 40 inch.
I'm after some thoughts on hearth/insulation. I plan to lay a reinforced 3 and half inch hearth. Insulation on top probably vermiculite a little over 3 inch thick. I have considered silicate board on the hearth under the vermiculite but I'm concerned whether the remaining vermiculite will be thick enough to be stable. Will it crack and if so the silicate board could become soggy with rain/moisture and compress. I'm not really thinking I'll be doing bread much so whilst I fully accept more insulation is better than less I think a moderate amount will do.
So I guess with 6.7 inch to play with what would you recommend for hearth and insulation thickness and type of insulation? Does vermiculite compress when drying or under weight, ie the dome?

Many thanks in advance.

[mpmckenzie]

Hi,
I'm no expert but here's my advice....
I don't think your allowed depth for slab/insulation is deep enough. With 3" of vermicrete on top of a reinforced slab, the slab will be 3.7" thick max. You only have a max 40" span so you'd probably be OK. If you use 2" calc. silicate boards you have 4.7" for your reinforced slab. I believe the calc silicate boards compress a lot less than vermicrete and have a better insulation value for a given thickness (twice that of vermicrete). The cost is not massive and I'd go for the calc silicate boards.
I don't know what thickness your hearth material will be (? 3" firebricks??) but you can see that the floor of the oven will not be flush with the granite slabs and you'll have to consider some type of transition if you don't want to see the edge of the hearth brick floor.
There's a fair amount of weight in a completed oven though a lot depends on the dome cladding chosen.
I built mine in a very similar way to you except I allowed 3 bricks from the support (see picture) and wish it had been 4 - I had to devise a slate "landing" to the oven entrance.
Keep asking the questions and I hope this has helped. Good luck,
Mark

[mpmckenzie]

Brainwave......
Looking at your picture, you could easily remove the top layer of inner bricks to gain more room for the slab/insulation/oven floor - while still keeping the oven floor at a reasonable height. You'd need to angle the slab form up at the front but you'll hardly notice it when finished. If you do go down the vermicrete hearth insulation route I think people will advise at least 4" thickness of vermicrete under the oven floor bricks.
Don't be afraid to take your work down to get things right - I had to do that at least twice on mine!
Mark

[P3 Stoaker]

Thanks for the info mpmckenzie. I just said to my wife I need to build a practice pizza oven !!!! I'll probably be ok at it by the time I finished one.
In reference to your points above;
1 I realised using 1 and quarter inch (i'm metric, 30mm) granite would mean a step at the opening. Having seen numerous pictures of ovens with steps I was ok with having one, hope I don't regret it. Whilst I haven't really applied myself to a solution to not having one, I don't expect it would be easily solved.
2 When looking at silicate and fibre boards I have read they can become wet and possibly crush or sink under load although I think this is more with fibre board. I have found it hard to obtain info about these. I can get 1 inch board locally and the 2 inch is a lot harder to get here and more costly, although I just want to get things right. I could have 3 one inch boards under the dome and then fill the edges with something to support the granite, or 2 boards with thicker hearth, in which case the vertical height I currently have would work. Are your thoughts that its ok to have dome and oven floor directly on the silicate board? I thought about having 1 inch of board under the vermiculite but wonder if that would leave a thick enough and stable enough layer of vermiculite, 2 inch?? I could put the board on vermiculite??
3 I see what you mean about lowering the hearth but it would be nice to support it on the lintel across the front areas.
It was after the base was finished I began to question the thickness. The plan was from the downloaded instructions where I was wanting insulation sure but don't really plan on breadmaking so thought this would be ok as the plan says, 3 and half hearth and 3 vermiculite. Once I read more i began to question things.
Kind regards

[Chach]

Yes the dome and cooking floor can be on the silicate board and I agree with Mark don't be afraid to take things apart to get it right. If you feel like your not happy but are thinking oh I'll live with it thus means take it down make it right and you not only won't have to live with it but it will be exactly what you wanted. Foamglas is also a non absorbing insulation that some people put down first so water doesn't wick into the calsil board. I did this as well and used a 1" foamglas and 2" calsil board so I have 3" of insulation under the dome and floor.

Ricky

[P3 Stoaker]

Gday Mark and Ricky,
Thanks heaps for taking the time to reply to my questions. I think the most important message there is take it down/remove some work to redo it better/different if warranted.
I'm not generally known to rush things and whilst I definitely agree with you both on this one I will just have a little look and think before moving ahead.
So in my mind I'm happy to remove an inner course of bricks,,,,,, But;
Mark, what would you possibly do with 4 courses vertically, that's a lot of insulation. If 3 inch of board is ample what could you do with the rest?

I'm more than happy to accept that insulation is very important. I don't plan to cook breads and of course if I can get away with my 6.7 inch I will, simply to keep the hearth on the three lintels across the front. Acknowledging I'm challenging your feedback, and not wanting to be difficult, what do you guys think about having 50mm or 2 inch of silicate board on the hearth? I would simply make the hearth thicker under the board, or I could even lay 3 boards one inch thick each on the concrete hearth. Actually this would be 0.2 inch too thick if keep the 3 and half inch hearth. I guess my thoughts are I can create additional thickness above the existing 6.7 inch but is it justified? Not concerned with baking bread, would appreciate your thoughts on 2 inch of silicate board?
Also, is it porous? Do we need to be concerned if a little moisture ( rain leaks through) gets through to the board? As you said Ricky, it's ok to put dome etc on the board.
Thanks again.
Kind Regards

[mpmckenzie]

Hi,
In my case I built the surrounding structure with the aim of the top of the blue bricks being level with the surface of the oven floor.
In my excitement I cast the concrete slab 35mm too thick and then had to come up with an acceptable solution for a working area at the oven entrance. I'd read several builders who liked the extended working area and the lack of a sharp oven floor edge which would get chipped etc. I used an offcut of slate slab for this and it has worked OK.
My base thickness was oven floor (3") + insulation (2") + slab thickness (5 1/4") + levelling fireclay layers (1/4"). I also included a layer of aluminium foil (space blanket?) as a vapour barrier between the slab and the insulation board. (Aluminium foil being heat resistant).
With 4 courses I would have had the space for 3" of insulation board and an oven floor level with the brickwork - hindsight is a wonderful thing.
There is a thread on here somewhere which asks builders what they would do differently next time. A common theme was more insulation between the slab and the oven floor.
In my case I was always going to enclose the inner dome in a brick dome outer structure - I just fancied the challenge. This weighs a lot (it ate an entire pack of bricks - approx 500 - plus the vermicrete and slate top cover) - hence my belt and braces approach to the slab thickness.
Insulation is there to limit heat loss - more insulation = more rapid heat-up times and longer cool-down times with higher thermal stability and less fuel use. It's all a compromise and a lot depends on total thermal mass but, if you can go 3" board insulation I'd strongly consider it.
Yes - the board is porous (or, at least, the stuff in the UK is). You could consider trying to waterproof it before use but my approach was to stop water getting to the board in the first place. Some builders go to the lengths of casting/drilling holes in the concrete slab to facilitate water vapour loss from the bottom of the slab/insulation/oven floor. Have a think about how you will stop water ingress.
Please don't worry about challenging my feedback - I'm certainly no expert!!
Best wishes,
Mark

[mpmckenzie]

Here's the thread I was thinking of...

https://community.fornobravo.com/for...uld-you-change

Mark

[P3 Stoaker]

Thanks Mark. I have been right through that thread recently and found some useful comments, mainly as you say regarding insulation. I'm happy to go ahead with a hearth thickness of 3 and half inch considering max span 40 inch and I'll use 4 inch concrete mesh rather than the standard 8 inch. I am now considering either going ahead with the vermiculite ( less likely) or what might be better is 3 inch of silicate boards laid right over the top of the hearth and the dome/floor directly on that. Did you use the levelling fireclay you mention between the silicate board and oven floor bricks? That would help where the hearth wasn't perfectly flat to fill minor deviations.
Regards Greg

[david s]

Greg,
I always recommend casting a few weep holes in the supporting slab to allow moisture to escape from under the floor. As this always seems to be the hardest and the last place for moisture elimination, the holes do a great job of assisting the escape. In your case, where your insulation is below the level of your bench height the entry of water and it’s removal, from the underfloor insulation from weather, becomes even more important. If you did not take this step you can still drill a few from underneath.

[mpmckenzie]

Hi Greg,
Sounds like your design is coming together.
I found that, to get a dead-flat oven floor, I needed to use some kind of levelling material - hence the sand/fireclay mix. The oven bricks are good but not perfectly uniform here in the UK. I tried laying them without the fireclay and had a floor with some brick edge variation of 3-4mm despite me moving them around to try to eliminate this.
If you do go down this route and use a calcium silicate board note that the board sucks the moisture out of the mixture very quickly so work fast or devise some way of slowing down the moisture ingress into the board. I ended up using the mixture almost dry!
Under the calcium silicate boards I just used kiln-dried sand to fill any irregularities in the top of the concrete slab and to get the calc. silicate boards dead level.
Good luck,
Mark

[P3 Stoaker]

G'day All,
Thanks again for the comments and feedback.
For info the plan will be, 3 and half inch hearth and 3 inch silicate board on the hearth with floor bricks on the board.
Thinking I'll cut the silicate board in the shape of the dome and entry and he floor bricks within the dome, rather than dome on them. I'll fill the rest level with the silicate board, probably with perlite, light and easy to work with. The granite slabs covering everywhere outside the dome are 30 mm and that does mean a step bit I can go double layer to the front thereby having the steps to the side of the opening.
Issues to consider and sort out;
1 I will slightly slope the granite surface away from the dome so water doesn't pool, minimising water ingestion. Probably raise the slab slightly in the centre (under the dome/floor) to allow this.
2 I'll look at sealing/waterproofing the boards and try to have a protective moisture barrier protecting the edge of the boards, as well as holes drilled around the hearth just wider than the dome.

Obviously this means I'm not creating more vertical space overall and I will have the step. Pro's and con's are there.
Kind regards