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Lardylad 07-22-2009 05:15 AM

Fibre cement base for hearth ?

Have an oven ready to go, pouring concrete in a couple of days and blocks on order. Just need some advice on the suspended slab please....

Will be building a bigger base than plans suggest and area under oven will be enclosed with open wood storage to the right hand side of the oven, under a food prep area.
Is it OK to use fibre cement board to make the base of the hearth form as I won't be able to remove the plywood (base enclosed) ? I'm thinking of using the cement board as I'm not sure whether leaving combustible formwork/ plywood enclosed under the oven is a good idea - although there would seem to be plenty of insulation etc between oven floor and wood ?

Also, with the longer dimension of my hearth being 2.4 metres, what can I do to stop the outside formwork bulging out and leaking concrete/ forming a curve when I pour the concrete ?

Any help would be greatly appreciated.


Lardylad (Aus)

splatgirl 07-28-2009 08:53 AM

Re: Fibre cement base for hearth ?
in my experience with cement board, it's brittle and nowhere near as strong as plywood, but if you've got enough bracing under there it shouldn't matter what you use. That said, based on what I've read, a properly insulated cooking hearth wouldn't let anywhere near enough heat transfer to your bridge slab to have to worry about having plywood under there. Seems like I read a thread by someone who measured the temperature under the slab during/after an active cooking fire and it was not even 100F.

For your slab, you'll need to brace your form with angled "kickers" that are screwed to the form boards and angle down (or out) to whatever other available surface you have. In my case, that was the ground but ajacent walls work, too. Here's what mine looked like with the bracing placed. Note that you'll want to put a screw or two into the tops of those kickers to attach them to the form boards:

RTflorida 07-28-2009 10:26 AM

Re: Fibre cement base for hearth ?
Cement board has been used by many. Personally, I used HardiBacker. My hearth slab has been complete for 2 1/2 yrs and I believe the HardiBacker is permanently bonded to the support slab concrete. If I tap it with a log or hammer, it sounds very solid with no hollow sounding spots from a lack of adhesion.
Brace it like the above photo and you will be good to go.


jmhepworth 07-28-2009 05:46 PM

Re: Fibre cement base for hearth ?
I used cement board cut to sit just inside the holes of the concrete blocks, braced in the middle. Because I divided the stand in half with concrete block down the middle making two small wood storage areas instead of one larger one, I was able to cover each wood storage area with a single sheet of concrete board. It worked just fine and was a lot easier than framing plywood. At some point I'll get around to posting pictures.


Lardylad 07-30-2009 04:00 AM

Re: Fibre cement base for hearth ?
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Thanks for the great advice - the picture was just what I needed to show how to brace the top formwork - feeling a lot more confident now.
Think I'll go with the fibre cement board spanning the blockwork as I was aiming for a qucik solution without having to put together a lot of form that wouldn't be able to be removed.
Also like the idea (from the pic) of leaving some rebar proud of the blockwork to lock the hearth into.
Had the concrete down in a couple of hours and blockwork dry stacked the next day. Just waiting for the loan of a mixer to core fill and then (based on replies) forming up the hearth.
Found that time taken to get the concrete exactly level made stacking blocks a breeze.

Nev Grady 07-30-2009 12:41 PM

Re: Fibre cement base for hearth ?
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Why oh why do you guys in the US build your oven bases as though you were expecting it to hold up a structure as big as the Empire State Building? Why all the rebar in your slabs, and massive concrete blocks also filled with rebar and concrete? An insulating hearth of a 5:1 vermiculite/cement mix ain't all that heavy. Neither is a dome structure and cooking floor built out of a couple of hundred firebricks. Over-engineering? You bet!

The photo shows the start of my oven. The stand is out of 4" concrete blocks and hearth support out of 2" thick paving slabs. The insulating base is 6" of vermiculite/perlite cement. Not a hint of rebar, angle iron or Hardibacker or anything else you guys use. It's withstood a good shaking from severe earth tremors on a couple of occasions so there's not much wrong with it!

splatgirl 07-30-2009 06:55 PM

Re: Fibre cement base for hearth ?
I'm not going to argue that the by the book block and hearth stand isn't ridiculously overbuilt, but I think it's a fairly cheap and easy means to making sure everything stays where it's supposed to over the long term. Certainly a structural engineer could provide better commentary on this than I, but...

I'm sure your stand is adequate, and I might even be fine with a 3 or 4" (poured) slab supporting my oven if it had rebar and mesh in it with that center support you've got, but that's as far as I'd go.

The functional strength of concrete is almost exclusively in compression. IT HAS VIRTUALLY NO TENSILE STRENGTH or ability to flex, hence the need for rebar. Reinforced concrete is an amazing thing. Unreinforced concrete is brittle and surprisingly weak under anything other than evenly applied compressive force.
In a rebar reinforced slab, a crack is nothing more than a cosmetic issue. Without rebar, a cracked slab is structural problem that will eventually get worse and/or cause the slab to fail.
Eventually, your slab and mine will crack. All concrete does to one degree or another. I'm happy with knowing the rebar I've got in mine will keep everything structurally sound and all the hard work that sits above it will stay in one piece over the long term.

Lets compare notes in 10 or 20 years, shall we?

RTflorida 07-30-2009 08:50 PM

Re: Fibre cement base for hearth ?
Well said splatgirl.
Not wanting to start a major argument, I would just like to add that there are several variables here. Nev, in your case (and maybe many others), what you have done may be more than is necessary, BUT, many builders have built pretty massive structures of stone and brick around their ovens, to say an oven is relatively light above the hearth slab is a generalization that could not be further from the truth in many cases. In my case, I am certain my stand is overkill.....I have a 36" igloo finished with mosaic tile. Not heavy by brick oven standards, maybe 1500 lbs. sitting on the hearth slab.
Build a house type enclosure and chimney out of stone/brick and you could easily top 5000 lbs. I think most of us have built our ovens to stand the test of time, and stay structually sound for decades (or centuries). I think the Pompeii plans are MEANT to be overkill, taking into account the different materials and weights involved.

Lets not forget we are trying to recommend a building method that will withstand - weather (frost heave), soil movement (a lot of that in FL, you can't pour a concrete slab here, even rebar/fiber reinforced, that won't get multiple cracks), and seismic activity (as you have experienced).
I too, would like to have this dialog again in 10 or 20 years and compare the structure and stability of our ovens......WE ALL may be way off base in our thinking.


Lardylad 07-30-2009 08:54 PM

Re: Fibre cement base for hearth ?

Coming from Queensland Australia, lots of houses, sheds, walls etc are built from 8" blocks, so I chose the material easiest to get hold of. I notice that you have used what looks like breeze blocks (I'm an ex pom). I don't think they would stack like the besser blocks I have used.
The main appeal to me was that I can't lay blocks, so avoiding mortar was good, and the large size means I can get the required height easily and they are very stable. It's common practice to core fill in Aus, even when using mortar, and you may as well slip some rebar in for a little extra cost.
I thought of using precast concrete pavers/ slabs (easiest option), but given the size of my structure I would have needed extra central block supports and would have had to cut pavers. Casting a suspended slab is the easiest, and most cost effective way for me - concrete is cheap.
As pointed out, my structure will be over engineered, but the real cost is in the price of my oven (pre cast refractory kit), so the last thing I wanted was to damage that through lack of a strong enough base.
In the end I guess it comes down to the cheapest/ easiest solution for me and big blocks plus concrete slab seemed the way to go.


RTflorida 07-30-2009 08:59 PM

Re: Fibre cement base for hearth ?
One last thought

Nev, really my only concern for you is the use of the 2" non-reinforced paver slabs. Just my opinion, but I think any more tremors could spell trouble (I hope I'm wrong).
I think the 4" blocks are more than enough support and really helps to open things up below. I wish I had the access and space.


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