Old 01-22-2009, 12:30 AM
Join Date: Jan 2009
Location: Dubbo, NSW, Australia
Posts: 14
Default opinions on strength requirement of base

I am about to construct my base this weekend and would like some opinions to save me about AU$120.

The base of my oven is 1400 x 1300mm. The inside measurements (inside the brickwork) is 1200x1100mm

Spanning across the void (1100mm) are four 30x30x5mm lengths of angle iron and in the middle is a brick pier. There are another 2 courses of bricks around the base giving a depth for insulation of 160mm.

I had originally thought I would need a piece of compressed fibre cement sheeting (1200x1100x15mm) to use as the base for pouring the insulation mix on. But now with 160mm of insulation mix over the top I'm wondering if I should just reinforce the insulation mix and put a piece of ply formwork underneath.

I'm unsure of the strength properties of the insulation mix and would like some informed opinions of this.

The insulation mix will be vermiculite and portland cement.

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Old 01-22-2009, 10:44 AM
Jed Jed is offline
Join Date: Apr 2008
Location: Bend, Oregon; West Coast USA
Posts: 428
Default Re: opinions on strength requirement of base

Ahoy Capt'n,

I can't comment on your planned structure, but do have an opinion on the vermiculite - Portland cement combination.. = vermicrete.

Although the vermicrete will tollerate a compression load, ie. the oven built on top of a layer of vermicrete, I think you can only plan on this if the vermicrete is appropriately supported from below. The vermicrete itself does not have any 'structural' strength. Don't plan on any structural support from this insulating layer.

Hope this helps with the plan..

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Old 01-22-2009, 12:10 PM
Master Builder
Join Date: Mar 2008
Location: Washington State USA
Posts: 803
Default Re: opinions on strength requirement of base

I agree with Jed the vermicrete or perlcrete has some compressive strength but as to shear strength or tensional strength I think one would be well advised to consider it nil/non existant/too small to be worth considering. IMHO the time spent messing about with a sheet of plywood as a form as opposed to a sheet concrte product (Hardibacker or wonderboard) which is cut oversize of the hole it is to bridge and incorporated into the structure is time and material better spent elsewhere.

Regarding whether or not the steel and brick pier structure is sufficient to support the weight of the oven and insulation above it I would have to see a detailed drawing and some further data before I would have any sort of reasonable opinion. What is the spacing of the angle iron? How are the ends of the angle iron supported and the location of these angle iron supports would be needed to be known. I am of the opinion that many think these WFOs are heavy and it depends upon the point of reference...on one's foot indeed yes; relative to other structures built indeed no. It is all in the strength of materials and the design. These are domes and they carry their weight on the circle created by the base of the dome, if the support structure is under that circle then the center under the hearth is carrying just the weight of itself which one could figure and guess-timate at only a couple hundred pounds at max. Not much in the way of support needed to accommodate that sort of load.
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Old 01-22-2009, 01:30 PM
Join Date: Jan 2009
Location: Dubbo, NSW, Australia
Posts: 14
Default Re: opinions on strength requirement of base

Cheers for the opinions. I had a long conversation with a buiilder last night and I have determined from this that the cheapest (and one of the best options) is to use a few sheets of roofing iron. It has the right characteristics for what I'm doing, and it's alot cheaper that FC sheeting
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