Okay to use granite or bluestone for the slab?

[jonk4423]

Excited to be building my first WFO! My plan with the base pictured is to lay 2" granite or bluestone slabs (stair treads, capstones etc) across the concrete pillars to be used as the weight-bearing load and overall base for the oven floor. I have insulating CalSil board to place on top of the slabs, and was planning on putting 2" firebrick on top of the CalSil board. Does anyone see any structural problems with using granite or bluestone (i.e. they might not be able to support the weight of the oven or may crack under the heat)?? Also, do I need more thermal mass between the firebrick and CalSil board? Thanks for any input!!

PS: the length between each of the 4 stands is approx 18" each.

[Toomulla]

Sounds like a good plan given the close centres of the supports. I would add a couple of inches of vermiculite concrete under the fire brick

[david s]

Sounds like a good plan given the close centres of the supports. I would add a couple of inches of vermiculite concrete under the fire brick

Sorry, can’t agree. The consensus for adequate underfloor insulation is 4” of 5:1 vermicrete or, as it’s a superior insulator, 2” cal sil board. More is marginally better (subject to the law of diminishing returns), less you can get away with but will reduce performance. Vermiculite concrete has poor strength (see attachment) and should not be relied on as a structural layer. (There is an error in the table it should read compressive strength for 5:1 vermicrete 175-225)

You don’’t say how big your oven is and therefore the weight the stand has to support. Looking at your pic it appears that the piers at spaced at 18” centres which means the actual span between them is about 14”. If it’s a small oven you could probably get away with using a thick cement sheet over those closely spaced piers, on top of which you have your adequate underfloor insulation.
A 2” thick firebrick floor is the usual thickness. If it’s thicker it will provide more thermal mass but take longer to heat. Thinner does not provide enough thermal mass and recharging the floor has to be more frequent.

Vermicrete PHYSICAL PROPERTIES 2 copy.docx.zip

[Toomulla]

That appears about the same 2 inches of ceramic fibre and 2 or more of vermiculite all or both as per IP same with the Granit

[david s]

That appears about the same 2 inches of ceramic fibre and 2 or more of vermiculite all or both as per IP same with the Granit

Toomula, I think I misunderstood what you meant, apologies. So you meant 2” of denser vermicrete over 2” thick calcium silicate board. That would work to add a little more thermal mass, to the floor as the vermicrete is a poorer insulator than the calcium silicate. The usual is to use the better quality insulator closest to the hot face.

[Toomulla]

Not a problem we all miss read poorly worded responses regardless We should have now provided plenty of advice