FB Board under floor AND dome?

[dottavio]

I just received my FB board and I am ready to build my Pompeii oven, finally.

However, the FB board seems softer than I expected. I am not sure that this material will support the weight of the dome and the vent. Maybe it is only supposed to be under the floor and the dome rests on the concrete foundation.

Any insight would help, I am ready to go if I can figure this out...

[Les]

Re: FB Board under floor AND dome?

The board can support the weight. You need the dome insulated from the hearth. When you say softer, what exactly do you mean? In no way is the stuff like concrete but it should be fairly firm.

[dottavio]

Re: FB Board under floor AND dome?

By soft I meant that my finger can leave an imprint. I did not expect it to be like concrete. Not exactly sure what I expected, but the dome will probably weight a half ton or so and all that weight will be on the 4 1/8" ring of my fire brick. Just didn't seem logical to me.

Did you use FB board without any issues?

[GianniFocaccia]

Re: FB Board under floor AND dome?

The board you got will be fine. I'm sure the FB folks would not send out an unsuitable product. Yes, the board will seem too soft to support the entire weight of your oven, but this is a common perception. Just make sure you don't let any portion of your dome (and entry) come into direct contact with anything other than insulation. Any contact will cause heavy losses of heat through conduction which translates into excessive wood requirements during heat-up and rapid loss of residual heat.

[dottavio]

Re: FB Board under floor AND dome?

I thought that contact with the concrete might act as a "thermal short", but I was more worried about the weight.

Since it looks like its OK I will proceed. Thanks

When you lay down the FB board should you level with sand or fire clay?
or
Maybe the bricks should be leveled on top of the FB board?

I noticed that the fire brick thickness seems to vary enough to show lips...

[GianniFocaccia]

Re: FB Board under floor AND dome?

The recommended solution is to level the bricks on the board with a 50/50 blend of fine sand and fireclay, although there's nothing that says you couldn't fill voids between the board and your concrete suppoort slab with the mix as well.

You can mix water with the blend and (notched) trowel it on wet or do what I did: just trowel on dry and tap the bricks with a hammer until you're satisfied. If you wet the bricks from the top it will set your 'mortar' underneath.

You may want to level your floor bricks with a slight slope towards the entryway so if any water gets in it will run out.

[dottavio]

Re: FB Board under floor AND dome?

Thanks for the pointer Gianni.

I'll see how much flex the FB board has when I lay it down and then I can determine if I need to level underneath it. Since I will be using the FB board I'll probably mix in very little water as to not saturate the board. Just enough to get the mix to flow.

I've spent years thinking about this build and just as many getting my site ready. Now that I am finally at the point where I can build my dome I don't want to leave anything to chance.

Wish me luck