Should I pull up the hearth bricks (not flat, level or smooth and with gaps)?

[scottc]

Hi Everyone,

I'm building a barrel vault oven that has a firebrick hearth layer that is 1.38m (55") front to back and about 1.05m (41") across.

I poured a concrete slab under my firebricks, above my insulation, for additional thermal mass. The plan was simple: pour it level and smooth, trowel on a thin layer of soft fireclay+sand+water mix and then tap the firebricks into it to level them... perfectly square, perfectly flat and no gaps between. Done!

... however, that didn't happen, and now I have the following problems:

1. The concrete slab wasn't completely flat, and it was slightly lower in the centre than the edges by a bit more than a centimetre (~1/2"). I figured I could fix this with a bit extra clay/sand mix between the concrete and bricks. That didn't work out quite perfectly so there's still a slightly concave cooking surface... not much, but not perfectly flat either.

2. I didn't realise that I hadn't properly leveled the first brick I layed at the front of the hearth, and now the bricks towards the back have progressively more clay+sand mix under them (like a wedge instead of a flat even layer), because I took the level of each subsequent brick from the first. There's a difference of about 1.5cm (1/2") from front to back, which is a distance of about 1.4m (~55"). The front is sitting on about 0.5cm (1/5") of sand+clay and the back is sitting on a bit more than 1.5cm (~1/2").

3. My bricks aren't exactly the same size. This has caused some bricks to have gaps between them, and some to be slightly off-level so that they are touching/flush at the top, but have a 1-3mm (1/10") gap at the bottom (or vice versa)

So I guess my questions are these:

1. Will the slightly concave cooking surface cause any issues?

2. Will the different thickness of fireclay+sand mix under the bricks cause problems, say, with differential expansion or shrinkage as it heats or dries, or movement over time?

3. Should I just pull the whole thing up and start again, or should I just grind off all the lips and wait for all the gaps to fill with ash? I won't bother pulling them up if the 'problems' I listed aren't going to cause structural or functional issues.

Cheers,

Scott.

[cobblerdave]

Re: Should I pull up the hearth bricks (not flat, level or smooth and with gaps)?

G'day
If you haven't finished your oven arch vault yet it might be best to attend to it now as it still easy to do.
If you have finished and still able to pull the hearth up, that will not change so why not use it and see if it is a problem. Attend to it then.
Regards dave

[guiriguiri]

Hey Scott, I'm in a similar situation. What did you do and what were the results?

[UtahBeehiver]

Post is quite old and he may not be following.

[scottc]

Hi,

I am still following, although I didn't know it :-)

Anyhow, in the end, I left the bricks where they were. I used an angle grinder to take of any lips that would catch and stop a peel from sliding along the floor, and I left the gaps. I figured the gaps would fill with ash eventually because, in the long run, they weren't that big. I also decided that pulling up the bricks wouldn't help because they are all very slightly different sizes, so some small gaps are unavoidable with the bricks I was using.