Concrete board vs. Plywood form

[james]

I've been thinking about the concrete board idea and wanted to ask everyone if they think we should consider making that the standard installation method?

Can you post your thoughts on the trade-offs between using concrete board (and leaving it) and plywood (and dropping it out) -- in terms of time, energy and cost. I don't think there is a "quality" issue, as both methods are equally stable.

As DMUN noted, it's too bad you can't buy hollow terracotta beams in the states.

Let me know what you think.
James

[CAM]

1 vote for plywood

O.K., I haven't built my oven yet. I have, however, used concrete board in a lot of bathrooms. Perhaps it's paranoia, but I don't think the 1/2" board will stand up to time and exposure like real poured concrete 4" thick. It will probably last a long time and I'm just goofy, though I have seen crumbling and deterioration. It will essentially be a shim, so I would worry about it. It will bond with the slab and this may be its saving feature. I'll build mine with removable wood. Being a newie, I look forward to other opinions and you guys changing my mind.

[james]

I'm not being clear

My fault. I can see that I wasn't clear on this.

You still pour the 3 1/2" concrete with a 1/2" rebar grid set on 12" centers, and set a full course of concrete blocks across angle iron for the stand. The only question is whether it is faster, easier and cheaper to use the concrete board or plywood as the form to hold in the concrete while it sets and cures.

Sorry about that.

Yep, no way concrete board could hold up an oven.

James

[maver]

In favor of concrete board, it is cheaper than heavy plywood and if you place the board just inside the block walls (rather than on them) it is not performing any structural function (it is just hanging on by itself, not as a shim), other than perhaps being more waterproof than poured concrete (which is not really a function in this location). If I had an unneeded piece of plywood that was substantial enough I'd use it to reduce the bracing needed, but after the form function is done I'd have to dispose of it. I really think both are reasonable options depending on what is readily available for the builder.

[CAM]

I gotcha. It's not going on the block, so it's not between block and hearth and thus does nothing but save you the time of removing it. I'm starting to like it. My bad in thinking you also used it to stop the block openings.
James: I understood one would still pour, but I was suggesting the pour would be best if directly in contact with the block and not "shimmed" by
concrete board. Apparently this is just what was done. That I did not understand.

Thanks to both of you, Chris.

[wlively]

I agree cement board

I used Durock cement board on my pour and I am very happy I did.

I placed the board directly on the blocks, rough side up, overlapping the opening 3 inches or so. You could very well use it to cover the holes in the blocks, but I did not want to go thru that much trouble to exactly cut the board, after all it was a time saver for me. The sheets of Durock was $10 less than equivalent ply-wood, so $$ wise it is not much difference. But time savings, I think would be a great deal. All I had to do was cut 3 2x4's the length of the opening, close enough so that they were held in by friction when placed in the opeing, flush with the top of the block. I then measured and cut each of the 9 legs, and taped them to the braces to temp hold them. Lay the durock on top, tape the seems and you are done. I had no problems with sag, for my 4" (22 bags) of concrete + 2.5" of vermiculite concrete.

This was my experience anywway.

[mrpbjnance]

also like concrete board

I also used the Durolock concrete board layed right on top of my blocks. Since I did not do the plywood I can not say how har it is but it was very easy to lay the concrete board on top and build a few 2x4 supports underneath. I watched the board and checked underneath during the pour and there was no sag whatso ever.