Questions about plywood and concrete

[sarah h]

Can anyone tell me whether it's necessary to remove the plywood supports after the hearth has cured or can I just size the plywood to overlap the concrete blocks by a few inches, position some middle supports and then frame only around the outside of the blocks? Is there any specific reason not to do so?

As well, does anyone know if fibre-reinforced concrete is a reasonable option for the bottom layer of the hearth? Someone suggested it would be better than regular concrete due to our harsh winter climate but I don’t have a clue if that’s correct.

Thanks,
Sarah

[Ken524]

Sarah,

I'm a little concerned about leaving the plywood in there. It will likely start to deteriorate over the years (humidity, temp, mold, etc). Instead of plywood, you can use cement board (Durock is one brand that comes to mind) and leave it in place.

[asudavew]

Originally Posted by sarah h

As well, does anyone know if fibre-reinforced concrete is a reasonable option for the bottom layer of the hearth? Someone suggested it would be better than regular concrete due to our harsh winter climate but I don’t have a clue if that’s correct.

Thanks,
Sarah

You can use it, but I would still use rebar as well.

I agree with Ken, just go with Durock and leave it in place.

[wlively]

I too did not want to go to the trouble of custom cutting the plywood and then fishing it out. I used Durock that overlapped the edges and was supported by 3 flat 2x4 (each supported by 3 2x4 legs). Worked great. I would think that you could use treated plywood that was painted or primed on the concrete side. It probably will deteriorate anyway gradually over the years (5,10, 20, ??), but will not cause any structural issues, just visual.

The FB plans, at least original, had you use the fiber reinforced concrete. So, that is what I used. It's not really that much more $$ so why not use it.

[asudavew]

Originally Posted by wlively

The FB plans, at least original, had you use the fiber reinforced concrete. So, that is what I used. It's not really that much more $$ so why not use it.

Did you also use rebar?

Just curious.

[wlively]

Originally Posted by asudavew

Did you also use rebar?

Just curious.

Yes. The first post didn't mention rebar/steel so I assumed the question was only in regards to concrete type. The fiber is just there to help the concrete be more resistant to cracking, it does not provide any significant structural support.

[El Puaco]

I can only speak to the question of the plywood. The plywood bottom that frames the hearth was held in place nicely by the 2x4's and when the supports came out the plywood simply came right down. Not much of an event and not much to plan around. You could leave it in place if you wanted I suppose but I found another project for the pieces anyway.

[sarah h]

Thanks everyone for your helpful suggestions. I think I know our direction now - Durock and fibre-reinforced concrete. I'm not sure if the rebar query was directed at me or 'wlively' but I've always planned to use it no matter what other modifications I make.
I'm off to Home Depot ...