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Old 06-09-2012, 04:39 PM
Peasant
 
Join Date: Apr 2012
Location: Phoenix, AZ
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Default Durock platform for hearth pour

I have purchased two 3x5 sheets of Durock to be used as my base for the hearth pour. Two questions:

1. It looks like I can avoid cutting the two sheets if I simply build my wooden support stand flush to the top of my concrete blocks, and then lay the sheets on top - then fasten them to the wood. I am using 1/2 inch size. I will have overlap of a few inches over the concrete block. I assume this is ok?

2. IF I do (1) above and let it overlap. Can the Durock handle the concrete pour without buckling, or do I need a plywood sheet for support for the pour (instead of tying directly into the wood)?

Thanks!
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Old 06-09-2012, 05:02 PM
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Default Re: Durock platform for hearth pour

No you need to support the Durock with plywood and support the plywood from underneath. A cubic foot of concrete weighs about 150 lbs. So if you just put 6 inches of concrete on your two sheets of durock 15 cubic feet weighs 2,250 lbs.

Being that you can snap Durock with your hands is does not have any supporting strength at all.

Hope that helps.
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Old 06-09-2012, 05:11 PM
Peasant
 
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Default Re: Durock platform for hearth pour

thanks. to be clear: I am already building a full temporary wooden 2x4 stand on the interior of my design. So, even though the Durock will be supported by the edge of the concrete blocks, the 2x4 stand (edges and interior supports) - it won't work? I must have a plywood support?

Sorry, just want to be 100% sure.
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Old 06-09-2012, 05:47 PM
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Default Re: Durock platform for hearth pour

Correct. I don't know what your spans are between your 2x4 stand but I would tent to use a 3/4 inch plywood or OSB on top of your temporary stand. Nothing worse than having a form fail and all your concrete falls in your oven base. Never underestimate the power ( and weight )of wet concrete.
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Old 06-09-2012, 07:30 PM
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Default Re: Durock platform for hearth pour

I got by with 1/2" 4ply, but I did have a lot of support underneath.

I have seen threads where cement board was used and left in place as you are planning, but this in my opinion. If you have to support it with plywood, then just use plywood. You can recover it for other uses .
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Last edited by Gulf; 06-09-2012 at 07:45 PM. Reason: Grammar
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Old 06-09-2012, 07:45 PM
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Default Re: Durock platform for hearth pour

Nice picture Gulf, that's a great example of what I was talking about. If he has less framing he would need the 3/4 inch plywood.
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Old 06-09-2012, 07:53 PM
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Default Re: Durock platform for hearth pour

You guys over build this stuff to a crazy level. 1/2" concrete board with under support 24" OC is overkill. Call it 5'x5'x4" thick=1035# of concrete or 3.5# per square inch, an order of magnitude or 3 below the sheer strength of concrete board. Bending and breaking it in your hands is irrelevant.
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Old 06-09-2012, 08:05 PM
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Default Re: Durock platform for hearth pour

Quote:
Originally Posted by Tscarborough View Post
You guys over build this stuff to a crazy level. 1/2" concrete board with under support 24" OC is overkill. Call it 5'x5'x4" thick=1035# of concrete or 3.5# per square inch, an order of magnitude or 3 below the sheer strength of concrete board. Bending and breaking it in your hands is irrelevant.
24" OC may be overkill, but I do love it when you talk Pie-R Square .
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Old 06-09-2012, 08:11 PM
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Default Re: Durock platform for hearth pour

Given that different areas have different requirements for footings, seismic, etc, 90% of the bases on this site are overbuilt by 80%. I grouted my cells with beer cans on top of the 3" slab I poured with leftover masonry cement and a couple 3/8 rebars I had used for stakes. My hearth is composed of 30x30x2" pads that I scavenged and cut to fit, they had been roof ballast for 20 years. There is no tie from the slab to the base or from the base to the hearth. It ain't going anywhere.
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Old 06-09-2012, 08:26 PM
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Default Re: Durock platform for hearth pour

And just to be clear, I did calculate the loads. I don't remember exactly what they were, but the slab dead load was less than 300 psi (equivalent to packed dirt), the hearth dead load less than 200psi.
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