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Durock platform for hearth pour

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  • 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!

  • #2
    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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    • #3
      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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      • #4
        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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        • #5
          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 .
          Last edited by Gulf; 06-09-2012, 07:45 PM. Reason: Grammar
          I don't care what folks say behind my back........They are either braggin' or.......lyin'


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          • #6
            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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            • #7
              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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              • #8
                Re: Durock platform for hearth pour

                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 .
                I don't care what folks say behind my back........They are either braggin' or.......lyin'


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                • #9
                  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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                  • #10
                    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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                    • #11
                      Re: Durock platform for hearth pour

                      I used unsupported Hardibacker over an ~18" x 44" span on my hearth pour with no problems. Overlapped the edges about 3" on each edge

                      Front part of the form in the picture is where I used the Hardibacker
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                      • #12
                        Re: Durock platform for hearth pour

                        Gee... feeling a bit like I need to defend my statements here. You talk about over kill and psi loads is all well and good. I don't believe that in the original questions made any mention of the actual framing. Therefore if you stretch out the spans under your durock (that is being used in a way that was not intended) it will fail. If yours did not fail then consider yourself lucky. Durock when used on horizontal surfaces such as floors and counter tops requires a solid structure beneath the durock.

                        I for one without having the actual spans of framing would hate to make recommendations and tell the guy "go ahead it's all good" then have the thing fail as he fills his form. An ounce of prevention is worth a pound of cure.

                        I will tell you though that I get a lot of work fixing mistakes that home contractors and home do-it-your-selfers that don't understand how to do it properly. So my customers appreciate my so called overkill practices. I would rather have it over kill then to go back and explain later why the thing failed.

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                        • #13
                          Re: Durock platform for hearth pour

                          Nothing wrong with overkill, especially when placing concrete. Massive overkill on the other hand is a waste of time and money. It is all calculable even if only roughly.

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                          • #14
                            Re: Durock platform for hearth pour

                            In all my time in construction and many concrete pours I have never, ever heard anyone say:

                            "Damn, I guess I overbuilt my form work"

                            However, failure of your form work during a pour is a major pain in the ass. Not only have you wasted time and money, you also have to clean up the mess while working against the clock.
                            Last edited by Neil2; 06-11-2012, 10:28 AM.

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                            • #15
                              Re: Durock platform for hearth pour

                              True that. See that huge monolithically poured concrete structure in the middle? We did 500 yard pours every third day for better than 18 months, not including the spiral ramps or freestanding monolithic staircases. We had one blow out on an edge beam that set us back a week.

                              https://maps.google.com/maps?q=Hobby...uston,+TX&z=18

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