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How do I pour a hearth and extension?

Pertz | Posted in Construction Techniques on November 18, 2003 02:06am

Hearth and Hearth Extension: I need advise on constructing this. I’ll be building the fireplace, and I’ve built several. But they’ve otherwise been on solid concrete foundations that also constituted the hearth and extension. This time, the hearth and extension need to be poured on a ten inch poured foundation wall that is a rectangle to support the fireplace; the hearth extension will cantilever over the foundation wall about two feet to the joist system. There will be an ash dump/combustion air system coming in through the hearth

The hearth and extension are specified to be four inches thick. The size of the foundation is 6′ x 2’6, and the extension is 2′ further out. How do I support the pour for the extension? I visualize screwing angle iron to the joists and foundation and laying corrugated sheathing on it to support the extension. I intend to place re-bar and/or mesh in the pour, but need to know at what spacings. What kind of concrete mix should I use (since this measures up to about a third of a yard of ‘crete, I’ll mix it myself)?

Any pitfalls to look out for?
I’ll appreciate any experienced advise.

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  1. User avater
    Mongo | Nov 18, 2003 04:03am | #1

    First, I'm no mason. I really don't know what it's formally called...but we call it a "hearth pan." It's essentially corrugated sheet steel, shaped something like this:

    __/__/__/__/

    When the masons build the chimney/fireplaces, they leave a horizontal notch/recess in the bottom back of the firebox.

    Lay the hearth pan it out over the chimney/foundation base, extending backwards to the notch/recess, and extending/cantilevering forwards into the living space of the room where the proposed hearth will be.  Where the front of the pan meets the floor framing, a 2-by is nailed off on the framing under the pan to support the pan during the pour. Add additional 2-bys, as needed, to ensure you end up with proper spacing between the concrete and the framing.

    Set a wood form, or something similar, where you want the ash cleanout to be.

    Rebar is tied off in a grid, and set back into the notch/recess. I've always seen 12" oc, which is likely excessive, but it's the way my mason works.

    I can't promise you anything regarding the mix, I believe though that it's a standard mix, which is normally something like a 5 or 6-sack.

    1. Pertz | Nov 18, 2003 06:20am | #2

      I had in mind to pour a 4" slab over the foundation and onto the extension and then build the fireplace up from that. Do I understand correctly that what you're describing is the fireplace built up directly on the foundation, then the hearth and extension poured after the firebox is constructed?  does the pan and the 2x stay in place permanently, or do they get removed after the pour?

      1. User avater
        Mongo | Nov 18, 2003 04:51pm | #4

        Lemme try to give more detail:

        The chimney is brought up to 5 1/2" below the level of the first floor finished height. At that height, the notch, or recess, is built into the rear of the chimney brick or blockwork. The recess is about 1" deep and 3-4" high (one brick heighth?), and runs across the width of the rear of the chimney.

        From this point you can do one of two things. Most common is to continue the rough brickwork of the chimney and rough brickwork of the fireplace, all the way to the cap, to expedite completion of the weathertight shell of the house. Then you come back and do the hearth and the finish brickwork in the fireplace. Or, you can stop, do the hearth, and simultaneously build the rough and finish fireboxes, then continue upwards.

        Assume you did the rough work only and finished the chimney to make the house weathertight...once doen, come back to the hearth:

        The hearth pan is cut to width and depth, and is laid in place. The rear of the pan slides into the recess, the front of the pan cantilevers over the front of the brick/blockwork and extends out to the framing of the house's floor platform. A 2-by is nailed off under (from the basement) the front lip of the pan, it's nailed to the floor's framing, to support it during the concrete pour. Add additional woodwork...scraps of 2-by...so the pour stays 2" away from the framing and honors the 2" gap required between the chimney/firebox and wood framing.

        Make a wood block sized to the ash pit assembly. Place that on the pan.

        Add rebar, tied off in a grid. Size it to the pan as well, and insert the ends of the bar into the notched recess in the back of the opening.

        Make the pour. Again, it's a standard mix. 3:1 will work. The pour should fill the notched recess in the back of the box, locking the hearth in place.

        Once the hearth has cured, build out the finished fireplace wth firebrick, etc.

        When all is done, go back in the basement and remove the 2-by that supported the lip of the hearth pan during the pour. That will re-establish the 2" clearance between the fireplace and the framing.

        the hearth pan stays in place forever.

        In a nutshell...the chimney rest directly, and bears directly, on the foundation/footings. The hearth is nested in the notch/recess, and bears on, the rear of the chimney (in the notch), it butts against the inside sidewalls of the chimney, and it rests on and cantilevers over the front of the chimney.

        Hope this helps!

      2. User avater
        Mongo | Nov 18, 2003 05:39pm | #5

        Despite all I've written, please do try to find another source for info regarding this subject. Again, though I've watched a few be built, I'm certainly no mason. And, a fireplace/chimney combo is tough to redo once done. There is a big potential downside for errors made in the process.

  2. doctord | Nov 18, 2003 03:18pm | #3

    I made a new hearth for my home after discovering the hearth was only about 1/2" thick. The house was moved from it's original location and the cement slab sat on stone which was since lost in the move.

    Since the firebox was supported, all I had to do was provide a form for the hearth. I chose wood to do this, since it would be entirely visible from the basement, and out of fire's reach. I drilled holes in the brick firebox below the slab finished height, and inserted rebar every 8 inches and tied them off. They were also inserted in the form wall.

    The pour was 2/3 mason's sand and 1/3 portland, which was what a lot of the old books called for, apparently.

    There have been a few really hot fires in there so far, and one microscopic crack has surfaced, but it's really very stable. 

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