Hearth CONSTRUCTION

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Old 03-01-10, 06:08 PM
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Hearth CONSTRUCTION

Help: We are installing fake stone around our gas fireplace. It will go from the top of the new hearth to the ceiling. We want to add a hearth, as previously we didn't have one. I am making the box myself and still have a some questions. This will go on a concrete slab floor.

1 - I'm using 2x4s to build the frame, should I put a vapor barrier down? ( the whole thing will only be about 82" x 25")

2 - Should I use treated 2x4's for the part that touches the ground?

3 - I bought cement board, but should I put a layer of plywood underneath that? I am currently planning on stringers on 18" centers.

4- How do I (should I) attach my box to the house?

Thanks!
 
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Old 03-02-10, 04:22 AM
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Welcome to the forums!
1) Since the hearth will be hollow, basically, forgo the vapor barrier. It will just trap moisture.
2) Absolutely use pt lumber for the bottom of the frame.
3) Apply 7/16 or thicker OSB or plywood around your framing prior to the cbu, since cbu offers no structural support on its own. Use 16" centers, since it is industry standards.
4) You can attach the bottom plate to the concrete via PL400 adhesive, powder actuated pins, tapcons or concrete nails. Then build your frame on top of the bottom plate just like you would a regular wall.
Information you didn't ask, nor pay for: Calculate the height you want pretty carefully. You will want to build the hearth to reach back into the chase area and set your firebox on the OSB. I usually use 1/4" cbu on such a small job. Cover the remainder of the OSB with the cbu, then tile it. The tile will sit slightly above the bottom of the firebox to trim itself out. What size tile do you plan on using for the wall field? Will the chase be proud of the wall, or is it flat set into the wall with an external chase? If an internal chase, you will need to put in place a firebreak of framing and fire resistant 5/8" sheetrock at about the 8' mark.
I know, more questions than answers, but we want you to do it right.
 
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Old 03-02-10, 10:29 AM
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More questions

Chandler, thanks for response.

First off, is cbu the same as the concrete board I bought?

What do you mean by "The tile will sit slightly above the bottom of the firebox to trim itself out" and "Will the chase be proud of the wall, or is it flat set into the wall with an external chase? "

As you might have guessed, this is a modification to an exisiting house. The current gas fireplace stays, so we ripped off all the old trim, tile etc.

So on the wall side of things, currently there is nothing sticking out, everything is flush with the wall. The end product will be fake stones (about 2" deep" ) going from the top of the hearth to the top of wall.

On the floor side, I tore up the tiles and am looking at bare concrete.

For the hearth, we are using hearth stones, that are about 2" thick, so I've calculated the height of my box to take that into consideration. On the hearth, the top should be just about where the top of the first row of the old wall tiles were.

A contracter is doing all the actual stone and masonry work, I'm just building the box for the hearth.

Thanks again!
 
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Old 03-02-10, 10:48 AM
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Yes, cbu is concrete backer underlayment, sorry. Do you have a chimney? Are the gas logs vented or unvented? That is what I meant by having an internal or external chimney chase (box for the flue pipe to go up and be hidden by).
If the current fireplace stays, how will you build a hearth, or is there a hearth there already? Are you building up a hearth or just covering the space where an original one existed? How much space do you have between the finished floor and the bottom of the firebox?
 
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Old 03-02-10, 11:00 AM
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Fireplace info

Chandler,

There is an existing chimney.

This is a new hearth, there was not one before. There is about a foot to from the floor to the bottom of the firebox, but there are vents below that for the insert. My plan was to have the top of the hearth stones about a 1/2" below the lowest vent. From top to bottom, this hearth will only be about 8" tall.
 
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Old 03-02-10, 01:00 PM
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Gotcha! Then forget what I said about the tile going up to the front of the firebox, since you will have space there. Sounds as if you have it under control. I was asking all the questions to make sure there weren't gonna be any other unexpected things for you.
 
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Old 04-03-10, 06:26 PM
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Hearth and Stone wall are finished

Chandler, thanks for your help. Wish I could post pictures of the finished product. It looks great!
 
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