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Unread 03-13-2003, 06:30 AM   #1
stevep
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Hearth Extension

I am doing an addition to my home and it includes 2 zero clearance woodburning fireplaces. I am at the stage of needing to put in hearth extensions with a plan of finishing them with ceramic tile.
My subfloor is 3/4" ext plywood over 2 x 10 joists 12" o.c. with a span of 12 ft. (although the 2 fireplaces are both over a steel beam) I am putting a finished floor in of 3/4" cherry t&g.

The fireplace manual requires a certain K rating for the hearth extension and recommends a product called Micore from USG. I can get the required K rating by using a 1/2" thick Micore.
Question: can I simply put down the Micore and then thinset the tile over that. The local supplier here of the Micore says yes - they do this type of installation all the time. I am trying to get the hearth height as close as possible to the finish wood floor.

The Micore does seem fairly dense and seems to have bits of ceramic in it. USG could not give me an answer when I called them.
Any suggestions?
Steve
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Unread 03-13-2003, 07:13 AM   #2
tileguytodd
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OK now that I know what Micore is (its a mineral enhanced wallboard)
No dont use it... Use USG's Durock. Its a cement based product rather than gypsum based.Its firproof rating is similar,(not that it would matter because the ceramic you use has a higher fire rating than any of it).
We use Durock for hearths up here all the time.
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Unread 03-13-2003, 07:18 PM   #3
John Bridge
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Hi Steve, Welcome.

I agree with Todd. Use Durock, but perhaps you could use the 1/4 inch. I'm not sure if that would blow your fire rating, but I don't think it would. The zero clearance f.p. is up against all sorts of things that are combustible, including the wood framing.
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Unread 03-14-2003, 06:29 AM   #4
stevep
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Todd and John

thanks for your replies. the fireplace manual I mentioned in my first post has a chart showing layout of a hearth extension and possible materials for it. and it shows that the K value of cbu is very high (bad) and would require 3 layers of 1/2" to meet the requirement. that is why I was trying to find a material which would work that was not as thick and that I could apply thinset to.

the manual also states that while being non-combustive (like the cbu's)- ceramic tile and many stones conduct heat pretty good and must have something underneath (between them and the plywood subfloor) which doesn't.

I'm thinking maybe I'll stay away from thinsetting directly onto the Micore as Todd suggests and use 1/4" Durock on top of it. I was hoping for all the layers to add up to no more than 1/2" but that might not be possible.

thanks, Steve
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Unread 03-14-2003, 07:20 PM   #5
John Bridge
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Steve,

Woe be to anyone who would argue with the "book," but a flush hearth receives just about zero heat from the firebox. I suppose if you lit a fire on it, it might heat up.

The idea, as least around these parts, is that if an ember should jump out, it'll have something non-combustible to die on.

How will you attach the CBU?
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Unread 03-14-2003, 08:28 PM   #6
stevep
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John
it's not "the book" - it's "da man" who I can't argue with. the bldg inspector has already indicated that he will make sure that the installation is per the fireplace manual relative to clearances, etc. I agree with you about the hearth never seeing much heat.

I've noticed on some foyers around here that the subfloor was "dropped down" to be flush with the top of the joists (the plywood was cut up and nailed to 2x cleats on the side of the joists) and then a mud bed is floated over that. would this have been done to keep the finished height as low as possible? and would this be a solution for me??

I plan on screwing the durock down thru whatever I use and into the plywood with thinset under the durock.

thanks again for the response. Steve
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Unread 03-14-2003, 08:42 PM   #7
John Bridge
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Yes, I think the mud bed would work, but I ain't da man.

That would certainly be a way of getting the thing lower.
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Unread 03-15-2003, 08:29 AM   #8
tileguytodd
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Well when i did my fireplace,Da Man was my insurance company and durock with tile was good enough for them.I built it according to the book.Thier Book!! By doing so my insurance didnt go up.
Good enough for them, good enough for me

Are you sure you have to have this product on the wall when you are using a tile face.Or do you need to use this product if it is going to be your finished wall?
Check your book again, i think you will find that with tile as a finished face,the mineral board is mute.
Ive never seen a fireplace where durock couldnt be used with a tile finish.
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Last edited by tileguytodd; 03-15-2003 at 08:34 AM.
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