Help, hearth extension being built now....

[nlittle]

HI-
After much research I spoke with my tile guy and determined my hearth extension for my hampton i300 (insert) would be made from Micore 300, durock and slate tile. This gives me my Rvalue etc. We expected our depth to get level with the brick to be 1".
We made an error and only have 3/4" of an inch to work with to get flush to the existing brick.
Micore is in place and durock doesn't fit because it is 1/2 and we only have 1/4 left to become level.
Only option seems to be 1/4" hardibacker to stabilize the top of the micore and bring me level for my tile.
I know this is not ideal but do you think it will be ok since this will be the very edge of the extension?

The current brick goes out 19" so there will be 10" of existing hearth beyond the insert window/ door and then 11" of new extension...
I don't think have any other choice but to go with hardibacker here.

Please tell me this is OK because I do not see any other 1/4" product to use here...

 

[nlittle]

Well, the tile guy is out to trade the 1/2" durock for the 1/4" hardibacker. I have done a bit more reading and since my manual says "non-combustible" and I'm getting my R value via micore, I think I am ok.

Not really any choice I guess!

 

[Wet1]

Be careful, not all 1/4" hardbacker is noncombustable! This is one reason why people use the 1/2" Durock. The other is support. Micore is far from being a good subfloor, you need something more than a 1/4 hardbacker for a subfloor. I can almost guarantee you will have cracking slate/tile/grout using only 1/2" Micore and 1/4" hardbacker.

Unless I'm misunderstanding you, I'd rethink this.

 

[nlittle]

As Charlie Brown would say... AAAAAARRRRRGGGGHHHHHH
It's already down on the OSB sub floor and they are starting to tile. I guess I will have to hope this works out.

It turned out to be from the subfloor-
morter, 1/2" micore 300, screws, morter, hardi 1/4", screws, morter and 1/4" slate tile.

The Hardibacker site for 1/4 inch says it is non-combustible: http://www.jameshardie.com/homeowner/products_backerboard_quarterInch.shtml So I'm not sure why you say it is not all is non-combustible.
So, I guess I will have to live with this and see how it goes. I know I will have a sub-par hearth in the eyes of the forum but I only had 3/4" to get to an Rvalue of .6, not many options for me.

My piece of mind is there because our local inspector said I just needed spark protection (even just a hearth rug!) which I did not feel comfortable with...so I did better that.
Plus I know that 10" out from the stove door will be actual masonry hearth.

Thanks for all the help from the forum though! Wish me luck in the long term for no cracks...

 

[Wet1]

Some brands of the 1/4" board are said to be combustible. It sounds like the stuff you used is not.

Since you have a subfloor under the Micore (correct?), you MIGHT have enough structure there to keep from getting cracks, but I'll bet the grout at a minimum will crack in time. I would watch what gets placed on that pad in the future...

 

[nlittle]

Thanks Wet1-
Luckily it is an insert so the weight will be in the fireplace not on the pad. The extension will only have some light foot traffic and me kneeling to load the stove...
I will however make sure that when they deliver the insert next week (the day before the install) they do NOT leave it on the extension part.
Great tip!

 

[meathead]

Well...you won't have to worry about anything catching on fire and that's the important thing. I bet you'll be fine. Hope it comes out well

 

[latitude45]

Your project sounds similar to mine.

From bottom up..
1/2" micore 300
1/4" Thinset
1/4" hardibacker
1/4" Thinset
1/4" Slate

I hope everything works out!! Thing that eases my mind is that stove will sit back into the fireplace opening on "solid ground"..

 

[greythorn3]

what kind of mortor you used is it fireproof? im building a hearth and need to know what type thiset or what ever its called to use to glue the wonderboard to the plywood..


Ray

 

[Wet1]

Standard thinset is fine.