Backer Board and Mortar for Stove base

[camhead]

Hey y'all, first post here.

I'm building a tile hearth/base for my free standing wood stove (a US Stove 1100): two layers of 1/2" concrete backer board for a heat shield on top of a hardwood floor in the corner of the living room, then topped with some tile.

All the online guides I've found for installing backer board over wood floors mention laying down thinset mortar and then screwing the board in place. But here's my question: do I need to use thinset underneath the SECOND layer of backer board as well? All the info I've found online is more addressed to laying tile for bathrooms or kitchens– not stoves– so it doesn't address using multiple layers of backer board.

I hope the question makes sense, and thanks in advance for the advice.

-PN

 

[jetsam]

A little thinset between boards might make it less likely that your screws squeak as the two pieces try to slide against each other. It will also make it more likely that the top piece isn't as flat as it could be, and it adds weight.

I don't think I'd put any between boards, but I don't think it would be bad if you did.

You may want to think about whether you want double cement board. Unless it's required by some regulation, you are adding a bunch of weight and not gaining much strength. If there's no wooden frame underneath, you also face the problem of how to screw the two together (grind down cement board screws to 3/4"?).

As far as ember protection goes, I do not think you are gaining anything from the second board, especially if the top one is tiled.

 

[edyit]

if all you need is ember protection then 1 sheet will be enough, if you need an R value for the hearth then look at this link to help figure what you need

https://chimneysweeponline.com/horvalue.htm

 

[camhead]

if all you need is ember protection then 1 sheet will be enough, if you need an R value for the hearth then look at this link to help figure what you need

https://chimneysweeponline.com/horvalue.htm

Yes, I do need an R value of 1.03. It looks like the backer board that I'm using (Finpan protec 1/2" concrete backer board) is not listed, and I can't find any info about its R-value online. Even Hardibacker would require me to stack FIVE on top of each other!

 

[jetsam]

Yes, I do need an R value of 1.03. It looks like the backer board that I'm using (Finpan protec 1/2" concrete backer board) is not listed, and I can't find any info about its R-value online. Even Hardibacker would require me to stack FIVE on top of each other!

1/2" Micore has you covered in 1 board, if you can find it. Home Depot can special order it, really not sure who carries it in stores. Honestly, plywood would work the same, but Micore is rated for fire spread and has a published R value.

 

[Doc C]

I glued and screwed my layers of durock. Used fuze it by liquid nail which is my go to for almost everything needed to be glued.

Durock is .39 r value and readily available

 

[Nateums]

Using mortar under and in between sheets of cement board is to create a solid substrate for the tile. Not mortaring the boards into place could cause tile to pop or grout to crack.

I don't think you should apply the mortar directly to the hardwood floor. It would be better to screw down exterior glue plywood and then mortar down the cement boards on top of that.

I'm not a professional tiler but I have set a lot of tile and done a lot of building. I also have the Tile Council of NA Handbook which I always try follow their methods.

 

[Doc C]

Using mortar under and in between sheets of cement board is to create a solid substrate for the tile. Not mortaring the boards into place could cause tile to pop or grout to crack.

I don't think you should apply the mortar directly to the hardwood floor. It would be better to screw down exterior glue plywood and then mortar down the cement boards on top of that.

I'm not a professional tiler but I have set a lot of tile and done a lot of building. I also have the Tile Council of NA Handbook which I always try follow their methods.

I thought about that as well. My reasoning for doing it was twofold. One I had to stack 5 layers to get the R value I needed and it would of added a couple inches of height to the hearth if I used thin set between. The other reason was the hearth doesn not get walked on to cause the flex that normally occurs with tile floors. I framed my hearth and set it in place. It is not attached to the floor so it’s not subject to the movement of the house during the winter and summer since the hearth is always in a temperature controlled environment. Also the humidity is always in a 20% range so there is not a whole lot of movement from moisture.

I might regret it down the road but it think it will be just fine.

 

[Nateums]

I thought about that as well. My reasoning for doing it was twofold. One I had to stack 5 layers to get the R value I needed and it would of added a couple inches of height to the hearth if I used thin set between. The other reason was the hearth doesn not get walked on to cause the flex that normally occurs with tile floors. I framed my hearth and set it in place. It is not attached to the floor so it’s not subject to the movement of the house during the winter and summer since the hearth is always in a temperature controlled environment. Also the humidity is always in a 20% range so there is not a whole lot of movement from moisture.

I might regret it down the road but it think it will be just fine.

Yeah I wouldn't lose any sleep over it. Just trying to give 'by the book' advice. I have had tile pop before, it is not a fun experience.

 

[jetsam]

Yeah I wouldn't lose any sleep over it. Just trying to give 'by the book' advice. I have had tile pop before, it is not a fun experience.

I had all the tiles pop the first time I made one of those. I had a half bag of type S mortar left over from some brickwork and thought, "How different can thinset possibly be? They're both mortar." I used the type S for thinset and grout, which went okay.

The next day I went to check it, and found a loose tile.... and another loose tile... and another... they all lifted easily off the cement board, leaving a tidy perfect grid of raised "grout" lines.

So I got to scrape all the mortar off, buy a bag of thinset, and do it again.

I would say that this is how we learn, except that I COULD have just googled it in the first place. :/

 

[begreen]

Yes, I do need an R value of 1.03. It looks like the backer board that I'm using (Finpan protec 1/2" concrete backer board) is not listed, and I can't find any info about its R-value online. Even Hardibacker would require me to stack FIVE on top of each other!

You'll need 3 layers of Durock NexGen to get to R=1.03. Two layers is R=.78. Regular concrete backerboard is about R=.26. Hardibacker 500 is R=.20. I wouldn't put any thinset between layers. With a screw every 8" the sheets are locked in place and are not going to move.

 

[camhead]

Ok, so now I'm debating whether to do a layer of Micore 300 under the tile (which would just BARELY make the R-value), or just to give up and get one of those pre-made stove pads (which would be cheaper) and calling it good.

 

[edyit]

the pemade pad would be quicker and they come in an assortment of styles, just make sure it meets your R value requirement since they can vary as well

 

[begreen]

Ok, so now I'm debating whether to do a layer of Micore 300 under the tile (which would just BARELY make the R-value), or just to give up and get one of those pre-made stove pads (which would be cheaper) and calling it good.

Micore will still need a layer of 1/2" cement board on top. Thinset will not bond well to the fibrous micore.

 

[jetsam]

You could do 1/2" Micore plus 1/4" Hardibacker and be comfortably above the R-requirement, and only at 3/4" before the tile. That should run roughly $30 in micore, $12 in Hardibacker, $15 in thinset, plus whatever tile you pick. You will also want tile spacers, hardibacker screws, and a notched trowel if you don't already have- plus either a wet saw or bullnose if you plan to trim out the perimeter.

I put mine on wooden framing to raise it up an extra 1.5", but that's not needed if you don't want it.

 

[begreen]

I'd go 1/2" for the backerboard. It will add rigidity and help prevent grout cracks. The cost difference is trivial.

 

[jetsam]

Eh, when you do a tile floor over joists and don't want to make it too tall, it's usually 3/4" osb/plywood with 1/4" cement board on top. I don't think 1/4" cement board will be much less rigid than 1/2 when screwed to plywood and laid on top of an existing hardwood floor.

On the other hand, there's also no downside to using 1/2" unless you are just trying to go as thin as possible. It's even the same price.

 

[begreen]

Rereading the OP sounds like he is doing this on a hardwood floor, not flush. If it were me I'd put down a layer of 3/4" plywood first, then the micore, backer board then tile. That way you are not messing up the hardwood floor and have an independent hearth pad. It can be edge-trimmed nicely for a good look.