Hearthpad Construction

After more homework and thanks to some proper guidance I chose to go with the heritage. . Now I need to make my hearthpad. I have my half inch micore 300 to meet the required r-value and I am starting on the 3/4 inch plywood subfloor. I have read here the proper sequence is : 1. micore 2. cement board 3. thinset and 4. tile.
My question is all the cement board manufactures suggest to use thinset both above and below the cement board. Now, my better judgement tells me that thinset and micore don't mix. Does anyone have experience with this scenario? What did you use Rhonemas? Should I place a layer of sheet metal and metal lathe between the micore 300 and the cement board? Should I just set the cement board down on the micore then thinset then tile and hope when I put my 500 lbs stove on top, I don't get cracks at the seams of the cement board
By the way the size of the pad is 60 wide by 42 deep. cement board comes in 3' x 5' sheets therefore I need to use two pieces and make a seam. I considered laying "1/4" cement board after the micore then thinset, then stager another 1/2" cement board, more thinset then tile.
Does anyone know how they construct the UL listed hearthpads??
Thanks!

Thanks Coasters, that sounds like it might work. So, the screws are meant to take the place of the bottom layer of thinset, is that correct? I suppose, the objective of each is the same. (to make sure the board dosen't move after tiles are applied) Do you know if I should use the same thinset with tape to bond the seams for the two pieces of cement board? Also a tile guy recommended versabond for this application. Is that the same as fortified acrylic ?

Hi Guys,

I have a more interesting setup.

I was planning to make a hearthpad but the Quadrafire Isle Royale I have requires such a large pad, that it stick into the path of walking in the room. So my nephew in law (a tile installer) suggested to take everythnig out of the room and install micore near the stove and plywood in the rest on the room. Then cover the entire floor with durock and then tile the intire floor. That way I will not have the 2 inch height difference the pad would create. Flush floor all the way.

Sounds good, or not???

Carpniels

I cut into my floor to make my hearth more flush because I have a small room. In the end my hearth only comes up about 1/2", not enough to trip over or block the room off. Also, I used micore and (I know someone will not approve of this) I put mortar onto the micore and laid one single piece of slate on it. It has not been a problem so far.

Thanks for the replies. Carpniels, that sounds like a good idea. Eventually, I want to work in that direction, if only I can get my wife to go for that. Either raise the rest of the floor and place tile or hardwood over the whole thing. For now, i think I'm just going to live with the extra 1/2 or 3/4 inch above the carpet where the new hearth will go. I'm just not ready to do that much work yet. Nokoni, I like your idea of doing what it takes to keep the floor flush with the rest especially if the stove portrudes into a walk space(which mine does)but, I'm just not ready to start choping into my subfloor. I don't like the idea of messing with the load and weight distribution members of the house. The fact that the carpet and pad stick up one inch really helps.
Thanks for all the great links Coaster! I'm certain they'll come very handy for future refrence. I did change the model of stove I was considering from the homestead to the heritage. The homestead reqires soo much insulation, that by the time I build the pad, I'm already out of clearnce for the rear smoke vent chimney. At least the heritage only requires 1.10 r value which is significantly lower than the homestead.

Hi, Not much differnece but I think the Heritage requires a pad with an R of 1.2 if over a combustable area, not R of 1.1....might want to check this

http://hearthstonestoves.com/documents/Heritage8021Manual.pdf

What??? No one wants to build a pad like mine?!?!?!?!

I like your pad, Don. I think the steel studs are a really nice feature. A friend of mine is thinking about putting in a stove, and I'm going to put him onto your design. Thanks for providing the link so I don't have to dig it up.

Need some paint on the back walls but the pad looks great...I was just saying they require R of 1.2. Nice stove!

Don,

You're on the "borderline" as far as using the 1/2" Micore 300...if that's what you used.... ....1-in Micore 300 has an R of 2.33 so 1/2" is about 1.165, just shy of the 1.2 required for that stove.... don't know if the R-value of the tiles can add to it but tile is very very low......R value of about 0.02 for a 1/4 inch thick tile.......so, your hearth is probably about R = 1.185......close enough to 1.2 I guess........

Coaster,

I agree, in general, that this is good advice in these articles however the big "Gotch Ya" is assuming even 1 inch (let alone the 1/2 inch they generally talk about) of Durock or Micore 300 is good enough....depends on the stove. A 1/2 inch thick layer of Micore 300 gives an R-value of about 1.165 but some stoves require hearth pad R-values of much higher. So, my advice would be this: read the stove manufacturers minimum R-value requirement and then plan accordingly.

Example: I think the Hearthstone Homestead model requires a hearth pad have an R-value of 6.6 if no heat shield is used and an R-value of 2.5 with a heat shield. This would require about 2.9 inches and 1.1 inches, respectively, of Micore 300 to achieve these.

We have a raised hearth, and I mean raised way up. We keep wood underneath it and can use it like a table during the summer. There is a frame made mostly from 2 x 4s, then a base of plywood, a sheet of Micore 300, then cement board on top of that and the ceramic tiling on top of that. We have construction adhesive between the Micore and the cement board, also many screws. Our hearth is larger than it needed to be for the official clearances, so we just have the screws put in outside the area where the R value hearth is required to be, so mainly near the edges. (We thought the screws through the Micore would undermine the R-value by putting long metal screws through it, so we only have them outside the required clearance area.) We only needed an R value of 1, so the one sheet of Micore 300 did that part.

My VC Dutchwest also has bottom heat shield, when I put my hand on the hearth pad directly under the stove while its burning, the tiles feel cold. But they STILL say in the owners manual I needed a 1.2 R value hearth pad. Seems like total overkill.

Anyway - thinset (with NO additives) works fine on micore 300 and cementboard. I put some cement board screws though it too. I think the plywood bottom layer is unnecesary. Are you really going to move it? Even if you are its probably going to be more portable without the plywood bottom. I was able to slide my finished pad around on the carpet a little for final placement.

Personally, I think most of this hearth construction regulation is sponsored by the "Hearth Builders Association" (I made that up, but you know what I mean). That, and the stoves of yesteryear have created the requirements most new generation stoves simply don't need. Burning my stove really hot still doesn't get the hearth tile over 94 degrees. I am all for spark protection and non combustibles, but having to pay through the nose for micore is a bit overboard. But, like everything else, we have to live within the confines of the worst case scenario.

With that being said, I went above and beyond with my hearth construction. Since I had no idea how hot it would get, I played it safe to insure that State Farm wouldn't have any issues with my installation. For the record, here it is:

Starting from the floor up.

1. 1/2" particleboard, sealed
1.5 - Tile mortar
2. 1/2" Micore
2.5 - Tile mortar
3. 1/4" Hardibacker
3.5 - Tile mortar
4. Ceramic tile

Edgeband - Surplus natural cherry floorinng from the floor instal, ripped and planed to my specifications.

Tile mortar worked fine for laminating layers together. If you use mortar, you have to put a lot of weight on layers 1-3 to insure the hearth doesn't bow while the mortar dries. Once complete, the entire hearth structure was solid as a friggen rock.

All that for sub 100 degree surface temps...

Holysmokers I have a hearthstone homestead that requires an r value of 2.5. I made a portable pad with 3 ply then micor, durock,micor,durock scewed it all down to the plywood every 8 in. than thinset and tile. I kinda worried about the grout cracking so I butted all the tiles together and used the colored caulk with sand in it. so far so good with no problems.

Forgot one thing, I bought 2 4x8 sheets of durock and cut them with a diamond blade on my skill saw.

Actually, my hearth is closer to r 10

It's -

Steel 2x4 framed

Sheathed w/ 2 layers of 5/8" Gypsom & 2 layers of 1/2" Durock

Topped with 1/4" ceramic tile

Add the air space and I think I met the minimum protection :coolsmile:

And I updated my signature with the "Almost" finished product. As for the standard pad design, I don't think there's any absolute right way of making it. You build it to meet the protection requirements using the materials available to you. If you don't have Micore, use Durock.

Of course you will need to post pictures of any final pad design