Securing Hearth Pad Layers

[Tfin70]

Hi all,

My new stove calls for a hearth pad with an R value of 2. I'm planning on using two layers of 1/2" Micore 300 (R value of 2 right there) with a layer of cement board on top, which will ultimately be covered by tile.

My question is, how do I secure the the two layers of Micore together, and also how do I secure the layer of cement board to the Micore underneath it?

Thanks in advance, this is an amazing site!

 

[Corie]

Very carefully?


LOL - you're going to have an all but impossible time trying to get the Micore to hold screws. You could put a piece of plywood on the bottom and screw into it to make a sandwich.

 

[Tfin70]

Yeah I thought of that too, but I didn't know if maybe there was some sort of adhesive I could use?

I just bought one of your Englander 13-NC's and can't wait to fire it up. Seems like a nice little unit. Already have the chimney installed, just need to get the pad built.

 

[backpack09]

I have no idea what micore is.... But I used heavy duty liquid nails between every level of my hearth.

 

[Michael6268]

I would be careful of using adhesive between the layers or cement board/micore. Im not sure liquid nails or other adhesives are fire retardent or can stand up to the heat the stove may generate. I know they do make a "fire rated" adhesive for use with fireproof wall board/paneling. Im all for the "sandwich" with plywood on the bottom.

 

[Gooserider]

I would reccomend using the same THINSET tile mortar between each layer as you would use to put the tiles down with on top of the Durock. I also like the idea of putting a layer of sheet metal in the middle of the sandwich - I used aluminum weatherstripping, or you could also use something like 26g steel. The metal acts as a heat spreader to dissipate any hot spots.

The Durock submittal sheets on how to make a hearth protector call for a Thinset that meets ANSI 118.4 or 118.1, which I was told amounts to a "Latex Modified" thinset, which deals better with the thermal cycling of a hearth. I used Flexbond, which meets both standards, and was suggested by the Durock tech support folks.

I would probably go something like thinset, Micore, thinset, metal, thinset, Micore, thinset, Durock. Use a 1/4" floor notch trowel on most of the thinset, but a wall notch on the two layers with the metal. I would put down just a few screws in each Micore layer to hold it in place while getting the next layer down, then use plenty of cement board screws in the Durock to hold everything together (go all the way into the subfloor)

Hope this helps

Gooserider

 

[Tfin70]

Great, thanks for the responses everyone. One last question......so there's no concern of the Micore (being a soft fiber board) absorbing to much of the moisture from the thinset then?

 

[JohnnyBravo]

you could use quick/speed set, it only takes 15 to dry. less time for the micor to soak up the water. altenativly you could do a quick scratch coat on all the faces that you are going to bond. morter and thinset stick to themselves real good. i have also sprayed down realy pouris cemement with a bit of water befor i layed the thinset just so it would not rob all the moisture.

 

[mayhem]

I used a sheet of 1/2" plywood as my base and then ran 3" sheetrock screws down through all layers around the perimeter. No need to do any anchoring under the stove IMHO since the mass of the stove will take care of that...you just need to make sure your materials are flat and smooth so you cna ly your tiels down nicely.

Are you constructing a raised platofrm or is this going on top of an existing solid surface? The reason I ask is becuase the micore is not a strong material (or so I've been told...I've never seen the stuff and nobody carries it wihtin a 60 mile radius so I used 6 layers of 1/2" durarock) and if you lay it down over an open frame and then top dress it with cement board, you won't have any strength to it.

 

[JohnnyBravo]

6 layers of durarock, i hope it was built in place. if not that sounds like a 24 beer move.

 

[Tfin70]

I used a sheet of 1/2" plywood as my base and then ran 3" sheetrock screws down through all layers around the perimeter. No need to do any anchoring under the stove IMHO since the mass of the stove will take care of that...you just need to make sure your materials are flat and smooth so you cna ly your tiels down nicely.

Are you constructing a raised platofrm or is this going on top of an existing solid surface? The reason I ask is becuase the micore is not a strong material (or so I've been told...I've never seen the stuff and nobody carries it wihtin a 60 mile radius so I used 6 layers of 1/2" durarock) and if you lay it down over an open frame and then top dress it with cement board, you won't have any strength to it.

The hearth pad (platform) will be laying flat on my lamanent floor, so yes, the weight of the stove will be supported safely. Yes, I had to have the Micore specially ordered through my local lumbar yard (Hammond Lumber). It came out of southern Maine from a distributor down there.

So as it stands now I think I'm going to go with half inch plywood on the bottom, followed by two half inch pieces of Micore with cement board on top of that. I'll sink appropriate sized sheet rock screws down through it all into the plywood along the outside edge (maybe a couple closer to, but not in the center as well). Should I pre-drill the cement board to avoid cracking, or is that not necessary? Also is adding a layer of sheet metal (when using Micore) really needed?

And I said I only had one more question. ;-)

 

[mayhem]

You're not kidding. I figure with the stove and all its a solid half ton. Took me awhile to get it all done, but I'm happy with the results...still need to paint the outside edges to match the room's trim though.

 

[JimWalshin845]

We have a work in progress similar to this.

We used Durock which is rated for hearth floor protection embeded in thinset mortar.

BEFORE PHOTO:

BEFORE you start anything, check the floor for bellies, if a stove or heavy furniture has been there before it probably has one. Use a self leveling mortar mix to remedy it.

Dry fit the whole project before cutting the Durock, measure 4 times and cut once :coolgrin:

Durock cuts the same way as sheetrock, score it several times with a utility knife, snap it and cut the back fiberglass mesh. Corner cuts can be easily made with a circular saw and a masonary blade.
Spec Sheet:
http://www.usg.com/USG_Marketing_Co...d_Submittal_Sheets/CB198_Durock_Cement_Bd.pdf


We used the Durock Screws , they are 1 1/4" and I was a bit disappointed with the gripping power so I would suggest going to 1 1/2 Deck screws. DO NOT USE DRYWELL SCREWS. Screw the perimeter 8" OC and nail the field 8" OC with 1 1/2" roofing nails. Make sure all screws and nails are set below the Durock surface.

Tape the seams and patch the nail/screw holes with thinset.

Over that we used a PEI-5(extra heavy duty) rated porcelein tile also set in thinset.

Use a very straight board to make sure tiles are level and tile spacers for the non-pro are an essential.

We laid about 36sqft and the whole project was a little under $200. All the materials were purchased at Lowes. We got a great deal on the tile that we both loved, $1.97/tile and the boarder tiles were about $5/2 feet.

Be sure to have plenty of this on hand!

Grouting will commence right after I finish this post.

Good luck,
Jim & Kathy

 

[mayhem]

So as it stands now I think I'm going to go with half inch plywood on the bottom, followed by two half inch pieces of Micore with cement board on top of that. I'll sink appropriate sized sheet rock screws down through it all into the plywood along the outside edge (maybe a couple closer to, but not in the center as well). Should I pre-drill the cement board to avoid cracking, or is that not necessary? Also is adding a layer of sheet metal (when using Micore) really needed?

And I said I only had one more question. ;-)

I found my life was made easier by predrilling a pilot hole in the cement board, but you're not going through 3 inches of the stuff. My problem was it was killing my screw shooter, not with cracking.



Jim, why shuold we not use sheet rock screws? Obviouslt too late for me, but I'm curious as the the reasoning.

 

[Gooserider]

Moisture absorbtion from the thinset between the layers is not a problem, or so I've been told. The function of the thinset is to ensure that all the layers are bonded together to increase the overall stiffness of the structure, and to ensure that there are no gaps.

You should use screws intended for use with cement board, they have heads that are designed to properly self countersink into the surface of the board, without damaging it. Sheet rock screws are NOT LISTED for the application. No need to pre-drill, but choose a screw length that will give at least 3/4" penetration into the subfloor. Also acceptable as fasteners, 11g, hot dip galvanized roofing screws, 1.5" minimum length, 7/16" nominal head diameter, must penetrate at least 3/4" into substrate. The Durock instruction sheets specify fasteners on 8" centers, and make no mention of not putting them under the stove. Again, it helps to ensure that all layers are bonded tightly together. If you use the sheet metal, it will dissipate any heat conducted into the pad by the screws.

The sheet metal is not required by code, however it is IMHO a "good practice" - does no harm, and cost is pretty nominal so why not?

Gooserider

 

[mayhem]

Thats interesting on the screw head design. The only difference I've seen is that sheet rock screws ae philips and cement board screws use a square peg driver and are waterproofed. Maybe I've been using the wrong cement board screws all these years.

Uh-oh.

 

[JimWalshin845]

So as it stands now I think I'm going to go with half inch plywood on the bottom, followed by two half inch pieces of Micore with cement board on top of that. I'll sink appropriate sized sheet rock screws down through it all into the plywood along the outside edge (maybe a couple closer to, but not in the center as well). Should I pre-drill the cement board to avoid cracking, or is that not necessary? Also is adding a layer of sheet metal (when using Micore) really needed?

And I said I only had one more question. ;-)

I found my life was made easier by predrilling a pilot hole in the cement board, but you're not going through 3 inches of the stuff. My problem was it was killing my screw shooter, not with cracking.



Jim, why shuold we not use sheet rock screws? Obviouslt too late for me, but I'm curious as the the reasoning.

Predrilling the Durock is definitely NOT needed. But you do want to stay back atleast 3/4"-1" from the edges and 1 1/2" - 2" from the corners. Sheet rock screw heads are smaller than deck screws and a deck screw has a broader shank for anchoring. Durock screws are phillips head type screws but most other deck screws are diamond head so you have a better grip on the screw head. When screwing into floor joists the 1 1/4" screws are NOT long enough to get into the floor joists with any beef to it so we would have gone to 1 1/2" long screws but we went with the specs as per Durock.

As per adding a layer of metal over the underlayment, the guy who built the home (we are second owners) was a commercial building contractor and built some of the casinos in Atlantic City. The underlayment is a fireproof board and NOT plywood. I beleive it was used as a underlayment for poured concrete floors. I wonder how much material in the house came off AC jobs because much of it is commercial grade. If we had plywood underlayment I would have defintely gone for the steel over the underlayment.

The old hearth board he used (which was covered in tile) was 2 pieces of 1/4" asbestos sheeting. When we pulled the boards up (just nailed thank God) you could see scorch burns on the bottom. I took a photo of it and will post it later on.

We have an old VC Defiant that does not have a firebrick base nor a heatsheild. I built a heatshield that we will suspend from the leg bolts of the stove to add safety.

We are back to cleaning up the grout,
Jim & Kathy

 

[Gooserider]

Thats interesting on the screw head design. The only difference I've seen is that sheet rock screws ae philips and cement board screws use a square peg driver and are waterproofed. Maybe I've been using the wrong cement board screws all these years.

Uh-oh.

The Cement Board screws that I used (RockOn brand IIRC) were definitely Phillips head (#2 size) as are the sheet rock screws I've used, and some of the deck screws. I also have some "DeckMate" brand deck screws that use a special "DeckMate" bit which is sort of a wierd hybrid between a square and a Phillips head, but only that brand uses that style bit. There are other specialty screws that use odd bits, but I would say most of the stuff I run into in non-automotive / motorcycle uses is a Philips head. I certainly would not consider it useful to ID a screw type based on the bit needed to drive it.

Gooserider

 

[JimWalshin845]

Thats interesting on the screw head design. The only difference I've seen is that sheet rock screws ae philips and cement board screws use a square peg driver and are waterproofed. Maybe I've been using the wrong cement board screws all these years.

Uh-oh.

We also have some "DeckMate" brand deck screws that use a special "DeckMate" bit which is sort of a wierd hybrid between a square and a Phillips head, but only that brand uses that style bit.
Gooserider

We used DeckMates (2 inchers) this past winter doing some repair work down in Florida. The 'diamond heads' are the same as other diamond head deck screws, but it is nice that they supply one in the box of screws. In past decking projects, one 'diamond head' nub equals one box of screws.

J&K;

 

[rdrcr56]

I used 3/4 in ply with alternating layers of micor and durock with everything screwed down every 8 in. Did not use thinset except for the tile and have not had any problems. Oh yeah, two layers of micor and two of durock, the pad is not screwed to the sub-floor.

 

[JimWalshin845]

I used 3/4 in ply with alternating layers of micor and durock with everything screwed down every 8 in. Did not use thinset except for the tile and have not had any problems. Oh yeah, two layers of micor and two of durock, the pad is not screwed to the sub-floor.

I looked for a Micor dealer and the closest was over an hours drive. The set up you have should be able to support a battle ship's boiler. Just wondering though, why didn't you spend the 10 bucks and put down thinset?

The set up we had was not in thinset and the tiles were starting to pop and the grout was almost non existent. When we pried up the 2 layers of 1/4 asbestos board it was WAY too easy to pry up and with heat expansion/contaction tile will not hold up. Now our home was built 30 years ago and the tile job failed... but I try to build things so they will last atleast one lifetime!

-Jim-

 

[Tfin70]

Well, my hearth pad is well underway now. I have 3/4" plywood for the base (this sits on top of my lamanent flooring) then two layers of 1/2" Micore, followed by 1/4" Wonderboard. I secured the "sandwich" with 1 5/8" Durock type screws. My tile will be secured to the Wonderboard with thinset. I only have to cut three tiles (I went with 12x12 tiles) then grout. I will be trimming the edges with some sort of hardwood like oak.

The reason I didn't secure the pad to the floor is because I want it portable. Our living room is small, and I'm planning on disconnecting the wood stove and storing it in the basement (along with the hearth pad) during the summer months. I didn't go with thinset between all layers for fear of the increased weight it would produce (the pad already is quite heavy and will require three grown men and a small child to maneuver it. Thanks again for all the helpful information. I definately would of gotten this done without the knowledge from the contributors of this site. Heck, I'd never even heard of Micore until I came here.

 

[JimWalshin845]

Before you move it with the 3 wise men and child, buy them all trusses and make sure one of the wise men is an orthopedic surgeon

 

[Tfin70]

Done, and done! ;-)

 

[fjordrr]

Well I am struggling with this question too, this morning. How do I put my "sandwich" together?

Raised level, 3 layers: Durock - Micore - Durock (this sits on top of metal studs)

Flat level, prob 3 layers: Micore, Micore, Durock (this sits on plywood with 2x6 car decking below).


Has anyone actually used thinset inbetween layers of Micore & Durock? Or should the layers all just be screwed together, & the only thinset used would be on top of the last layer (Durock) for the tile?