How to bond Micor to Hearth?

[parksplace5454]

Hi,
I am a newbe to this forum. I am recently retired and building my retirement home in the White Mountains of Arizona. I have an Osburn air-tight and have have constructed a hearth structure with a plywood platform. I plan to put 1/2" Micor over the top of the plywood platform and then 1/2" Durock over the top of the Micor. The final hearth surface is porcelain tile over the Durock. My question is this:
How should I bond the Micor to the plywood; with mortar or construction adhesive? Secondly, how should I bond the Durock to the Micor; with mortar or construction adhesive?
Any advice or input would be greatly appreciated. Thanks and Merry Christmas!

 

[Hogwildz]

You can screw it down, there was a screw made for it, but forget what it was.
The screws you use to secure the Durock will also secure the micore at the same time.

 

[begreen]

Yes, just get longer 1 3/4" or 1 5/8" cement board screws and screw right through the micore/durock sandwich. Screw it every 8" and don't worry, the micore is going nowhere.

 

[EatenByLimestone]

I used drywall screws with a fender washer to keep it from pulling through. Then I used a piece of hardibacker screwed down with a washer.

 

[CenterTree]

You can screw it down, there was a screw made for it, but forget what it was.
The screws you use to secure the Durock will also secure the micore at the same time.

I am wondering though if you use screws to go THROUGH the micore and into the plywood,,,,then is that still up to "code"?? Wouldn't the metal screws be an avenue for heat transfer into the wood?

If seeking a particular "R" factor, the screws may throw a monkey wrench in the mix.

Would it be better to use adhesive to attach the micore and then use the screws to bond the Durock to the micore (but NOT through to the wood)??

 

[rwhite]

I guess I must be overkill...I used both. And I'm jealous that you get to retire in the White Mountains. I grew up in Happy Jack and have made my way across the world and the US. Would love to be in that area again.

 

[begreen]

I am wondering though if you use screws to go THROUGH the micore and into the plywood,,,,then is that still up to "code"?? Wouldn't the metal screws be an avenue for heat transfer into the wood?

If seeking a particular "R" factor, the screws may throw a monkey wrench in the mix.

Would it be better to use adhesive to attach the micore and then use the screws to bond the Durock to the micore (but NOT through to the wood)??

Adhesive is combustible. Stop obsessing over this stuff. To my knowledge there has never been a hearth fire due to heat transference through screws.

 

[Hogwildz]

If those screws transfer enough heat to light off the plywood, I am thinking you have other major issues.
If they are that hot, there is more other things that are prolly burning to worry about.

Screws ain't going to bite into the Micore alone.
I agree with BG, overkill is great, I am the same way, but there, your going off the deep end.

 

[Brick (ware)House]

I am wondering though if you use screws to go THROUGH the micore and into the plywood,,,,then is that still up to "code"?? Wouldn't the metal screws be an avenue for heat transfer into the wood?

If seeking a particular "R" factor, the screws may throw a monkey wrench in the mix.

Would it be better to use adhesive to attach the micore and then use the screws to bond the Durock to the micore (but NOT through to the wood)??

Screws through multiple layers are thermal conduits to the combustibles below. I'm not 100% sure but I don't think that type of assembly would meet code in a strict interpretation. I used construction adhesive between the plywood and micore and between the micore and durock. My larger concern was that in the unfortunate event of a fire the insurance company would use the heat transfer through the screws theory to attempt to deny coverage. Considering the durock will be covered by thinset and brick and then have an 800 lb. stove on it, I'm not worried about anything moving. Used screws to secure the durock facing the raised hearth in the vertical plane.

 

[Hogwildz]

Screws through multiple layers are thermal conduits to the combustibles below. I'm not 100% sure but I don't think that type of assembly would meet code in a strict interpretation. I used construction adhesive between the plywood and micore and between the micore and durock. My larger concern was that in the unfortunate event of a fire the insurance company would use the heat transfer through the screws theory to attempt to deny coverage. Considering the durock will be covered by thinset and brick and then have an 800 lb. stove on it, I'm not worried about anything moving. Used screws to secure the durock facing the raised hearth in the vertical plane.

Construction adhesive isn't flammable or combustible?
What is the flashpoint of the adhesive?
I'd go with screws over adhesive any day.
To each, their own.

 

[Brick (ware)House]

Construction adhesive isn't flammable or combustible?

Material Safety Data Sheet indicates that it is nonflammable when dry.

 

[Hogwildz]

I ain't buying that.
Anyway, as I said, to each, his own.
Enjoy.

 

[CenterTree]

Adhesive is combustible. Stop obsessing over this stuff. To my knowledge there has never been a hearth fire due to heat transference through screws.

Well, FIRST, I am NOT obsessing. I am just trying to add some caution to the OP's setup. He can decide.

Notice in my original post I ended each of my statements with a ? mark. I thought this was a place where we all join in and bounce thoughts, ideas, and QUESTIONS off each other so that we would all walk away more informed. Is it?? Or is it NOT??

If those screws transfer enough heat to light off the plywood, I am thinking you have other major issues.
If they are that hot, there is more other things that are prolly burning to worry about.

Screws ain't going to bite into the Micore alone.
I agree with BG, overkill is great, I am the same way, but there, your going off the deep end.

I wouldn't say that trying to abide by any possible CODE specs is "Going off the deep end".

I myself would not be deeply concerned with the screws, but an inspector might. The OP would need to check HIS local code to ensure a "Clean" install.

I know metal is NON combustible but ...Doesn't metal conduct heat?? (ie into plywood) Isn't stopping/slowing HEAT transfer one of the reasons we build a hearth at all?

Is it possible that the screws may possibly cause an inspection fail? So why not discuss ALL the details up front BEFORE any issues arise from an inspection/insurance claim?

Again, see those are questions guys.

 

[Zulu45]

Hi

I am also in the process of building a hearth with micore 300 to meet the required R value. I will be using 2 layers of 1/2" micore / 1 layer 1/2" durock/ vermont slate.

I have read alot of previous posts on hearth.com and other sites, but none of them specifically address how to assemble the layers of micore and durock ... " drystack" the layers or use thinset- versabond- between each layer?

The plan at present is to place the first sheet of micore directly on the plywood subfloor (no thin set), followed by layer of 24 ga galvanized sheet metal, 2 nd sheet of micore, then durock sheet. This construction/ "dry stacked" sandwich will all be screwed to the subfloor. Then will spread some thinset on the durock topside and tile.

I am hoping the sheet metal between the first and second layer of micore will function like a heat sink and strip the screws of any heat that might be transferred top down.

Sound like a good or poor hearth design????....., I really don't want to have redo because of cracked tiles, etc

Thanks for the help!








I wouldn't say that trying to abide by any possible CODE specs is "Going off the deep end".

I myself would not be deeply concerned with the screws, but an inspector might. The OP would need to check HIS local code to ensure a "Clean" install.

I know metal is NON combustible but ...Doesn't metal conduct heat?? (ie into plywood) Isn't stopping/slowing HEAT transfer one of the reasons we build a hearth at all?

Is it possible that the screws may possibly cause an inspection fail? So why not discuss ALL the details up front BEFORE any issues arise from an inspection/insurance claim?

Again, see those are questions guys.[/quote]

 

[begreen]

Can anyone show me a case where heat transference through hearth screws cause ANY issues? I mean any issues, even with an inspector. This is bordering on obsessiveness.

 

[Zulu45]

ok, I am with you on the screws. sheet metal is cheap. Do I need thinset/adhesive between any of the micore/durock layers? or just screw the crap out of it?

 

[begreen]

No adhesive or thinset. Micore is fibrous. You will not get a good bond to it. Just screw through the sandwich and it will be totally captive. Durock recommends a screw every 8".

What stove is going on top of this hearth?

 

[Zulu45]

Thanks Begreen! love this site, just too lazy to post much

Have been using a Hearthstone heritage for the past four years or so in a different part of the house. It will get moved to the hearth I am building now. Don't really care for the performance of the Heritage, will upgrade to bigger Woodstock cat stove in the future.

thanks again

 

[begreen]

I would buildup protection a bit differently. The Heritage needs R=1.2. so I would put down 1/2" of micore, sheet metal, then 2 layers of Durock NexGen. Why? Increased rigidity. You will still be over the hearth R value requirements but will have a stiffer hearth. This will keep the grout from cracking and will have more strength if you chose a big rock like the Progress Hybrid. If you choose to do this stagger the seams of the Durock layers.

1/2" microre = R .52 + 1" Durock NextGen = R. 78 --- total R= 1.3

 

[Zulu45]

Begreen , I Like your thoughts on the hearth construction. Thanks for the input.

Yes, I know that I am considerably above the R value required, but Menards online had a little data entry error and offered 4 sheets of 1/2" X 24" X 48" micore 300 for ~ $18.00..(I got two, 8 boards for ~36) They have since corrected their error.

The previous hearth dates back to 1826, basically a box/crib filled with sand suspended between a floor joist and the fireplace masonry.. So, I have plenty of room to stack material without making the new hearth too high...and, the old floor joist likes the lightweight Micore!

Think I will need to add another layer of 1/2" Durock to my original plan.... wife has informed me that the new hearth should be a little higher of the floor....won't be as rigid as the scheme you outlined, but should be a step in the right direction.

thanks again

 

[begreen]

Sounds good. The extra layer of cement board will still stiffen things up.

 

[fire_man]

I would buildup protection a bit differently. The Heritage needs R=1.2. so I would put down 1/2" of micore, sheet metal, then 2 layers of Durock NexGen. Why? Increased rigidity. You will still be over the hearth R value requirements but will have a stiffer hearth. This will keep the grout from cracking and will have more strength if you chose a big rock like the Progress Hybrid. If you choose to do this stagger the seams of the Durock layers.

1/2" microre = R .52 + 1" Durock NextGen = R. 78 --- total R= 1.3

begreen: What is the purpose of the sheet metal on top of the micore?

My hearth plan for an R value = .59 is: 1/2" micore + 1/4" Hardibacker + thinset + 1/4" tile. (my total thickness cannot exceed 1")

Micore and Hardibacker will be screwed down (no adhesive or thinset).
BTW isn't the R value of 1/2" micore 300 = 1.09?

 

[begreen]

A sheet metal layer is not a requirement unless the top brick, tiles, stones are dry fitted without grout. The intent then is to have the metal as an ember barrier under the cement board.

 

[fire_man]

That's what I thought, but 2 layers of Durock seem like enough of an ember barrier, especially if the seams don't line up, or if a single sheet of Durock is continuous.