Securing Hearth Pad Layers

[Tfin]

I simply sunk Durock screws through the Durock, Micore and into the plywood base. Then applied the thinset to the top of the Durock to adhere the tile to it.

This design has worked well and looks great.

 

[fjordrr]

Thanks so much for the quick reply, Tfin70. Screws it is then - that's a lot quicker/easier too!

 

[firefighterjake]

I simply sunk Durock screws through the Durock, Micore and into the plywood base. Then applied the thinset to the top of the Durock to adhere the tile to it.

This design has worked well and looks great.

Not that I needed it (due to my low R value requirements and fact that I've already built my hearth), but where did you get the Micore Tfin . . . I looked around in a few places in Central Maine and couldn't locate any source.

 

[G-rott]

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?

Why are you using Micore on two levels? If the flat level is continuing under the raised level can you use the micore there and build the raised level using the Metal studs and Durock? Just trying to simplify. How much R-value do you need?

Garett

 

[Gooserider]

Per the EXPERTS over on the John Bridge "Tile your world" website, you DO want to put a THIN layer of latex fortified thinset (as spec'd in the Durock data sheets) between each layer!

IIRC you mix up the thinset and spread it evenly with a 3/8" square notched trowel. Then put the next layer in place and push it down from the center, working out, and screw it down (If doing multiple layers, you can go light on the screws in the middle layers) then repeat with the next layer as soon as possible. Optimally you should get all the layers down and screws in before the thinset on the bottom sets up completely.

The purpose is to bond all three layers together into one solid block, as opposed to letting them work against each other and flex... This is especially important with Micore in the mix which tends to be slightly spongy. It also fills any voids or low spots between the layers, again making a more solid construction. The way the folks on the JB website put it, "The screws hold the subfloor down, the thinset holds it UP"

Remember that ANY flex in the sandwich is unacceptable, and will lead to tile failure. Thinset between the layers will greatly reduce the potential for flex

 

[Tfin]

I simply sunk Durock screws through the Durock, Micore and into the plywood base. Then applied the thinset to the top of the Durock to adhere the tile to it.

This design has worked well and looks great.

Not that I needed it (due to my low R value requirements and fact that I've already built my hearth), but where did you get the Micore Tfin . . . I looked around in a few places in Central Maine and couldn't locate any source.

Jake, I had to get Hammond Lumber to order it from a company out of Portland for me. It was only $20-$30 or so for a 4x8 sheet of 1/2".

 

[Tfin]

I guess my setup is a little different. I only have a 3/4" base plywood sheet, followed by two 1/2" sheets of Micore, then one layer of Durock with the tile adhered to it via thinset. I guess I don't see how thinset between the Durock/Micore/Micore/Plywood was going to make that much difference. I used a lot of Durock screws, and the pad isn't going to move from its location, so ultimately no flex.

Tile
Thinset
Durock <---------screws from here down into the plywood
Micore
Micore
Plywood

 

[firefighterjake]

I simply sunk Durock screws through the Durock, Micore and into the plywood base. Then applied the thinset to the top of the Durock to adhere the tile to it.

This design has worked well and looks great.

Not that I needed it (due to my low R value requirements and fact that I've already built my hearth), but where did you get the Micore Tfin . . . I looked around in a few places in Central Maine and couldn't locate any source.

Jake, I had to get Hammond Lumber to order it from a company out of Portland for me. It was only $20-$30 or so for a 4x8 sheet of 1/2".

Thanks . . . good to know . . . both in my line of work and if I ever decide I want to build another hearth.

 

[Gooserider]

I guess my setup is a little different. I only have a 3/4" base plywood sheet, followed by two 1/2" sheets of Micore, then one layer of Durock with the tile adhered to it via thinset. I guess I don't see how thinset between the Durock/Micore/Micore/Plywood was going to make that much difference. I used a lot of Durock screws, and the pad isn't going to move from its location, so ultimately no flex.

Tile
Thinset
Durock <---------screws from here down into the plywood
Micore
Micore
Plywood

This is the sort of setup that the John Bridge folks are very emphatic about needing thinset between the layers. Their argument is essentially that you are likely to have slight irregularities in the surfaces that cause gaps between the layers - probably somewhat more likely if you are going over decking boards than ply, but still... The gaps could potentially allow very slight flexing between the layers and the tile, eventually leading to the tile either popping or cracking. In addition, by not bonding the layers together, each one can move, again by tiny amounts but enough to cause eventual tile failure. By putting thinset between the layers, you eliminate all the gaps, and you bond all the layers together into one solid laminated structure, which is much stiffer and stronger than the sum of the individual layers - sort of like making a home-brew version of plywood.

If you look at the Durock Application notes, you will see that they explicitly call out for thinset between the Durock and the substrate, as well as between layers if multiple layers are used.

IMHO using the thinset between layers may not be absolutely essential, but it doesn't make the job that much harder, and the end result will be stronger and more durable - so why not do the job properly....

Gooserider

 

[EatenByLimestone]

I used plaster washers like these:

http://www.kilianhardware.com/ceilbutplasw.html

I picked them up at Lee Valley and used standard drywall screws to hold them down. I have read that screws can transport heat straight through the insulator to the subfloor below. I don't know if this is true but it sounds reasonable. I placed most of my screws around the edges where there would be less heat.

Matt

 

[Gooserider]

I used plaster washers like these:

http://www.kilianhardware.com/ceilbutplasw.html

I picked them up at Lee Valley and used standard drywall screws to hold them down. I have read that screws can transport heat straight through the insulator to the subfloor below. I don't know if this is true but it sounds reasonable. I placed most of my screws around the edges where there would be less heat.

Matt

That would probably work, but again, the Durock application sheet specifically calls out a special cement board screw, and says NOT to use standard sheetrock screws... I don't know if there is a material difference, but the cement board screws looked like they were a larger diameter, and had a slightly different head profile that was designed to self set into the board without damaging it...

As to the heat transfer question, it is possible, though there don't seem to be any documented cases of problems caused by it. Two approaches that I've seen suggested to reduce the risk of any problems is to put a layer of sheet metal (I used aluminum roof flashing) in the bottom or center layer of the "sandwich" (Between two layers is best, but if you are only doing one layer of material like I did, then between the decking and the Durock is OK) - this acts as a heat spreader to dissipate any localized hot-spots. The other is to go light on screws in the area under the stove, with more around the edges to make up for it. If you have multiple layers, I've also seen it suggested to use shorter screws that only go between a couple layers and stagger them. Again, how necessary this is, I'm not sure....

Gooserider

 

[Tfin]

I guess my setup is a little different. I only have a 3/4" base plywood sheet, followed by two 1/2" sheets of Micore, then one layer of Durock with the tile adhered to it via thinset. I guess I don't see how thinset between the Durock/Micore/Micore/Plywood was going to make that much difference. I used a lot of Durock screws, and the pad isn't going to move from its location, so ultimately no flex.

Tile
Thinset
Durock <---------screws from here down into the plywood
Micore
Micore
Plywood

This is the sort of setup that the John Bridge folks are very emphatic about needing thinset between the layers. Their argument is essentially that you are likely to have slight irregularities in the surfaces that cause gaps between the layers - probably somewhat more likely if you are going over decking boards than ply, but still... The gaps could potentially allow very slight flexing between the layers and the tile, eventually leading to the tile either popping or cracking. In addition, by not bonding the layers together, each one can move, again by tiny amounts but enough to cause eventual tile failure. By putting thinset between the layers, you eliminate all the gaps, and you bond all the layers together into one solid laminated structure, which is much stiffer and stronger than the sum of the individual layers - sort of like making a home-brew version of plywood.

If you look at the Durock Application notes, you will see that they explicitly call out for thinset between the Durock and the substrate, as well as between layers if multiple layers are used.

IMHO using the thinset between layers may not be absolutely essential, but it doesn't make the job that much harder, and the end result will be stronger and more durable - so why not do the job properly....

Gooserider

Well, I built the pad last year and its been in service since then without any issues. I can appreciate the additional info you provide, but I'm not going to tear the pad apart to put thinset in between all the layers at this point.

We'll see how things go as the years progress. I will report back on this though if any issues arive in the future, but my personal feeling is everything should remain intact.

 

[Gooserider]

I guess my setup is a little different. I only have a 3/4" base plywood sheet, followed by two 1/2" sheets of Micore, then one layer of Durock with the tile adhered to it via thinset. I guess I don't see how thinset between the Durock/Micore/Micore/Plywood was going to make that much difference. I used a lot of Durock screws, and the pad isn't going to move from its location, so ultimately no flex.

Tile
Thinset
Durock <---------screws from here down into the plywood
Micore
Micore
Plywood

This is the sort of setup that the John Bridge folks are very emphatic about needing thinset between the layers. Their argument is essentially that you are likely to have slight irregularities in the surfaces that cause gaps between the layers - probably somewhat more likely if you are going over decking boards than ply, but still... The gaps could potentially allow very slight flexing between the layers and the tile, eventually leading to the tile either popping or cracking. In addition, by not bonding the layers together, each one can move, again by tiny amounts but enough to cause eventual tile failure. By putting thinset between the layers, you eliminate all the gaps, and you bond all the layers together into one solid laminated structure, which is much stiffer and stronger than the sum of the individual layers - sort of like making a home-brew version of plywood.

If you look at the Durock Application notes, you will see that they explicitly call out for thinset between the Durock and the substrate, as well as between layers if multiple layers are used.

IMHO using the thinset between layers may not be absolutely essential, but it doesn't make the job that much harder, and the end result will be stronger and more durable - so why not do the job properly....

Gooserider

Well, I built the pad last year and its been in service since then without any issues. I can appreciate the additional info you provide, but I'm not going to tear the pad apart to put thinset in between all the layers at this point.

We'll see how things go as the years progress. I will report back on this though if any issues arive in the future, but my personal feeling is everything should remain intact.

While I report on, and reccommend doing what the experts suggest, I'm not sure just how much of a difference it makes in the "real world", and how much the issues raised are theoretical as opposed to practical.

Hopefully you won't experience any problems, but I certainly agree with your attitude that given a finished pad, there is no sense it tearing it apart to put in thinset between the layers... I wouldn't tear it apart either...

Much of the time I try to mention issues like this for the benefit of future readers as much as I do for the current participants -

Gooserider

 

[fjordrr]

G-rott - you asked why I was using Micore on both raised and flat layers. Well, I think I was over-engineering things - wanted to get an R-Value of 2 on both portions - but I think the flat level will not need to meet this, (outside min. pad rqt dimensions) so I did not use any Micore at all there - just Durock (not screwed down yet in case the stove installers say I need to up this.)

Gooserider - for the raised level, I did do Durock-Micore-Durock and, since the neighbor helping us is an old tile guy, he wanted to use Flexbond inbetween each layer anyway, so that is what was done in my case, in addition to the Durock screws. I was worried the (minimal) moisture content of the Flexbond might somehow harm the Micore because it is so soft, but everything seems to be OK & very solid.

 

[Tfin]

Much of the time I try to mention issues like this for the benefit of future readers as much as I do for the current participants -

Gooserider

Understood, and appreciated!

Strangely enough, I actually had a dream over the weekend that I was standing on my hearth pad and it suddenly began to flex and pop off some tiles. :lol: Weird.

 

[Gooserider]

G-rott - you asked why I was using Micore on both raised and flat layers. Well, I think I was over-engineering things - wanted to get an R-Value of 2 on both portions - but I think the flat level will not need to meet this, (outside min. pad rqt dimensions) so I did not use any Micore at all there - just Durock (not screwed down yet in case the stove installers say I need to up this.)

Gooserider - for the raised level, I did do Durock-Micore-Durock and, since the neighbor helping us is an old tile guy, he wanted to use Flexbond inbetween each layer anyway, so that is what was done in my case, in addition to the Durock screws. I was worried the (minimal) moisture content of the Flexbond might somehow harm the Micore because it is so soft, but everything seems to be OK & very solid.

If you are outside the minimum protected area called for in the manual, then there are NO real requirements as such, so you are fine with just Durock. I like going a bit over on the minimum dimensions with something non-combustible just in case of those "OOPS" moments like a burning log rolling out of the firebox, or the occasional popping ember, but there isn't a concern about heat from the stove getting the floor to hot.

What gets more interesting are those setups where the owner puts the stove itself on a raised area, but has the lower level inside the protected area. Since the lower level is further from the stove, it presumably would need less insulation material under it, but there is very little that says how much less - this is definitely a case of being reasonable and negotiating with the inspector types... My own estimate is to treat an inch of air as about an r-1.0 insulation equivalent and say never go less than a 1/2" of Durock plus tile...

FWIW, my setup is a brick hearth that was about one brick above the floor level, with a carpeted floor. The smoke dragon that was originally installed was a side load, so I had no problems with clearances, but when I put in the Encore, I would BARELY meet the minimum under the old 16" front clearance standard, but not the current 18" rule. I ended up adding an 18" wide extension at floor level. I lifted the carpet, and cut out the particle board subfloor, leaving the decking in place. I then built back up with a layer of roof flashing aluminum, a 1/2" Durock strip, and random size slate tiles, with flexbond between all the layers. (to match an existing adjacent entryway floor that the extension tied into...) The extension is essentially flush with the carpet, and gives me a hearth that is far wider than required, but looks nice. With the stove cranking I find the brick raised portion is a bit uncomfortable to stand on for long, but the slate is really comfortable...

(You can find pictures if you look back about a year)

Tfin - Didn't mean to give you bad dreams, just hope it isn't your subconcious telling you to go on a diet.... ;-P

Gooserider

 

[donkarlos]

great thread here, but now I have a question : for my basement stove, which sits on boring 2" blocks, I want to build a hearth with tile and such. I imagine I'll just layer durock 3-4 times and tile but really don't want the layer on plywood on the bottom because it would sit on the bare cement floor. I thought of maybe using dricore panels but it alll might be too heavy ....
any ideas on this?

 

[firefighterjake]

great thread here, but now I have a question : for my basement stove, which sits on boring 2" blocks, I want to build a hearth with tile and such. I imagine I'll just layer durock 3-4 times and tile but really don't want the layer on plywood on the bottom because it would sit on the bare cement floor. I thought of maybe using dricore panels but it alll might be too heavy ....
any ideas on this?

If I'm reading this right you have a cement basement floor and simply want to build up the stove by using 3-4 sheets of 1/2 Durock without using plywood. Seems like a good plan to me . . . I would definitely screw and thinset the layers of Durock together to avoid any shifting . . . and if you really wanted to go hardcore you could even screw and thinset the bottom layer to the cement pad . . . although I would think that would be overkill. I think the only reason many of us have used plywood as a base for the Durock is as a way of reducing flex when building the hearth pad on a strung floor (vs. cement pad which would carry all of the weight and would have no flex, unlike a wood strung floor.)

 

[Gooserider]

Exactly... No need for plywood if the substructure is strong enough... The plywood is only needed if building a raised hearth where you need something to keep the Durock or other material on top from flexing, and really isn't the greatest thing to have under a hearth because it's combustible... If you don't need it, don't use it, just layer up the Durock instead.

In the interest of keeping everything anchored, I would probably put a layer of thinset between the basement and the first layer of Durock, and anchor it down with a few flat-head Tapcons, then do a layer of thinset and Durock screws for each layer above that. Probably overkill, but this is one of those jobs where it isn't much extra work to do the overkill approach to begin with, but you can't easily go back and change stuff later.

Gooserider