Extending existing fireplace hearth for wood-burning insert

Kate1569 · Jan 20, 2011

Hello Everybody,

I'm new to the forum (have been lurking here for a while), and now wanted to see if anyone can help us with the specific dilemma we face with our wood-insert installation.

We bought a Drolet Escape I-1400 months ago, and are only now getting around to tackling the installation (this is part of a larger DIY renovation). We actually recently decided to let the installation of the insert itself be done by a professional, but we first have to deal with the hearth and hearth extension. I am attaching some pictures so you can see what we are up against.

The current hearth our 1925 house is a concrete slab that extends about 14.5" in front of the fireplace opening (there used to be a travertine slab over it that we ripped out). The Drolet insert specifies a hearth with an R-value of >1 to extend 16" in front of the blower. That means we will need to extend the current hearth by about 11". To reach the required R-value beyond the concrete slab, we are planning to cut out the hardwood and insulate the combustible subfloor with 1/2" Fiberfrax Duraboard LD.

Our main issue is how to finish the surface. We would like to keep things as close to floor level as possible (although we are flexible on that if necessary, we might able to raise the insert a few inches), and tile both the hearth and the fireplace surround (we already ordered the tile, so I can't really change that plan). I have been told that tiling over two different substrates, even when covered with a continuous sheet of backerboard is a problem because of movement issues.

As I imagine that people who retrofit wood inserts into old houses deal with this issue quite often, I wanted to see if anyone here has any ideas on how to handle this situation, or what they did.

Also, I am wondering if I need to use special heat resistant materials to set the tile.

Thanks in advance for any advice you might give!

kettensÃ¤ge · Jan 20, 2011

I'll give this a try and welcome to the forums. I recently tiled and added stone veneer to an existing brick fireplace into which an insert was already installed. Used reular thinset to attach the tile to the brick surface. I used tile adhesive to attach the tile to the wood subfloor in front of the fireplace. I cut a picture frame border into the hardwood floor at the same time In both cases I used sanded caulk for grouting. It can be found near the regular tile grout in the home improvement stores and comes in a tube just like regular caulk. It is a little more difficult to work with than regular grout. I have no issue with expansion between the hardwood floor and the tile in front of the hearth or with heat related expansion and the tile on the face of the fireplace. This product looks just like grout but is flexible so it won't crack and loosen like regular grout. Colors are limited clean up the grout lines with acetone.

I think raising the insert a little is the best bet to create a grout line or edge between the two. My hearth is 3 brick courses high above the floor.

Eaglecraft · Jan 20, 2011

Kate1569 said:
Hello Everybody,

I'm new to the forum (have been lurking here for a while), and now wanted to see if anyone can help us with the specific dilemma we face with our wood-insert installation.

We bought a Drolet Escape I-1400 months ago, and are only now getting around to tackling the installation (this is part of a larger DIY renovation). We actually recently decided to let the installation of the insert itself be done by a professional, but we first have to deal with the hearth and hearth extension. I am attaching some pictures so you can see what we are up against.

The current hearth our 1925 house is a concrete slab that extends about 14.5" in front of the fireplace opening (there used to be a travertine slab over it that we ripped out). The Drolet insert specifies a hearth with an R-value of >1 to extend 16" in front of the blower. That means we will need to extend the current hearth by about 11". To reach the required R-value beyond the concrete slab, we are planning to cut out the hardwood and insulate the combustible subfloor with 1/2" Fiberfrax Duraboard LD.

Our main issue is how to finish the surface. We would like to keep things as close to floor level as possible (although we are flexible on that if necessary, we might able to raise the insert a few inches), and tile both the hearth and the fireplace surround (we already ordered the tile, so I can't really change that plan). I have been told that tiling over two different substrates, even when covered with a continuous sheet of backerboard is a problem because of movement issues.

As I imagine that people who retrofit wood inserts into old houses deal with this issue quite often, I wanted to see if anyone here has any ideas on how to handle this situation, or what they did.

Also, I am wondering if I need to use special heat resistant materials to set the tile.

Thanks in advance for any advice you might give!

Kate:

We used a totally different approach to extend our hearth - one that I haven't seen on hearth.com. But first I might suggest that you extend your heart lastly, after the stove is installed. That way, if anything is dropped on your extension - like your stove, for instance, - nothing gets damaged.

We had the same issue that you have now. Our fireplace hearth was fine - all brick and mortar - when it was used only as a fireplace. But when we installed our Hearthstone Clydesdale in its far forward position, we needed to extend the hearth another eight inches or so. In front of the brick/mortar hearth was beautiful red oak 3/4 inch hardwood flooring, which didn't meet the requirements for ember protection or insulation. What to do??? We didn't want to create a tripping hazard by installing anything that would stand above the existing oak flooring. Whatever we did would have to be flush with the existing surface. We chose to use 1/4 steel.

We had our local steel fabricator cut a piece of 1/4 steel using his "hydro-jet" cutting table. With this machine he can cut as fancy a design as one might image - even spelling out your name in steel - if that's what you want. We chose a simple trapezoidal like piece measuring 44X40 inches. So when the door of the Clydesdale is opened, the door is completely over the steel.

Our Clydesdale has the blue/black enamel finish. So I took the steel over to the local "powder coating" service and we choose a color and texture to match the Clydesdale surface. The steel was $54 and the powder coating was about $30 (we had our 13 forced air vents powder coated at the same time so I can't determine an exact price).

I traced the steel pattern over the wood flooring that had to go and cut the wood flooring out using a Sears 3 inch circular saw. Over the existing sub-floor I placed 26 gauge sheet-metal just to use a "shim" to get the final surface perfectly level with the hardwood floor. Over the sheet metal I placed Micore 300 cut to size to get the R value of 1.1 that I needed. Then on top of the Micore 300 I placed the 1/4 inch powder-coated steel. To reiterate: Over the sub-floor I have 26 guage sheet metal, 1/2 inch Micore, and 1/4 inch steel. This assemblage is perfectly flush with the hardwood floor.

Around the steel, I inset in a piece of 2 and 1/4 inch oak (the same size as our floor) stained with "Gunstock 231" stain. I think the installation looks pretty nice. I'll add a photo to this post Saturday when my wife the photographer returns from a plant symposium.

This approach isn't for everyone, but it is a solution to making a hearth extension flush with an existing hardwood surface.

Good lucj with your installation.

Kate1569 · Jan 20, 2011

Thank you both for your replies. I am thinking if we should raise the hearth, because if it is 4" raised, we would only need 6" of R-1 out from the blower, and could use stacked cement board to achieve this. I am unsure, however, if that might cause clearance issues with the insert inside the firebox -- it is pretty tight as it is, because the sides curve in. I'll have to do some more measuring for that scenario.

If we had a step in the hearth extension like in kettensÃ¤ge's picture, it would be easy to do an expansion joint between the raised part and the rest of the hearth extension, which would only need to be spark protected. However, given how small our living room is, I would prefer not to have any raised section.

If we (have to) stick with the floor level installation, we would need Micore, and we might have to do an extension joint within the tile field, along the outline of the old hearth. Volemister, your solution does sound similar to what we have in mind, but I really want the hearth to be all one material, and that is ceramic tile. We are trying to achieve a craftsman-style look. I already ordered the tile, because while I knew I had to insulate, I didn't foresee the problem with setting the tile over two different substrates underneath the backerboard (I thought a continuous layer of backerboard spanning the two zones would do), and it was expensive. I have to find a way to install it.

Where did you buy the Micore? I just can't seem to find a retailer that carries it in small quantities.

jocam · Jan 20, 2011

Same situation here as yourself. We chose to cut out the hardwood a full 25 inches and lay the new hearth down. We used 1 1/4 inch marble.
The issue unfortunately is radiant heat protection and what R value is required of your hearth for your insert.
I banged my head up and down over this one. I looked into using micore, durock backerboards etc. All from excellent advice from the members here on the forum. But realistically putting all this together without having the hearth stick up was impossible. In addition, these fibreboards are as hard to find as Michael Vic in a playoff run.
I had our local reputable dealer involved who swore up and down that all that was needed was ember protection and a solid surface. No one understands the R value from an inspection standpoint. At least not in Ontario.
Needless to say, under my hearth like yours is concrete over which I laid the marble. I have kept a very careful eye as to how warm my hearth gets. I really can't fathom how this could be a fire hazard. If I start losing sleep I will have to jack it up and start over.
Whatever you decide,tile slate marble or what ever, if you want it flush, it likely would meet the R rating of the unit.
I expect some wrist slapping. I gave up trying to reach code when it became clear that the R value/K value required was astronomical.

Kate1569 · Jan 20, 2011

Hi Jocam,
Thanks for your response. Do you mean you have concrete all the way under your marble? Then I'm sure your installation is fine. It also looks like your insert is slightly raised. Our dilemma is that our concrete slab doesn't extend nearly far enough. That means we have to extend the hearth beyond the concrete, and therefore we have two issues:
1. getting an R-1 floor insulation for the extended part, which could only be achieved with Micore if we want to hearth more or less flush with the floor
2. tiling over a substrate that has two different subfloor areas underneath (concrete and wood). I realize now that a slab would have been the better solution for this subfloor situation, but now I already bought the tile. So apparently we will have to incorporate a joint into the tile field, to allow for any movement between the two subfloors. Or find a way to isolate the tile substrate from the subfloor.

I'll will stay curious to hear what other people have done, this is all very interesting.

Eaglecraft · Jan 20, 2011

Kate1569 said:
Thank you both for your replies. I am thinking if we should raise the hearth, because if it is 4" raised, we would only need 6" of R-1 out from the blower, and could use stacked cement board to achieve this. I am unsure, however, if that might cause clearance issues with the insert inside the firebox -- it is pretty tight as it is, because the sides curve in. I'll have to do some more measuring for that scenario.

If we had a step in the hearth extension like in kettensÃ¤ge's picture, it would be easy to do an expansion joint between the raised part and the rest of the hearth extension, which would only need to be spark protected. However, given how small our living room is, I would prefer not to have any raised section.

If we (have to) stick with the floor level installation, we would need Micore, and we might have to do an extension joint within the tile field, along the outline of the old hearth. Volemister, your solution does sound similar to what we have in mind, but I really want the hearth to be all one material, and that is ceramic tile. We are trying to achieve a craftsman-style look. I already ordered the tile, because while I knew I had to insulate, I didn't foresee the problem with setting the tile over two different substrates underneath the backerboard (I thought a continuous layer of backerboard spanning the two zones would do), and it was expensive. I have to find a way to install it.

Where did you buy the Micore? I just can't seem to find a retailer that carries it in small quantities.

Kate: Go to the US Gypsum web page and search for Micore 300. Once you find Micore, search for suppliers in your area using zip codes in places that you are willing to drive to. And you can phone US Gypsum (which I did) and ask for suppliers in your area - that's how I came up with Swanson Building Supply in Salt Lake City. It's too far a drive for you (obviously), but since there are no suppliers in Idaho that had any Micore in stock (it's a special order in large quantities only for them), I had no choice.

Swanson sold one 4X8 sheet, 1/2 inch Micore 300 to me for $33 including tax. I took my utility knife and cut 2 feet off the end so that the Micore would fit into my 6 foot 6 inch truck bed. Micore is somewhat fragile - not nearly as strong as gypsum board.

There was one person that posted that "hearth boards" sold by Home Depot or Lowes contains 1/2 inch Micore. I didn't check this out as a possible source since I already had a supplier in mind. You might give this angle a try. Anyway, you are right, it's a difficult product to obtain.

jocam · Jan 20, 2011

My insert is 10-12 inches above the hearth. The concrete under the marble does NOT extend the entire way. I think it is about 10-12 inches out as well. There is plywood overtop of the joists and a healthy layer of mortar on which the marble is laid.
My insert required a K value of 0.84 that was at least 1.5 inches thick. Micore is the only product that I could find but then that would need to be supported by cement board. This would raise the hearth too much for me to handle. As you can see it is not flush anyway.
The visual is personal choice anyway. It's the safety aspect that concerns me but at the same time the requirements seem more than a bit excessive. We don't burn 24/7. I know I sound like I'm rationalizing and minimizing the requirements but in all honesty, after an exhaustive search I could not find anyone in person who actually understood what I was talking about.
My advice is to build it to the safest you can manage. I would not worry about cutting out any of your hardwood floor. Just do it. The entire hearth will then be at the same level. A good installer can do this for you if needed.

Kate1569 · Jan 20, 2011

Volemister, you drove from Idaho to Salt Lake City?? Wow.

I just found out that I seem to be lucky: I talked to one of the places I found on the Micore website, and as they told me they only do wholesale, I asked if my local lumberyard could order from them. They said yes, they can order one 4x8 sheet. Last week when I had talked to the lumberyard, they said they couldn't get Micore (but I hadn't talked to "my" guy in the special order sales department, and the guy I talked to may just have been too lazy to check). So for others on the lookout for Micore, it might be worth calling some of the wholesale distributors (because only those seem to be listed on the Micore website) and asking them about dealers in the area who can order from them, even if those dealers don't even know about the product.

Kate1569 · Jan 20, 2011

jocam, if we could raise our insert that high, we wouldn't have a problem. If the hearth is a minimum of 4" above the floor, we only need a 6" extension. Beyond that, we would only need spark protection. Since it seems like I can lay my hands on Micore (I am still waiting to hear how much it costs, but if it's $33, it's great), I think we might try that. We will still have the issue of laying the tile over a two-part subfloor, though.

jocam said:
My insert is 10-12 inches above the hearth. The concrete under the marble does NOT extend the entire way. I think it is about 10-12 inches out as well. There is plywood overtop of the joists and a healthy layer of mortar on which the marble is laid.
My insert required a K value of 0.84 that was at least 1.5 inches thick. Micore is the only product that I could find but then that would need to be supported by cement board. This would raise the hearth too much for me to handle. As you can see it is not flush anyway.
The visual is personal choice anyway. It's the safety aspect that concerns me but at the same time the requirements seem more than a bit excessive. We don't burn 24/7. I know I sound like I'm rationalizing and minimizing the requirements but in all honesty, after an exhaustive search I could not find anyone in person who actually understood what I was talking about.
My advice is to build it to the safest you can manage. I would not worry about cutting out any of your hardwood floor. Just do it. The entire hearth will then be at the same level. A good installer can do this for you if needed.

Eaglecraft · Jan 20, 2011

Kate1569 said:
Volemister, you drove from Idaho to Salt Lake City?? Wow.

I just found out that I seem to be lucky: I talked to one of the places I found on the Micore website, and as they told me they only do wholesale, I asked if my local lumberyard could order from them. They said yes, they can order one 4x8 sheet. Last week when I had talked to the lumberyard, they said they couldn't get Micore (but I hadn't talked to "my" guy in the special order sales department, and the guy I talked to may just have been too lazy to check). So for others on the lookout for Micore, it might be worth calling some of the wholesale distributors (because only those seem to be listed on the Micore website) and asking them about dealers in the area who can order from them, even if those dealers don't even know about the product.

Kate:

I see that you live on the right side of this great nation. Those of us who live in the western states thinking nothing of driving several hundred miles just for the fun of it. Driving to SLC from our home was about 400 miles round trip - did it one day on I-15. It gave my wife and me the opportunity to enjoy a nice dinner in SLC and check out a few other things. Next month we are driving to Seattle to visit the Seattle home show - 1600 miles round trip. By the way, the big Hearth Expo is in SLC the first week in March. We will be attending.

Hope that you get your Micore.

Kate1569 · Jan 21, 2011

I didn't realize that Idaho was that close to Utah (I'm not American, so not as familiar with the geography).

On the subject of Micore, I wonder what is better: Micore 300 or Micore 160? I read somewhere here that 300 has greater insulating value, however, according to this table here: http://chimneysweeponline.com/horvalue.htm, Micore 160 has a greater R-value at 1/2" thickness than Micore 300. Now I'm confused.

Eaglecraft · Jan 21, 2011

Kate1569 said:
I didn't realize that Idaho was that close to Utah (I'm not American, so not as familiar with the geography).

On the subject of Micore, I wonder what is better: Micore 300 or Micore 160? I read somewhere here that 300 has greater insulating value, however, according to this table here: http://chimneysweeponline.com/horvalue.htm, Micore 160 has a greater R-value at 1/2" thickness than Micore 300. Now I'm confused.

Kate:

The Micore 300 product has a slightly smaller R value (1.03 for a 1/2 inch board) compared to Micore 160 (R=1.27 for a 1/2 inch board). When I spoke to my supplier in SLC about which to purchase, he advised my to buy Micore 300, because he felt it was a higher quality product. This was substantiated by the comments made by US Gypsum themselves -please read the following product description.

So choose the size and type that meets your R requirements. I only required an R value of 1.1. so I went with Micore 300.

Micore 300

Micore 300 board does it all. Itâ€™s a superior substrate for fabric and vinyl-covered wall panels, office dividers and tackboards, and an excellent core for chalkboards, stove boards and other similar applications. With outstanding resilience, superior machinability, and high density, Micore 300 board has the best surface hardness of any Micore board. The boardâ€™s smooth surface laminates easily with various finish materials such as vinyl, fabrics and steel. It is compatible with most water- or solvent-based adhesives.

Thickness 3/8 inch (.375)
7/16 inch (.438)
Â½ inch (.500)
5/8 inch (.625)
Â¾ inch (.750)
Tolerances Thickness .015 inch sanded and coated
Thermal Conductivity k Factor = .49
R Value (1/2 inch board) = 1.03

Micore 160

For superior performance plus savings, Micore 160 board adapts to a variety of design applications. Ideal for open plan partitions, office screens, dividers, tackboards and various other applications. The board combines lightweight properties with the strength and stability of heavier boards. And that means easier handling, simplified assembly and lower freight rates. Micore 160 board also has good sound control, reducing noise in open plan interiors.

Micore
Mineral Fiber Board 160

Thickness 3/8 inch (.375)
Â½ inch (.500)
5/8 inch (.625)
Â¾ inch (.750)
Tolerances Thickness 0.015 inch sanded and coated
Thermal Conductivity k Factor = .39
R Value (1/2 inch board) = 1.27

Kate1569 · Jan 28, 2011

We received our Micore 300. I realize I am still not sure how to install it properly. Once we cut out the hardwood around the old hearth extension down to the subfloor, do we just lay the Micore in the cavity, do we screw it down (seems questionable to put highly conductive screws in there), or do we cement it down? I think the subfloor underneath our hardwood is wooden planks.

Then, should we use backerboard over it, or pour cement? If we pour cement, should it be refractory cement? Can anyone recommend a product for that?

I went to a brick and mortar store (an actual one!) today, and they were very confused about our whole hearth issue. The whole idea of extending it, insulating with Micore, etc. was very foreign to them. They said the proper way to do it would be to knock out the old concrete slab and repour it larger. Needless to say, this would cost thousands and is not in our budget.

The reason I had gone to the brick store was to ask them what product to use to repair the actual hearth, i.e. the floor of the firebox, and possibly buy some refractory cement. The hearth of our firebox has cracks, and the chimney guy who was here told me to just knock out whatever is loose and pour mortar over it before putting the insert in. So I started taking up the loose stuff the other day, and it comes up in about 3/4" thick chunks. Please see the attached pictures (showing firebox, closeup of demolition of loose mortar or concrete inside, and the hearth from below in the basement). The slab, even though it has one crack that runs all the way through from front to back, appears solid from below (no cracks visible there), and the chimney guy who looked at it said it was fine. (I still worry about its stability, but what can I do?) It appears to be one piece with our poured concrete foundation.

So at the brick store today, they told me there should have been firebricks on the floor of the firebox, and that they've never heard of a fireplace with a concrete hearth without firebricks over it. They said the refractory cement they sell (Heat Stop) is not meant for patching larger areas, just for pointing between the bricks. Even though they think concrete shouldn't be on the floor of the firebox, they said I should use regular concrete for repairing it. Do you agree with that? Or is there another heat resistant cement product that they don't know about? And what do you think about the idea of laying a piece of Micore over the concrete underneath the insert? I thought that might protect the non-heat resistant concrete, if that's what we pour into the firebox.

Back on the topic of tiling, I wanted to ask KettensÃ¤ge: Are you saying you used sanded caulk for all of your grout lines, even on top of your elevated hearth, near your insert? Have you noticed any deterioration of the caulk under the heat? I've been wondering about that for my expansion joint.

Thank you all for your advice!

Eaglecraft · Jan 29, 2011

Kate1569 said:
We received our Micore 300. I realize I am still not sure how to install it properly. Once we cut out the hardwood around the old hearth extension down to the subfloor, do we just lay the Micore in the cavity, do we screw it down (seems questionable to put highly conductive screws in there), or do we cement it down? I think the subfloor underneath our hardwood is wooden planks.

Kate: You won't want your hearth "sliding" around, so I think you will want to lay down a bed of thinset mortar over your subfloor, then place the Micore 300 over the thinset. You can pour the thinset right over the concrete, if your subfloor is concrete. If the subfloor is plywood, then use a latex modified thinset mortar (works as a water-proofing membrane) approved for use over plywood substrates. So your layer will be thinset over concrete (or plywood), Micore 300, more thinset, then cementitious type ceramic tile backer board, then your decorative tiles, also in thinset.

The Micore is so fragile that you can't put the tiles right over it. If you have a plywood subfloor, it would probably be OK if you put a few screws suitable for backer-board through the backer board, through the Micore and then into the plywood. If you have concrete as the subfloor, then you will have to rely on the thin-set holding everything in place.

Then, should we use backerboard over it, or pour cement? If we pour cement, should it be refractory cement? Can anyone recommend a product for that?

I went to a brick and mortar store (an actual one!) today, and they were very confused about our whole hearth issue. The whole idea of extending it, insulating with Micore, etc. was very foreign to them. They said the proper way to do it would be to knock out the old concrete slab and repour it larger. Needless to say, this would cost thousands and is not in our budget.

Kate: I think that "knocking out the old concrete slab and repour it larger" is unnecessary.

The reason I had gone to the brick store was to ask them what product to use to repair the actual hearth, i.e. the floor of the firebox, and possibly buy some refractory cement. The hearth of our firebox has cracks, and the chimney guy who was here told me to just knock out whatever is loose and pour mortar over it before putting the insert in. So I started taking up the loose stuff the other day, and it comes up in about 3/4" thick chunks. Please see the attached pictures (showing firebox, closeup of demolition of loose mortar or concrete inside, and the hearth from below in the basement). The slab, even though it has one crack that runs all the way through from front to back, appears solid from below (no cracks visible there), and the chimney guy who looked at it said it was fine. (I still worry about its stability, but what can I do?) It appears to be one piece with our poured concrete foundation.

So at the brick store today, they told me there should have been firebricks on the floor of the firebox, and that they've never heard of a fireplace with a concrete hearth without firebricks over it. They said the refractory cement they sell (Heat Stop) is not meant for patching larger areas, just for pointing between the bricks. Even though they think concrete shouldn't be on the floor of the firebox, they said I should use regular concrete for repairing it. Do you agree with that? Or is there another heat resistant cement product that they don't know about? And what do you think about the idea of laying a piece of Micore over the concrete underneath the insert? I thought that might protect the non-heat resistant concrete, if that's what we pour into the firebox.

Kate: I'm not sure how your hearth extension will interface with your actual hearth (the material directly under your insert), but in fact, you could just pour quick-set concrete under where your stove will be and let it go at that. You are not going to use this space as a fireplace again, right? Then concrete is OK. Why is it OK? Because it won't be subject to the very high temperatures that an open fire would produce. If you are concerned that the insert might come out in the future and that area would be once again used as a "fireplace" then you would need firebrick. This would be secured in place with Rutland Refractory Cement, which is "A pre-mixed, fiber-reinforced, high temperature silicate mortar for setting, coating or repairing firebrick. Used extensively as the mortar between firebricks when building fireplace fireboxes."

Back on the topic of tiling, I wanted to ask KettensÃ¤ge: Are you saying you used sanded caulk for all of your grout lines, even on top of your elevated hearth, near your insert? Have you noticed any deterioration of the caulk under the heat? I've been wondering about that for my expansion joint.

Thank you all for your advice!

Kate1569 · Jan 29, 2011

Volemister, thank you so much. I bought some concrete today. We haven't cut out the hardwood yet to look at the subfloor, but I imagine it is wood, probably the old planks rather than plywood, since this house was built in 1925. I was unsure at Home Depot which thinset to buy, maybe I'll get that at the tile store, where I get better advice than at HD.

Regarding the concrete backerboard: I read somewhere here on this board that the "next gen" Durarock is not non-combustible anymore. Is there a non-combustible concrete backerboard, or can I still use Durarock for this application? I've also wondered if I should just float the whole hearth, hearth extension (the concrete block), and the Micore-insulated extension to the hearth extension with the concrete, instead of using CBU. Is that a possibility? The layer would be thin, though, like 1/2".

Hogwildz · Jan 30, 2011

Next Gen Durock is non combustible.

Eaglecraft · Jan 30, 2011

Kate1569 said:
Volemister, thank you so much. I bought some concrete today. We haven't cut out the hardwood yet to look at the subfloor, but I imagine it is wood, probably the old planks rather than plywood, since this house was built in 1925. I was unsure at Home Depot which thinset to buy, maybe I'll get that at the tile store, where I get better advice than at HD.

Regarding the concrete backerboard: I read somewhere here on this board that the "next gen" Durarock is not non-combustible anymore. Is there a non-combustible concrete backerboard, or can I still use Durarock for this application? I've also wondered if I should just float the whole hearth, hearth extension (the concrete block), and the Micore-insulated extension to the hearth extension with the concrete, instead of using CBU. Is that a possibility? The layer would be thin, though, like 1/2".

Kate:

Durock - Next Gen, made by USGypsum would be a fine product to use. Here's what US Gypsum has to say: DUROCK cement board Next Gen is moisture and mold resistant and does not deteriorate, swell, soften, decay, delaminate, or disintegrate in the presence of water, making it the perfect choice for baths, showers, kitchens, and laundry rooms. It is also suitable for exterior applications, including fences, mobile home skirting, agricultural buildings, garage wainscoting, and exterior finishes.

The board is non-combustible and can be used in a variety of fire-rated designs. Its low thermaland hygrometric expansion help prevent finish cracking."

Kate, I'm quite not sure what you are suggesting with your second paragraph. Since you are extending the hearth over wood (probably planking or plywood), you will need the Micore 300 to reduce heat transmission to the wood. Could you pour the concrete directly over the Micore? Maybe. But you will need to waterproof the plywood, or even solid subflooring first, to prevent the wood from swelling from the water in the concrete, I think. There are waterproofing membranes that can be applied to plywood, or wood planking subfloors to make them suitable for top surfaces that are wet.

So you put the waterproofing membrane down over the wood subfloor, then the Micore 300, then the concrete over the Micore. Then you set the tile over the concrete with thinset mortar. I think that's what you are suggesting - maybe.

Kate, before you pour concrete over the Micore, I recommend that you check in with the folks at US Gypsum and see what they think - if that's what you are suggesting.

Hogwildz · Jan 30, 2011

I laid Micore down, then durock over top, screwed both down together using durock spec'd screws. Then thinset slate tiles overtop.
Why are you wanting to lay concrete in there? that thin a layer most likely will crack.

Kate1569 · Jan 30, 2011

Thanks for looking up the Durarock properties, volemister, I should have done that instead of just going by what I read somewhere! You are right about what I was suggesting, regarding pouring the concrete over everything.

Hogwildz, that the thin layer of concrete might crack is what I thought, and your comment confirms that it's probably not a good idea. The reason I was contemplating this is that we are creating a two-zone substrate situation by extending our hearth beyond the old concrete slab hearth extension, which may be problematic as a substrate for the tile. I've been told there may be movement between the concrete slab and the Micore-insulated area over the wooden subfloor, even if we lay a sheet of Durarock over everything before we tile. We may need to create an expansion joint between the tiles over the concrete slab and the tiles over the Micore/Durarock. I realize that if we poured concrete over everything in a thin layer, it would probably just break from whatever movement there may be, rather than serve to make our hearth this one solid piece I would like it to be.

Maybe this afternoon we'll get to cutting out the hardwood floor, and see what's underneath...

Kate1569 · Feb 9, 2011

Hello again,

We still haven't cut out the hardwood, but we received the saw to do it (Rotozip) yesterday. So one of these nights or this weekend...

Meanwhile, I am gathering the missing materials. I realize I am still unsure about which thinset to use. Latex-modified thinset has been mentioned, but does anyone know how that takes the heat, with the latex in it? And is there a particular product that works well? I hear of Laticrete products being the best, but I'd have to drive farther for that than to Home Depot, so I'd like to know if it's worth it. We are laying ceramic tile over Durock and (in the area of the old hearth) over concrete slab.

Thanks in advance!

Kate1569 · Feb 10, 2011

It seems that people here are using Flexbond on their hearths, so I assume it is heat-resistant enough.

After more research, I realize that any regular concrete requires at least 2" of thickness. We are not going to apply it that thick when we repair our firebox hearth. I read that in thinner applications, one should use a modified concrete, something like Quikrete Sand-Mix mixed with Acrylic Fortifier. Does anybody know if it is possible to use such a product in a high-heat situation like the area directly under the insert? I don't find any temperature limitations in the data sheets, but it seems questionable to me to use a mixture containing a plastic here.

Any advice is much appreciated!

Hogwildz · Feb 10, 2011

Kate1569 said:
It seems that people here are using Flexbond on their hearths, so I assume it is heat-resistant enough.

After more research, I realize that any regular concrete requires at least 2" of thickness. We are not going to apply it that thick when we repair our firebox hearth. I read that in thinner applications, one should use a modified concrete, something like Quikrete Sand-Mix mixed with Acrylic Fortifier. Does anybody know if it is possible to use such a product in a high-heat situation like the area directly under the insert? I don't find any temperature limitations in the data sheets, but it seems questionable to me to use a mixture containing a plastic here.

Any advice is much appreciated!

Directly underneath the insert is probably the coolest place around the insert.
You should be fine.

Kate1569 · Feb 11, 2011

Thank you, that supports what I learned when I called the stove manufacturer today. The customer service guy said that the blower blows the air around the bottom of the insert, so it shouldn't get too hot, but he couldn't tell me any temperature data. Also called Quikrete, and the customer service guy said their products are rated up to 300F. I really don't have a good feel for how hot anything around that insert is going to get, but if you think the modified concrete is fine, that's what we'll use.

Hogwildz · Feb 11, 2011

The hearth under my insert is almost cool to the touch. It is just over about a foot out from the front that the hearth gets warm from the heat radiating from the glass. But underneath, you should be more than fine.

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