Hearth extension construction: How to provide tile underpayment with the lowest profile

[Bend]

I am so happy to find this forum and really need some help.
I have purchased a new wood-burning stove. The requirement is a hearth of 25 inches in order to have 18 inches in front of the fuel door opening. It requires a hearth R-value of 1.1, so I plan to use 1/2-in Micore 300, then 1/2-in Durock, then Tile.
I would like to extend my hearth from the existing 19-in to the required 25-in by removing 6-in of the subfloor in front of the existing hearth. I have already removed the existing hearth stones and cleaned up the (fairly soft & crumbly) cement residue underneath them. Underneath the cement residue is a firm concrete base which is part of the fireplace construction & is in fine shape. It forms the floor that the hearthstones were set on and it measures 19 x 60-in. The subfloor is 1 1/8 inch plywood which stops short at the existing hearth area.
I crawled under the house & can see that this concrete butts up to a 4x6 girder that is set on pier blocks & posts. So, if I removed 6-in of the subfloor, it will expose the surface of the 4x6 and an additional couple of inches. I found a post at the John Bridge forum where Gooserider, CX & Rose are discussing beefing up the floor joist area in order to extend a hearth in this way, so I know that would have to be taken care of. I understand that Gooserider has had an accident and has not been a part of this forum for quite some time now; I wish he & his family well.
If all this was done, there would an area in front of the fireplace that measures 26 x60 & is recessed 1 1/8 inches below the surface of the existing subfloor. Once the 3/4-in hardwood floor is added to the subfloor, the hearth area would be recessed a total of 1 7/8 inches.
My question is: how can a hearth be constructed in this space which meets the R-value, provides adequate substrate for the tile in a way that does not transmit a crack to the tile surface where the wood 4x6 and concrete meet and is not above the surface of the hardwood floor. Or, if that is not possible, to keep the height to a minimum. There is the complication of the fact that the working surface would be made of dissimilar surfaces (existing concrete and 4x6, etc). What could you use as a substrate for the Micore/Durock/Tile that would make sense & not transmit a crack to the tile surface due to the disimilar surfaces underneath (concrete & wood). I read in this forum about using deck mud with lath over a moisture barrier like roofing paper. It said the mud should be 1-in thick to do that. But they were talking about putting the mud over a plywood subfloor (which is not what I would have). Would that even be an option for me? If I did use 1-in of concrete, the hearth would be about 1-in higher than the finished floor (after hardwood floor) & I might need to raise the level of the firebox floor a bit so that the stove sits level. Is there a way to provide adequate substrate with another product that is thinner and that would protect the finish tile flooring from cracking due to the dissimilar surfaces below the tile installation?
I hope someone has an answer for me; my brain is in a knot thinking about this. I love this forum.
Thank You, Bend

 

[begreen]

Welcome Bend. I personally would leave the 1 1/8" subfloor intact. That is a great substrate to avoid flexing of the floor under the hearth. Consider putting a nice, finished wood curb around the outside perimeter of the hearth extension. It can have a beveled edge that starts out at floor level and ramps up to the tile level.

 

[Bend]

Thanks for your quick reply BeGreen, if there is a way to do it, I would really like to keep the hearth low. My Hearthstone stove is quite large & the room above the stove is minimal so if I could maximize that space it would be a good thing. Also, Currently, there is already no subfloor where the old hearthstones were--the 1 1/8-in subfloor surrounds this area. So, if I removed another 6-in of the subfloor, it would bring that area down so that it is level with the existing lower concrete surface that supported the hearthstones originally. Then the entire required distance of 25 inches would be at the same (lower) level. Currently, the lower level is only 19 inches, but I need 25 in order to have 18 inches in front of the door.

 

[Bend]

I was just reading about Advantech which is a plywood that can be applied to concrete. Could I remove 6-in of the subfloor & then lay in a piece of this 3/4-in Advantech, then lay down the Micore, 1/2-in Durock, then tile? This would bring the surface of the finished hearth to about 5/8-in above the finished hardwood floor (not counting thin set between the layers). It seems like the Advantech would bridge the dissimilar materials and provide a consistent layer across the entire surface to lay the rest of the tile sandwich on so that the tile would not crack. I have no idea if my thinking makes any sense & would really appreciate comment from you'll. Thanks

 

[begreen]

I would need some pictures and confirmation to be sure I have the full picture here. But as I understand it, if this was the ONLY option, I would create a base at the desired level by screwing in support blocks for the depth of the hearth, screwed and glued to the floor joists. Then build up from that point with the plywood, micore and cement board sandwich and finally the tile to the floor level. If you are worried about flexing, use 1.25" plywood instead of 3/4".

 

[Bend]

Thanks South Puget Sound for your reply. I am a newby to this forum; I am going to figure out how to send a picture. Yes, that is what I want to do--create a base to build on. And i want to keep the height of the entire tile sandwich to a minimum because the bulk of the woodburning insert sits inside the fireplace--only about 7-in will extend out onto the hearth. It is a hearthstone clydesdale. I have minimal room above the insert, so i want to keep the tile sandwich as small as possible.
There are no floor joists in cement area to connect anything to to; it is solid cement.

The old hearth stones that I removed were 2-in thick. The finished surface of those old hearth stones was about 3/4-in above the subfloor & then there was carpet & padding on the subfloor which made the carpet appear level with the surface of the hearthstones. I removed the 2-in thick hearthstones and that surface underneath them (the concrete) is now flush with the BOTTOM edge of the subfloor. The concrete extends all the way down to the dirt below the house. This area held 3 hearthstones and measures 19 x 60.

The 4x6 girder butts up to this concrete & is parallel with the face of the fireplace. In other words the long edge of the girder buts up to the concrete. The top of the girder is level with the top of the concrete and it is level with the bottom of the subfloor.

The hearth will be extended about 6-in out onto that 4x6 girder and a tiny bit (a couple of inches) beyond the girder. With a bit of beefing up of the support structure to support a couple inches of hearth extension beyond the 4x6, I think the support issue would be taken care of. 2x support will be attached from under the house to beef up that couple of inches where the hearth extends past the 4x6.

I have done very little tile in the past, but i remember reading that if you lay tile so that it bridges 2 dissimilar surfaces (concrete & 2x6) without providing an expansion gap at that spot, it is likely to telegraph a crack to the surface of the tile.

I want to avoid an expansion gap in the middle of my hearth installation where the concrete meets the 4x6. So, I would like the bottom layer of the tile sandwich to provide a base for the tile sandwich which will bridge the dissimilar surfaces so the tile will not crack and also add minimal height to the tile sandwich so that the insert will fit comfortably into the fireplace opening.

I read on the net that Advantech is 3/4-in thick plywood that can be applied to concrete. I have no experience with this material, but I am thinking that this might be an effective bridging material so I would not need to provide an expansion gap. Then continue on this base with Micore/durock/tile. Does this sound like something I could use to solve the problem? I am going to try to send a picture--worth 1000 words. Thanks again for your help South Puget Sound!

 

[Bend]

Thank You BeGreen,
I just got your most recent post right noour new at navigation. Your advice is very much appreciated. Are you familiar at all with Advantech? Apparently it can be applied directly to the concrete. Is anyone reading this familiar with that product? Does it hold up well over time?
Thank You for reading.

 

[begreen]

OK, I think I'm with you now. If I have it visualized correctly, this shouldn't be a problem, whether you use AdvanTech or regular exterior-grade plywood. It sounds like the sandwich will be very well supported on a very solid base. You don't need to worry about a gap because there won't be any. You will use a single piece of plywood to create a base on top of the cement, girder and 2x6 gap filler. That gets anchored to the surfaces below and creates your first level of uniform surface.

To anchor, it would help to put in a few rawl plugs or tap cons into the concrete. Drill through holes for the screws in the plywood, then use the plywood as a template and mark the concrete. Remove the plywood and set the screw anchors in the concrete. Then, clean the cement off well, vacuum and use a damp rag to remove any dust. Lay down generous beads of heavy duty, liquid nails, construction adhesive on all surfaces (concrete, steel beam, wood). Screw down the plywood.

Now fit the micore layer on top of the plywood base. This can be one piece or a couple, it doesn't matter. Then make a single piece Durock cap. Use a 60" wide sheet for no trimming. Cut and fit it so that the micore is capped with this single sheet of cement board. Screw the Durock down using their 1 3/4" screws, one every 8", no adhesive here. Now you have a single unit that is not going anywhere. Set the hearth tiles using latex modified thinset per mfg directions.

(PS: Is the 4x6 girder steel? I am assuming so. If not, no big deal.)

 

[Bend]

Thank You PE,
I am so happy to have this information & I really appreciate your help. I have a couple more questions:
There is no thin set on either side of the Micore, is that right? Or should I use thin set? If I use thin set should. It be modified or not. You mentioned no adhesive here, but I wasn't sure if thin set is considered adhesive.
The 4x6 is wood (Douglas fir). So, is it better to leave this area without screws? I live in an area that has earthquakes. So would it make sense to leave the screws off here?
Thank You so much for your help; I will sleep well PE.
Best Wishes to you.

 

[Cueman]

I have a similar issue with my hearth install, maybe someone can provide some insight. I want the stove on the floor. The stove is going in the carpeted living room. Eventually I will replace the carpet with wood flooring, presumably 3/4" stock. I have stripped the carpet, and underlayment. So I'm down to ply sub floor on my hearth foot print. I mocked up the hearth with 1/2" micore, 1/2" Durock, and 1/2" soapstone tile. This looked ok and was about flush with the carpet, but I want it lower so it can match the flooring someday without toe stubbing trim work later on.
Has anyone set tile directly on 1/2"Micore. According to the spec sheet it should be able to handle 170psi, and with tile on top any weight (of course that constant 400lbs of the stove) should be more than adequately displaced to prevent distortion of the Micore or to cause tile popping. And I know proper adhesion of the Micore to the sub floor would be important, as well as fining the right way to bond the tile to the Micore. Perhaps a layer of wire lathe screwed to the micore, than topped with thin set, or mortar could work.

Any ideas or advice appreciated!

Need to get this damn stove working to keep the oil man as far away as possible.

 

[begreen]

Are you in Bend, OR? We are also in an earthquake region. If the house really gets shaken up, I wouldn't worry about the hearth. It certainly is not structural. Good to hear that the 4x6" is a wood beam and not a girder. That means you can anchor the plywood base to it easily with screws.

No thinset needed or helpful on either side of the micore. The screws through the durock to the plywood below are what are holding this sandwich together. With a screw every 8" in each direction, the final product will act as one unit. Micore is very fibrous so the thinset wouldn't bond well anyhow. Just leave it raw. The latex modified thinset is for bonding the tile to the durock.

 

[begreen]

I have a similar issue with my hearth install, maybe someone can provide some insight. I want the stove on the floor. The stove is going in the carpeted living room. Eventually I will replace the carpet with wood flooring, presumably 3/4" stock. I have stripped the carpet, and underlayment. So I'm down to ply sub floor on my hearth foot print. I mocked up the hearth with 1/2" micore, 1/2" Durock, and 1/2" soapstone tile. This looked ok and was about flush with the carpet, but I want it lower so it can match the flooring someday without toe stubbing trim work later on.
Has anyone set tile directly on 1/2"Micore. According to the spec sheet it should be able to handle 170psi, and with tile on top any weight (of course that constant 400lbs of the stove) should be more than adequately displaced to prevent distortion of the Micore or to cause tile popping. And I know proper adhesion of the Micore to the sub floor would be important, as well as fining the right way to bond the tile to the Micore. Perhaps a layer of wire lathe screwed to the micore, than topped with thin set, or mortar could work.

Any ideas or advice appreciated!

Need to get this damn stove working to keep the oil man as far away as possible.

Don't do it. Micore is too soft. It is not a suitable substrate for tile. Your stove will be heavy. You don't want any flexing at all beneath the tile. Also, micore is very fibrous so there will be a poor bond, if any, between it and the tile. You are far better off to have bullnose trim edge. Or how about having the edges of the outside stones chamfered so that they meet flush with the floor? Soapstone is pretty soft so this should be fairly easy to do. That would be a nice solution here.

Take pics. This sounds like a nice hearth. What stove will this be for?

 

[Cueman]

I don't think the bullnose idea will work. You see the tile is 1/2", and even if I sanded the edge, the 1/2"durok would still be exposed without trim work. I guess I understand that the Micore can flex, and the Durok would provide the rigidity needed for the tile. It baffles me because if you walk over Durock flat on the ground, it will crunch beneath your feet, and the Micore will support your weight.

Do you really think the Micore would flex if it was fastened to the sub flooring? Wouldn't that mean that the floor joists are also flexing? (a bigger problem). As a side note, I'm sistering the joists below the hearth for peace of mind.

will try to put some pics up. I hope it will be a nice hearth. Still searching for the right stone to veneer the wall.

thanks

 

[begreen]

Perhaps a beveled surrounding wood trim frame would stop the toe stubbing and still look attractive.

 

[Cueman]

Thanks for the insight. I'll go with the Durock. Liquid nails Micore to sub floor, 1 3/4 Durock screws through Durock and Micore to subfloor. And latex modified thin set for the stones. Sounds consistent with other post's project. Then I'll figure the trim work later when the wood floors go in.

Stove is VC Enore 1450 Non Cat, 8" pipe. Hope its worth the investment!

 

[begreen]

No need to use liquid nails at all, your situation and sandwich are different. In the other case the liquid nails is below their plywood to bond it to the cement.

I would also check to find the best product for setting the soapstone. I don't know if thinset is the best or not. The other poster is setting tiles.

 

[Bend]

I'm in central CA. Yes I know what you mean about bigger concerns than the hearth during an earthquake. That's great-- no thin set around the Micore. Less work & less height also. Thanks for helping me move forward with my project PE; I was really stuck on the hearth construction. I like the idea of the beveled wood trim to curb the toe stubbing.
Thanks PE

 

[Bend]

An afterthought/ additional question. When an R-value of 1.1 is required for the hearth, does this apply to the firebox floor also? Or is there a higher R-value requirement inside the firebox? Would it be OK to continue with the above installation right into the firebox so that the entire hearth is on one level? So that leveling of the insert would not be an issue? The firebox floor is currently firebrick. That would be 3/4-in exterior plywood attached with heavy duty construction adhesive & a Few tap cons into the concrete portion. Then 1/2-in Micore. The 1/2-in Durock. Then acrylic-modified thin set & tile. The Micore & Durock are on top of the plywood & exceed the required r-value. So, is it OK to run this entire installation right into the firebox? can I also use some tap cons to attach the plywood to the firebrick?
State of the job: 6-in of the subfloor has been removed in front of the hearth exposing the girder. The girder, the concrete in the hearth area and the firebox floor are all at the same level.
Thank you so much for your help!

 

[begreen]

The firebox floor is already much higher than the stove requirement. Don't sweat it. You could continue the cement board under the insert, but I would do it as a separate piece under the insert with a seam that is hidden under the front of the insert. For easy servicing, I like to have a sheet of sheet metal under the insert. It helps it slide in and out easily.

 

[Bend]

Thanks PE, I like the sheet metal for servicing the insert. It will slide easy in & out . Would it be ok to fit the cement board to the firebox floor tightly so it wont move and stack it up so it is approximately level with the hearth. Or do I need to screw it down & cement the layers together with thinset? Is it necessary to tile over it? Or could I use a piece of 1/4-in metal as part of the stack for weight disbursement/stability under the stove (use the 1/4-in thick metal under the thinner .26 gauge for sliding in & out).

 

[Bend]

It seems that it would be better to screw the Durock to the firebrick & then screw the additional layers down also. The tile I am considering is kind of expensive & it won't even be visible behind the insert surround; so I was thinking that I could save some money & work by using thick metal. Or if that is not a good idea, possibly I could just use a cheaper tile of a similar color.
Thanks PE, this forum is absolutely wonderful. I plan to learn a lot more as my experience evolves into burning next winter

 

[Bend]

Would it be best to lay in another layer of firebrick set in refractory cement?

 

[Bend]

Consistent with previous posts in this forum, you need thin set below and above the Durock along with screws every 8-in in both directions (every 6-in along the edges, is that correct?). This supports it from below and holds it down from above. Would it be a better job (and it seems like less work) to add a layer of firebrick instead of layers of Durock? There is an ash clean out at the back of the firebox floor. Would it be OK to cover this up? The fireplace will never be used again without an insert. Thank you for reading this and any advice you may have.

 

[Bend]

Excuse me please for having numerous posts; I appreciate very much your help. I would like to clarify my question. My insert will sit about 3/4 inside the firebox and about 1/4 outside the firebox. The hearth stack outside the firebox will be 3/4-in exterior plywood (over a firm level base consisting of one portion concrete & 1 portion wood), 1/2-in Micore, 1/2-in Durock, then thin set & tile. The tile is 5/8-thick so the total is approximately 2 1/4- in. This means the exterior hearth will be 2 1/4-in higher than the firebox floor. I would like to raise the firebox floor in order to eliminate or reduce any Hearthstone Clydesdale leveling issues. The Clydesdale has feet that can be adjusted to level the insert, but the manual does not state what range of adjustment there is available. Does anyone who has the Clydesdale know the range of adjustment on the feet?
Thank you PE for the information on the firebox floor. Since the firebox floor is made of firebrick it already exceeds the r-value requirements of the Clydesdale. So I am wondering how I should raise the firebox floor with non-combustable materials to resolve the insert leveling problem. After raising the firebox floor to the level of the hearth, the fireplace opening will still accomodate the insert comfortably. Use firebrick or Durock? If firebrick, lay in with refractory cement? If I used layers of Durock, how would they be installed? and, since it will not be visible, what is needed over the Durock stack to protect the layers from the point-loading of the insert feet? Thank you very much.

 

[Bend]

This forum is such a resource! I found the post about fireplace floor. I think I will use quick Crete mortar mix to level the firebox floor. Then put 22 gauge sheet metal over it for servicing the insert.
Thanks for all your help PE.
Also want to share my experience with ceramic insulation board. Before I found this forum, I had purchased some Fiberfrax Duraboard. I ordered it from Western Industrial Ceramics in Santa Maria CA. Then I read here that Micore 300 is better to use in the hearth construction. I found a local supplier. I am going to use the micro in the hearth construction. It seems to be firmer than the Fiberfrax.