Hearth extension - can I put concrete backer board over concrete hearth + subfloor?

[breathebeast]

This place looks awesome. So glad to find all the good information. We're getting a Regency I2400 for our fireplace. I had planned on just re-tiling our existing hearth originally, as well as the surround, but it turns out that the hearth needs to be extended about 7". The fireplace is on the second floor. The original hearth is concrete. I've been wrestling with what to do for the past week or so.

To extend the hearth I'm either going to have to put concrete backer board over subflooring (or, it sounds like, over another layer of plywood over the surrounding subfloor) AROUND the concrete hearth, and then tile over the two different substrates (the guy at the local tile store that does installation thought that would be fine since the house is over 50 years old and "should be done settling," but I'm still a bit worried about it in terms of tiles breaking over time). Or I'm thinking of breaking down the top inch or so of the concrete hearth using a rented SDS hammer drill (sounds like a pain, but doable), smoothing it over with concrete or thinset so it's level with the surrounding subfloor, and putting concrete board over the whole shebang so the tile is on one substrate. Any thoughts on what would work best? A little complication: new hearth width will be 61.5", so sounds like one piece of Durock (if I can find any) wouldn't be long enough, and I'm going to have a concrete backerboard joint in the new setup no matter which route is chosen.

The third alternative, of course, is to get an extension pad, and just tile over our existing concrete hearth. Pulling out the pad for when the woodstove is used. Not so keen on this, or a raised hearth, because the room involved is our main living space and the preschooler and dogs are pretty active there.

Just as an aside, also planning on tiling the surround, but didn't want to remove the mantel. The current surround is spray-painted brick. Going to try using a wire brush to remove the paint (wish me luck!). The same tile guy mentioned above said there was no need to move the mantel to put the tile in, but This Old House recommends it. Any thoughts?

Thanks so much for the help!

 

[Beetle-Kill]

breathebeast- Welcome to the forum. You're correct, this place not only looks awesome, but is! (OK Craig, pay up) I'm way too braindead from my own hearth project to answer you, but others with vast amounts of experience will chime in in short order, be patient. Good luck, JB

 

[firefighterjake]

I'm pretty dense . . . would it be possible to post some pics of what you're dealing with . . . this may help some of us visualize things better and be able to offer some good advice.

 

[breathebeast]

Thanks for the welcome! Can't believe i've got replies already - you guys rock. It's been hard to get info on hearths....

Here's the fireplace and current concrete hearth pad. And the view of the framing for the concrete hearth pad from below - it was poured after the hardwood floor was put in place and fits the hardwood floor, is level with the hardwood floor (for now). Someone suggested I take out the current concrete hearth and frame and pour a new concrete hearth! Which sounds lovely, only I fear i would break my house trying that project.

The current concrete hearth is 16" x 51". With the woodstove insert in place, we need 25" from the front of the fireplace (18" + the front door of the woodstove protrudes 5-7"). If I removed the wood floor just in front of the concrete hearth, I would need to remove the wood to the side of the hearth, because the wood framing just in front of the hearth protrudes ~2" beyond it to make a corner within the next plank of wood flooring. So, with 25" extending in front of the hearth, we'll also need to widen the hearth by 5" on each side (that'll make it even with the mantel pillars). Definitely more complicated than I had originally thought.

On the bright side, Home Depot says they have the Durock, though they did call it cement board....

 

[fire_man]

I'm pretty dense . . . would it be possible to post some pics of what you're dealing with . . . this may help some of us visualize things better and be able to offer some good advice.

Glad to see I'm not the only nimwit - the pictures really helped :-S . I have the exact same problem, need to extend the hearth with a similar situation in case I get the new bigger Woodstock Stove. There should be some good advice posted for this, I have been scratching my head, too. Messing with the hardwood floor is the tricky part for me.

 

[begreen]

The 2400i projects a good distance into the room. It needs hearth protection with an R = .6 value. If the desire is to keep the hearth flush with the floor, it's going to be tricky because it will still need a minimum of insulation board, cement board substrate and then the tile. An alternative is to not go flush and either raise the hearth enough to give it visual prominence or to raise it slightly and have the flooring frame it with a bevel from the flooring level to height of the tile. Let's go with the last option as a for instance. Lets say it takes 3/8" micore + 1/2" cement board + 1/4" tile and the current flooring is 5/8" above the subfloor. (It was 3/4" flooring, but let's say it's been sanded down to 5/8"). The hearth will have a layer thickness of 1 1/8". That means the hearth floor frame bevel will need to go 1/2" from floor level to the tile height. If this is done over a 2" wide bevel, it will rise 1/4" per inch.

 

[breathebeast]

Thanks for finding the data for the woodstove - and interpreting it for me! If the firebox itself, which looks to be brick, is only 1/4" higher than the hardwood floor, is it possible to create a raised hearth (even just 1/2", which doesn't sound too bad) without putting the insert out of balance? Any thoughts about whether it's necessary to break the concrete hearth down and use a single piece of concrete board versus building a "moat" of concrete board and insulation around the current concrete hearth?

I'd tried looking into edgings that would cover the edge of the cut wood floor, had decided on cutting it as well as possible and possibly extending bullnose tile slightly over it (knowing that will weaken the tiles). I couldn't find any tile bevels (besides the 1/4" provided by the bullnose tiles). When you talk about a bevel, would that be like wood molding or a metal bevel - possibly like what I see at the junctions of the hardwood and linoleum floors in our house?

Also, I read an earlier post which suggested that putting the cement board over 3/4" plywood subfloor is not enough support. Would putting down some plywood over the subfloor be necessary, or can it somehow go below the subfloor and still add stability?

 

[CarbonNeutral]

I had no issues with r-value, but I had a very similar situation. I have hardibacker over both the concrete and subfloor sections. No issues to speak of. My subfloor is panels, not plywood.

 

[breathebeast]

Carbon Neutral, great to hear that that solution worked for you. Did you use thinset under the cement board?

 

[midwestcoast]

I can't answer all your questions, but I have the same insert & some similarities in install. First I would try to determine the exact construction of the existing hearth so you know whether just keeping it as-is is an option. When I installed I drilled two small holes through my tile & concrete hearth to determine thickness of concrete, measured the heck out of things from above & below & made a sketch of it all to get my head around it. Next calculated the R-value of the existing hearth (concrete is something like R-0.08 per inch). My FP is on the first floor & I had some brickwork below it, so the R-Value came out okay & I didn't need to rip it up. Based one the photos, my GUESS is you won't have the req'd R0.6 on the existing hearth so better to take it all down to the subfloor or joists & build back up as a single unit with sufficient insulation. I know that sounds like a pain, but it's easier to do it now...

Now, about that mantle, I suggest you poke around behind it to see if there are combustible materials in the wall near the req'd clearance above the unit. Reason being it looks very similar to my FP & I chose the I-2400 partly because the mantle clearance is low & I could get away without pulling it off. Didn't work out so well: I pulled a piece of trim off top & sides to get the minimum clearance and started burning. A few days later the wife noticed a burning smell & I traced it to the lower facing of the mantle. Off came the mantle after all & I found that the exposed brick (looking rather like yours) was cased in with 1x3's and outside that was paper-faced gyproc over brick. The 1x3 across the top (behind the mantle) was charred black & would have certainly ignited at some point, starting a fire behind my mantle
:grrr: That was just about 1/4" PAST the req'd face (trim) clearance above the unit, but so much heat was being tranferred through the brick it was charring from the back side. I pulled all the plaster, gyproc, lumber.... off of the brick well past the required clearance, raised the mantle and reinstalled with an air gap behind it to the rough brick, detached the trim "feet" on the mantle legs & moved them down to cover the new gap below the legs. Looks good now with a bit of rough brick exposed below the mantle, & more importantly, it isn't threatening to burn my house down.
Best of luck, hope I didn't scare you

 

[begreen]

Thanks for finding the data for the woodstove - and interpreting it for me! If the firebox itself, which looks to be brick, is only 1/4" higher than the hardwood floor, is it possible to create a raised hearth (even just 1/2", which doesn't sound too bad) without putting the insert out of balance? Any thoughts about whether it's necessary to break the concrete hearth down and use a single piece of concrete board versus building a "moat" of concrete board and insulation around the current concrete hearth?

I'd tried looking into edgings that would cover the edge of the cut wood floor, had decided on cutting it as well as possible and possibly extending bullnose tile slightly over it (knowing that will weaken the tiles). I couldn't find any tile bevels (besides the 1/4" provided by the bullnose tiles). When you talk about a bevel, would that be like wood molding or a metal bevel - possibly like what I see at the junctions of the hardwood and linoleum floors in our house?

Also, I read an earlier post which suggested that putting the cement board over 3/4" plywood subfloor is not enough support. Would putting down some plywood over the subfloor be necessary, or can it somehow go below the subfloor and still add stability?

You could build up the firebox floor too with a sheet of 1/4" metal or 1/2" cement board if desired.

If you look at your current floor, there is a picture frame edging around the old hearth. That is the same idea I am talking about, but it would be thicker on the hearth side like a bevel. This would have to be custom made, but can be done pretty easily on a table saw. Or have a woodworker make them up for you. Or you could box frame it with an unbeveled, straight stock, frame.

 

[breathebeast]

Okay, so I measured the depth of the concrete hearth, and it goes from ~9" at the deepest near the fireplace to ~6" at the farthest point, would just about keep the r-value at 0.6 as required. Still, if I take off the top 1" or so and cover with micore and cement backerblock, even better!

As far as the mantel goes, good to know the heads up! Is a bit scary....

 

[midwestcoast]

You're looking pretty good at that thickness IMO. Metal on the fireplace floor to level it out will reflect some heat back at the stove too (can do the same of FP walls if you're ambitious). I guess it's just up to esthetics on how you finish it. Edging the heart could be as easy as some Ranch Stop or 1/4 Round cut with a Miter box.

 

[midwestcoast]

double post

 

[elmoleaf]

A couple of points:
1. Typically a concrete hearth extension is supposed to be integral to the structure of the chimney...it generally is cantilevered out from underneath the firebox. In other words, if none of that wood framing was there, the concrete shouldn't fall down. Not sure if yours is typical or not, but removal probably a bad idea.

2. Building codes today generally require that wood forms used to form the hearth extension be removed after the concrete ihas cured. This never seems to be done however, as is the case with yours. If possible you might investigate removal of formwork for added peace of mind.

I had the same issue as you...concrete hearth extension about 5/8" below level of surrounding white oak wood floor, existing footprint too small for new stove.
My stove only required ember protection, no r value. I cut out my finished oak floor to the sides and front of fireplace and exposed the plywood subfloor below. I then spanned the concrete hearth extension and surrounding subfloor with concrete board. This was topped by granite tiles, which raised top surface slightly above adjacent finished oak floor. I left a 1/2" gap between tiles and oak floor, into which I fitted some slightly beveled wood trim finished to match the floor.

 

[breathebeast]

Well, got some feed back from another tile specialist, who agreed with the owner of the tile shop that there's no need to remove the top of the concrete hearth for the tile's sake, which was what I was originally thinking (worried that the concrete and backer board would shift and cause tile cracks). Both tile guys now have recommended just making sure the cement backer board is stabilized with thinset and screwed, and that fiberglass tape is placed across the joints. Even with the 12" x 12" tiles we'll be using, they both still thought it would be fine. So, much less work for me! We'll be leaving our concrete hearth intact and placing backer board around it.

Elmoleaf, the only difference is that our concrete hearth is actually level with the hardwood floor. That was kindof why I was thinking of trying to break down the surface, to get it level with the subflooring, but now instead I'll be working around it since the tile guys said it shouldn't be a problem and the r-value is fine.

Interesting about the hearth form. I'm kindof scared it would fall down if the wood were removed. I assume the concrete was poured long after the foundation, since it was poured level with the hardwood floor and exactly fitting the picture frame around the hearth.

BeGreen (or anyone else, for that matter), wondering if I could use 1/2" micore + 1/4" durock cement backer board underneath the tile at the foremost 9" of the hearth, making my insulation/non-combustible layer equal the height of the concrete hearth (hopefully). Our Home Depot has the old-style 1/4" durock. Practically speaking, does the thinset between the backer boards and the sublayers take up height?

I also got some clarification on the issue of putting more plywood subflooring under concrete backer board, as was brought up in another thread I read late at night yesterday. Apparently, before putting tile down, you're supposed to have more than an inch (I think an inch and a quarter?) of subflooring (such as plywood) to prevent any bounce that might cause tiles to pop off or crack. This second tile guy wasn't too worried about that from a practical stand point, but did know where the concern arises from. Still wondering if one can reinforce the plywood from below somehow, but it doesn't make sense to me, more wishful thinking.

 

[traviswalken]

If this has already been covered, sorry.

I noticed in a prior post you were planning to provide 18" of hearth in front of the door.

I recently purchased a Hampton insert. It also requires 18". HOWEVER, if your hearth is flush with the floor, Hampton requires 19.5" in front of the glass. I don't know if this is a code requirement or a Hampton requirement.

I wouldn't want you to extend your hearth only to find out you were short of code.

**edit** I just noticed you were planning a Regency...Hampton is a Regency product. Be sure to read your hearth requirements closely. I know I missed this requirement the first couple times I looked at it.

 

[breathebeast]

Traviswalken, thanks for the heads up. I had noticed that some of their wood stoves required more clearance than 18", but not this one. However, read a few others posters on other threads recommending using whole tiles rather than cutting them if possible, so we could definitely extend it a bit further than planned just in case, or to expand future options....

 

[breathebeast]

Well, no source for ceramic board yet, will keep trying.

Read on the Hearth Wiki (great resource, sorry I found it late!) (1) that the ceramic board itself is kindof soft, so wondering if 1/4" cement board would be too little to stabilize for the tile, especially given that we'll be walking across it, and 1/2" as originally suggested by BeGreen would be best; and (2) that there is often decking below the subfloor (that often equals 1.5 inches), meaning that in many houses there would be no need for additional plywood to stabilize underneath tile and prevent bounce and tile breakage.

Of course, as soon as some areas start to seem clear, others get hazy. In this instance, flooring! I started wondering about the need for an expansion gap between our hardwood floor and tiled area. We did not have one before, but the hardwood abutted solid concrete. I read another thread where the poster didn't put an expansion gap in, but he poured a whole new concrete hearth so the hardwood might've been abutting concrete, not quite sure. Anyway, any guideline as far as when an expansion gap is needed? I bought before I researched on this one, so we're getting bullnose tiles that might have to be returned, but not the end of the world. The other benefit of doing an expansion gap and molding is it would help resolve the issue of how to get the concrete backer board to abut the hardwood planks on either side, which have little shelves on them for the nails - or can those be chiselled off without damaging the floor?

 

[begreen]

I'm pretty sure that Micore or Fiberfrax is available in CT. Hopefully someone local to there will chime in soon. It is soft and you'll definitely want to use 1/2" cement board on top of it.

 

[breathebeast]

Everyone I call for micore is either whole sale (no individual sheets sold) or haven't been able to get stuff from USF for a year (so they say). Time to start asking for fiberfrax duraboard....

 

[begreen]

Try:

E J DAVIS CO
10 DODGE AVE
NORTH HAVEN, Connecticut 06473
Phone: 203-239-5391
Fax: 203-234-7724
Website: http://www.ejdavisinsulation.com

 

[breathebeast]

Ah, EJ Davis is the only one I couldn't get through to about whether I could pick some up, but they DID say they carried it...will try them again tomorrow.

 

[breathebeast]

Can get it! It costs 3x the real price, 'cause there's a minimum order, but at least we can get it. And they carry 1/2" and 3/4" but not 3/8". Interestingly, saw that someone else got it off ebay, and sure enough, it's available there, too.

Which raises the questions. If the base of my tile is raised 1/8" over the wood floor, can I place caulk or grout around the hearth to hide the exposed edge of concrete backer board? Or do I need to look into shallow beveled wood? And is it possible to use the thinset to make up the 1/8" difference in height between the concrete hearth and the 1/2" ceramic board + 1/2" concrete board (the hardwood floor measures 7/8", or at least the concrete hearth measures 7/8" above the subfloor).

 

[begreen]

Finishing is up to you. A beveled frame as previously described is the way a good floor finisher might do it. But there are alternatives. Maybe look at 3/8 thick tile and use a small 1/4"round bead to trim it?