Some guidance on replacing a wood stove hearth

CeeDee

New Member
Dec 4, 2018
32
Northern California
We're getting a new wood stove soon, we hope, and I'm tearing out the old hearth today. It takes up a lot of space, and it's off center. The former owners did this to accommodate a large couch. We don't need an air space behind and under the hearth (not a code requirement), and we also aren't required to install cement board on the back wall. But I do intend to use it in the construction.

Any flex in the floor can crack tile, I know I can put the cement board directly on the wood floor and screw it into the joists. Our subfloor is a thick, jointed wood, no plywood. I'll probably need 3.5" screws. I have a trap door to the crawl space where I can measure this and make sure I've got it right. But how do I locate the joists?

If I decide to build a platform out of 2x4s on edge (not laid flat), would I need to screw the platform to the wood floor? With long enough screws it could pass through to the jointed wood subfloor. I wouldn't know how to hit the joists. Is this necessary?

Also, I've read that cement board can be brittle, so would it be best to first lay down some plywood on top of the frame?

As for tile, porcelain is stronger and less likely to crack. But the current hearth has been there for 25 years and is tiled with faux bricks that are maybe 1/2" thick. No cracks anywhere. I like the idea of larger tiles on top of the platform and smaller ones to cover the sides of the platform and for the back wall. I just need to get an idea of how to build a strong platform, spacing between the boards, and how to ensure the tiles won't experience any movement, and then I need to chose the tile.

I'd sure welcome some suggestions. I'm taking down the bookshelves and dismantling everything today. Once the hearth is done and we finally have some heat in the house, I'm going to build some floor to ceiling bookshelves on either side. It should be nice.

Thank you.

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webby3650

Master of Fire
Sep 2, 2008
10,291
Indiana
The current setup looks pretty nice to me. Are you sure want to get into this? Looks like it was setup for side loading stove.

The cement board or base doesn’t need to be screwed into the joists. You want to bed the cement board into thinset and screw it with cement board screws on a 6”x6” grid. If you raise it, you will need to use plywood under the cement board, most definitely. What stove is going on the hearth?
 

begreen

Mooderator
Staff member
Nov 18, 2005
78,714
South Puget Sound, WA
What about just moving the current hearth over to center on the ceiling support and removing the side ledge?
 

Hogwildz

Minister of Fire
Why not just go over top of what is there, and move the hearth over to center it?
You could make new edging trim to make up for the added height, or tile it to mate to the top tiles.
 

CeeDee

New Member
Dec 4, 2018
32
Northern California
We've considered keeping the hearth with no changes or trying to move it over. The back panel is screwed to the wall with an air space behind it of a couple of inches. It comes out 2.5" from the wall to the front of the brick. I think the design is cute and cozy, and I don't like wasting materials. If I decide to replace it, I'll try to salvage what I can. I've got some ideas on how I can use it outside.

Removing the back panel would require chiseling out the brick to access the screws. There isn't any other way to do this. The platform is 7" tall and comes out 57.5" from the brick wall panel. There's a large overhang all the way around that extends 6.75" out from the base.

We can feel cold air rushing up and out of the air space behind the brick wall. With a flashlight I can see some cutouts in the wall, and I can also see on the wood floor where the hearth used to be when the house was first build in the 60s. It was centered under the pipe, but it's possible there was a wood stove insert and the wall opening wasn't covered.

I've got my tools ready and I'm feeling twitchy to create something new. But you've said exactly what I've been feeling. If I can modify the hearth and save money, that'd be great. But what I have in mind for the new hearth would be really fun for this room.

Gah.
 

jetsam

Minister of Fire
Dec 12, 2015
4,229
Long Island, NY
youtu.be
As a connoisseur, I can assure you both that you appear to have on your hands a high quality Stove Dog, and that Stove Dog wants you to complete this project as rapidly as possible.
 

CeeDee

New Member
Dec 4, 2018
32
Northern California
As a connoisseur, I can assure you both that you appear to have on your hands a high quality Stove Dog, and that Stove Dog wants you to complete this project as rapidly as possible.
Stove Dog is curled up in a tight ball on a folded tarp, sound asleep.

On her behalf, I must say that I concur.

The 20,000 cats that call me mom are also in agreement.
 

CeeDee

New Member
Dec 4, 2018
32
Northern California
Demo is way too much fun. I'm now eyeing the entire house.

I'm finding black mold under the trim around the brick wall. I think maybe the cold air I feel drafting out from the air space back there is the culprit, because I didn't see it under any of the other trim.

Condensation when it comes in contact with heat?
 

webby3650

Master of Fire
Sep 2, 2008
10,291
Indiana
Demo is way too much fun. I'm now eyeing the entire house.

I'm finding black mold under the trim around the brick wall. I think maybe the cold air I feel drafting out from the air space back there is the culprit, because I didn't see it under any of the other trim.

Condensation when it comes in contact with heat?
Don’t confuse mildew and black mold, 2 very different things that are often misdiagnosed...
 

jetsam

Minister of Fire
Dec 12, 2015
4,229
Long Island, NY
youtu.be
I'd be tempted to leave that particular air leak alone, unless you are installing an OAK. Every cubic foot of cold air that gets pulled into the stove room is a cubic foot of freezing air that didn't get pulled in on the other end of the house where there is no stove.
 
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CeeDee

New Member
Dec 4, 2018
32
Northern California
I was able to chisel off a lot of the brick and save it. Unfortunately, nothing else was salvageable. The back panel was cement board only and it broke off in huge sections. It was kind of surprising, never having worked with it before. It was both strong and brittle. The platform had a couple of problems, too.

For the new base, I'm leaning toward a 2x4 platform instead of placing plywood directly on the floor. When I bed the cement board in thinset, do I wait to nail the two pieces together? I'm thinking the nails would create a tighter seal for the thinset, but I'm not sure.

Is thinset what I should use for the tiles? I'm maxed out on YouTube videos. The eyes are going fuzzy. Also, whenever I look at plywood at the hardware store, I feel like there are lots of options. Which one should I buy?
 

begreen

Mooderator
Staff member
Nov 18, 2005
78,714
South Puget Sound, WA
Put a 3/4" plywood base CDX or BCX on the 2x4s, then the cement board. The cement board gets screwed down with special cement board screws. No nailing needed, just screw as directed (every 8" I think). Use fiberglass tape over seams and latex modified thinset.
 

CeeDee

New Member
Dec 4, 2018
32
Northern California
Put a 3/4" plywood base CDX or BCX on the 2x4s, then the cement board. The cement board gets screwed down with special cement board screws. No nailing needed, just screw as directed (every 8" I think). Use fiberglass tape over seams and latex modified thinset.
I meant screws, not nails. Does the fiberglass tape help protect against swelling or shifting so the tiles don't crack? Thanks for clarifying things for me. I'm enjoying this project and want to do a good job.
 

CeeDee

New Member
Dec 4, 2018
32
Northern California
I forgot. For the wall section, should I use plywood under the cement board? I think the added thickness would look nicer and give the cement board a large area to hold it in place, versus how my old hearth was done with the cement board screwed into 2x3" boards.
 

begreen

Mooderator
Staff member
Nov 18, 2005
78,714
South Puget Sound, WA
The cement board can be mounted right on the wall, no need for plywood there. If you want a thicker look double up on the sheets, placing the top layer at right angles to the layer underneath so that seams don't line up.
 
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Chimney Smoke

Minister of Fire
Nov 24, 2013
674
Maine
Do you know what new stove you're getting? Some require ember only protection and others need various R-values for thermal protection to combustibles. Better to know before you start rebuilding.
 

CeeDee

New Member
Dec 4, 2018
32
Northern California
We ended up buying a PE Super 27 wood stove. I got hung up on some pretty big distractions, so I'm just now starting to build the hearth. We're using strong Italian porcelain tile that looks similar to a darker slate.

At first I was going to build the base out of 2x4 studs, and then 3/4" plywood and 1/2" Hardie Board. But we're now thinking a slightly lower platform would be nice in our small family room. Can I screw the plywood directly to the oak floor, and then layer two 1/2" Hardie Boards on top? I'm wanting more thickness. Or should I do two layers of plywood?

Is thinnest a must to secure the cement board to the plywood, or can I use something like Tough as Nails?

Thank you for any help with this.
 

jetsam

Minister of Fire
Dec 12, 2015
4,229
Long Island, NY
youtu.be
Check the clearances on the stove. Plywood may not be allowable (or it may be fine).

A double layer of cement board does not help much with the need for whatever R-value the stove manufacturer specifies.

If you do end up securing cement board to plywood, use cement board screws. They should be on offer right next to the cement board. Hardie has their own variant. Skip the Liquid Nails.
 

begreen

Mooderator
Staff member
Nov 18, 2005
78,714
South Puget Sound, WA
If you want to anchor the plywood to the floor, 4 screws - one in each corner will suffice. You can use 2 layers of cement board with longer cement board screws. There's no need for thinset between the cement board and the plywood. With a screw every 8" it will lay flat and is not going anywhere.
 

CeeDee

New Member
Dec 4, 2018
32
Northern California
Check the clearances on the stove. Plywood may not be allowable (or it may be fine).

A double layer of cement board does not help much with the need for whatever R-value the stove manufacturer specifies.

If you do end up securing cement board to plywood, use cement board screws. They should be on offer right next to the cement board. Hardie has their own variant. Skip the Liquid Nails.
But what about setting the cement board in thinset on top of the plywood? Is that necessary?
 

begreen

Mooderator
Staff member
Nov 18, 2005
78,714
South Puget Sound, WA
No
 

jetsam

Minister of Fire
Dec 12, 2015
4,229
Long Island, NY
youtu.be
But what about setting the cement board in thinset on top of the plywood? Is that necessary?
Thinset is the glue that holds the tile on the cement board. Then you grout between the tiles (my hearth pad actually has thinset as grout too, since I didn't care what color it was).

I like to use polymer modified thinset like Flexbond for stuff like that, because there is a lot of weight and a lot of thermal expansion and contraction. Having a thinset that can flex a little is a big help to keeping all the tiles in place long-term.
 

Highbeam

Minister of Fire
Dec 28, 2006
16,260
Mt. Rainier Foothills, WA
My house was built with the same west coast t&g fir subfloor 2x6 planks on top of big beams 5 feet apart. You can screw to the decking directly, no need to hit the beams underneath. I chose to add additional supporting beams, posts, and pier pads below the hearth for extra support.
 

CeeDee

New Member
Dec 4, 2018
32
Northern California
I've had to delay this project, but I'm ready to get it done.

The hearth pad came together nicely. I layered two pieces of 3/4" plywood together and topped it with one sheet of 1/2" cement board. The plywood had some warping, so I used a strong adhesive to bond them together and screwed it into the subfloor. I used the same adhesive under the cement board and screwed it to the plywood in several places.

Where I got stuck is the back wall. I'd really welcome some feedback on what to do.

From what I'm seeing, there are two studs in the wall that would be behind the tile. There are probably some cross braces, but I'll need to upgrade my stud detector to find them. Instead of drywall, there's old 1/4" wood paneling. We're layering tongue and groove pine boards over that entire wall once the hearth project is finished. It's just under 1/2" thick.

I can't attach cement board to that wall and then tile. The tile would end up recessed because of the pine. We also don't like the look of a hearth wall that's too thin. It tends to look like something we bought pre-made, rather than something more structural. So, I was going to hang some 2x4s on the wall and attach the cement board to them, but wouldn't that create potential weakness for the cement board? Could it warp and affect the tile or get punched through?

It seems better to secure the cement board to either the old wood paneling and screw it into the studs, or a sheet of plywood. But for the thickness to make sense relative to the hearth, we would need two layers of plywood, which is really heavy. Would this be a problem with just two studs available? Essentially I'd be creating a second hearth pad and securing it to the wall.

(Edited to add: We've thought about putting down the pine tongue and groove first, but it won't have any strength for holding anything. It would remedy the depth issue, though. Maybe I could layer two cement boards together, which wouldn't be as heavy as plywood and wouldn't have any warping issues?)

I've been so stuck on what to do. Meanwhile, the hearth pad has been a giant luxury dog bed. It's time to reclaim it and get our lovely new wood stove installed. Thank you so much!
 
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begreen

Mooderator
Staff member
Nov 18, 2005
78,714
South Puget Sound, WA
You could put a sheet of plywood behind the cement board to build it out. Don't worry about the weight. It is supported by the floor and sheer strength of screws is high. The plywood spanning the studs will strengthen the wall a bit.