Need to repair or replace my fireplace hearth before installing new woodstove insert. Ideas?

[Rome]

The fireplace in our new home is not currently functional, and we decided to install a woodstove insert. The Hearth is damaged and needs to be repairs or more likely replaced, and I'm looking for some ideas.

As you see in the photos, the current hearth is a large number of flat pieces of stone, all set together with wide mortar joints and they are almost all cracked. Some pieces are completely loose (maybe 7 pieces), example shown. Under the layer of stone appears to be a layer of concrete/mortar that was put ontop of the subfloor to level the area for the stone. This also appears to be cracked, broken.

I initially thought I would just get a clear epoxy and cover it all over as is to seal it all back up, but am now thinking replacing the entire hearth would be a better idea. I have done basic floor tile work before and could lay the tile. I don't know what I'm walking into though with attempting to demo the current stone and concrete base on the subfloor, and am worried I am biting off too much of an unknown project.

Looking for ideas, tips, or pointers. Thanks in advance!

 

[DAKSY]

I would recommend removal of everything in that area. You won’t know what you need to do until you get to the final underlayment. Once everything is out of there & you can see what’s left, a final course of action can be decided upon.

 

[Rome]

I would recommend removal of everything in that area. You won’t know what you need to do until you get to the final underlayment. Once everything is out of there & you can see what’s left, a final course of action can be decided upon.

Finally got around to getting this all out.

There is a 5" thick concrete slab in the center, and then wood subfloor around that as pictured. Where the granite mosaic was done over the wood, it all cracked. With my new clean slate my plan is as follows, but heavily comes from AI research so I want to verify before proceeding.

1. Cut out the remaining piece of damaged subfloor on the right side and replace with new plywood.

2. Circular saw the extra layer bits of subfloor on the far left side to be flush with the hardwood surround.

3. Use the concrete no sag sealant tubes to fill in all the little gaps between the wood and the concrete and the brick face

4. Use the polymer modified thinset across the entire space level

5. Lay the uncoupling membrane layer into the thinset, to prevent future cracking with the different subfloor types.

6. After that dries, run another thin layer of the thinset across the top to fill the bubbles of the membrane, making a smooth surface for brick and tile work.

7. After that dries, use type S masonry mortar to lay the new clay bricks against the current firebrick floor, extending that level plane out far enough for the front of the insert to sit on, should only take 1 brick row, perpendicular to the fireplace. Do this in the width to span the inserts metal faceplate so it sits on the brick and doesn't appear to float over the hearth.

8. After that dries, use my thinset to lay porcelain tile surrounding the new brick step and filling the rest of the hearth space, level with the hardwood floor. This will prevent toe kicking the hearth constantly if the whole thing was 3 inches higher to match the brick.

9. The faceplate for the stove we are buying is unfortunately giant, 50", which will partially cover the vent grates. Will need to modify the faceplate to make it less wide, since Drolet with the 44" faceplate is completely out of stock.

Does this all sound right? Questions I have are mostly about the membrane. I only became aware of this through the AI back and forth. I am also tempted to just break out the floor firebrick, hope it's concrete straight through, just lay the porcelain tile throughout and not have to worry about building the brick step to level the stove. But not sure if it runs under the sides of that current metal firebox sides.

 

[begreen]

I'm no mason, but isn't uncoupling membrane typically plastic? If so, that's a no-go here. The hearth will get hot. A sheet of cement board would be safer.

 

[megszinNH]

Masonry requirements are spelled out in woodstove insert manuals. For example, the Jøtul C 550 V2 requires a structurally sound, code-approved masonry fireplace, a Level II NFPA 211 inspection, no damaged masonry, and a properly installed clay or UL 1777 stainless-steel liner. The manual also says not to remove masonry unless it can be done without weakening the structure.

If you know which insert you’re installing, I’d check its manual. As a 48-year, still-working court reporter who has heard plenty of heartbreaking stories about fires and deaths caused by improper hearth installations, this is one job I would not DIY.

 

[EbS-P]

I’d fill holes and pour self leveling mix it’s quite runny. Measure my total depth. Add cement board if I hag it. Even then piece of it fits and thinset tile on top.

 

[ScottLewis]

The question is, Why did the hearth crack into pieces?
Improper subfloor.
Do what I did, go into the basement and double the three joists under, or, closest to, the hearth. Assess the plywood, it probably is 1/2 inch.
Add another half inch of plywood. My subfloor is 3/4 inch Advantech.
My hearth hasn't cracked in 11 years and a 430 pound wood stove sets on it. With the 2 inch thick rock my hearth alone weight 200 pounds.

 

[Rome]

The question is, Why did the hearth crack into pieces?
Improper subfloor.
Do what I did, go into the basement and double the three joists under, or, closest to, the hearth. Assess the plywood, it probably is 1/2 inch.
Add another half inch of plywood. My subfloor is 3/4 inch Advantech.
My hearth hasn't cracked in 11 years and a 430 pound wood stove sets on it. With the 2 inch thick rock my hearth alone weight 200 pounds.

The previous install poured mortar right over the concrete slab and surrounding wood subfloor. My understanding is as the wood subfloor expanded and contracted it cracked all the mortar joints in the granite mosaic. The pieces over the concrete slab were still literally rock solid.

That's why I was recommended the uncoupling membrane, to handle the disparite expansion of the 2 subfloor materials. If that's a bad idea I'd love to not have to do it. My original plan was to put a piece of hardibacker tiling board over the slab and subfloor parts, and then do the brick and tile in my plan on top of that. In the basement there is already 2 steel posts with structural iron supporting the floor underneath the concrete slab.

 

[Rome]

I’d fill holes and pour self leveling mix it’s quite runny. Measure my total depth. Add cement board if I hag it. Even then piece of it fits and thinset tile on top.

I think there's some typos in this post, but if I'm reading it right, you would fill the holes, self leveling concrete mix, then cement board (like hardi or duroc) over that, then tiles over that?

Do you use the self leveling mix as the thinset to place the cement board, or wait for the self leveler to dry, then use thinset to lay the cement board?

 

[ScottLewis]

Oops. I see now, the hearth sets on a 5 inch thick concrete slab. Disregard my post.

 

[Rome]

I'm no mason, but isn't uncoupling membrane typically plastic? If so, that's a no-go here. The hearth will get hot. A sheet of cement board would be safer.

So would the steps be instead then to:
1. fill the holes and gaps
2. self leveling concrete mix to make a fully level smooth surface
3. Then cement board (like hardi or duroc) over that
3. then brick and tile over that?

Do you use the self leveling mix as the thinset to place the cement board, or wait for the self leveler to dry, then use thinset to lay the cement board?

Here's an AI rendition of approximately what I plan to have as a finished project, ready for the stove install

 

[bholler]

The question is, Why did the hearth crack into pieces?
Improper subfloor.
Do what I did, go into the basement and double the three joists under, or, closest to, the hearth. Assess the plywood, it probably is 1/2 inch.
Add another half inch of plywood. My subfloor is 3/4 inch Advantech.
My hearth hasn't cracked in 11 years and a 430 pound wood stove sets on it. With the 2 inch thick rock my hearth alone weight 200 pounds.

On a fireplace hearth extension there should be no wood under it. It needs to be a self supporting slab

 

[begreen]

On a fireplace hearth extension there should be no wood under it. It needs to be a self supporting slab

I'm not positive without looking underneath the hearth, but in the picture, it looks like the original hearth has the 16" requirement met. He wants to extend past the required 16" to fill the gap left by the previous job.

 

[EbS-P]

I think there's some typos in this post, but if I'm reading it right, you would fill the holes, self leveling concrete mix, then cement board (like hardi or duroc) over that, then tiles over that?

Do you use the self leveling mix as the thinset to place the cement board, or wait for the self leveler to dry, then use thinset to lay the cement board?

You have to full hole sole the self leveling won’t run through. I’d use it as the base for the cement board. Secured in some way. The cement board is now fully supported by the self leveler and is the expansion break. It still might crack out of the floor deflection is substantial.

 

[Rome]

I'm not positive without looking underneath the hearth, but in the picture, it looks like the original hearth has the 16" requirement met. He wants to extend past the required 16" to fill the gap left by the previous job.

That 5 in thick slab in the middle is about 20" deep to the fireplace face., and then there is about 5 inches of subfloor plywood until it reaches the hardwood. That subfloor piece on the right is warped and damaged, so replacing that and making it level to the concrete slab

 

[Rome]

You have to full hole sole the self leveling won’t run through. I’d use it as the base for the cement board. Secured in some way. The cement board is now fully supported by the self leveler and is the expansion break. It still might crack out of the floor deflection is substantial.

Ok. What I meant was would you pour the self leveler and then lay in the cement board while its still wet, using the self levelr as a thinset of sort to secure the cement board? or wait for the self leveler to dry and then use a regular thinset on that to lay and secure the cement board?

 

[EbS-P]

Ok. What I meant was would you pour the self leveler and then lay in the cement board while its still wet, using the self levelr as a thinset of sort to secure the cement board? or wait for the self leveler to dry and then use a regular thinset on that to lay and secure the cement board?

Let the self leveler set and cure. I’m not sure how would bond the cement board. epoxy? Thinset? To the concrete screws to the subfloor.