Concrete Hearth Extension

[sykocus]

I need to make a hearth extension for my fireplace. I'm thinking to just pour a concrete slab. My wife and I both like the look. The slab would be about 6' wide, 5" thick and extend 16" from the fireplace. I'm thinking to drill some holes laterally into the existing hearth. Epoxy some rebar into it and to tie it into the hearth extension. My subsfloor is OSB. I've seen "Redguard" used as a sealant in some videos. That's sort of my game plan but any advice is appreciated.

 

[begreen]

Is this a Zero Clearance wood-burning fireplace or an existing masonry fireplace?

 

[EbS-P]

I was concerned about the weight but my math says less than 600 pounds. I don’t see an issue there. Personally I think if you are goin up 5 inches you have space to frame it out with metal studs. Durrock and finish with your choice of material. I have never poured concrete that looks good enough to have in my living room. Concrete maybe less work and cheaper. Floor will move some eventually so where do you want the cracks to be? Plan ahead.

What is your reason for the extension?
Evan

 

[EbS-P]

I've seen "Redguard"

It’s a waterproofing liquid applied membrane probably not necessary here.

 

[begreen]

There are some basic rules if this is a masonry fireplace. The extension depth and width depend on how large the fireplace opening is. 16" if for under 6sq ft, 20" for over 6 sq ft. Combustible materials must not be under the completed hearth extension after construction. The exception being if the fireplace hearth is >8" above the hearth extension.

 

[sykocus]

Is this a Zero Clearance wood-burning fireplace or an existing masonry fireplace?

Existing masonry

I was concerned about the weight but my math says less than 600 pounds. I don’t see an issue there. Personally I think if you are goin up 5 inches you have space to frame it out with metal studs. Durrock and finish with your choice of material. I have never poured concrete that looks good enough to have in my living room. Concrete maybe less work and cheaper. Floor will move some eventually so where do you want the cracks to be? Plan ahead.

What is your reason for the extension?
Evan

I thought about that but I have kids so at the very least it will be walked across. I thought the flexing of backboard and about 1/2" of mortar/thinset/something would lead to heavy cracking maybe hole sections chipping off. The slab, while I know cracking can't be completely eliminated can't be completely, would remain structurally intact.

The long story is a 1970's home that had a sunken living room with bout a 10"+ tall extension. We demoed the extension and the floor was leveled with the rest of the area.

There are some basic rules if this is a masonry fireplace. The extension depth and width depend on how large the fireplace opening is. 16" if for under 6sq ft, 20" for over 6 sq ft. Combustible materials must not be under the completed hearth extension after construction. The exception being if the fireplace hearth is >8" above the hearth extension.

I must have missed that rule. I saw where it had to be a min of 2" thick and thought I was good. What If I poured it over a layer of cement board?

 

[begreen]

Is the fireplace hearth now 10" above the floor level? If so that is the exception. Can you post a picture so that we see what you are working with?

 

[bholler]

Existing masonry

I thought about that but I have kids so at the very least it will be walked across. I thought the flexing of backboard and about 1/2" of mortar/thinset/something would lead to heavy cracking maybe hole sections chipping off. The slab, while I know cracking can't be completely eliminated can't be completely, would remain structurally intact.

The long story is a 1970's home that had a sunken living room with bout a 10"+ tall extension. We demoed the extension and the floor was leveled with the rest of the area.

I must have missed that rule. I saw where it had to be a min of 2" thick and thought I was good. What If I poured it over a layer of cement board?

No combustibles can contact the underside of the slab period. All framing and subfloor needs to be removed. And then either a removable wood form or one made out of metal needs to be put in place for the concrete to be poured in. And yes it needs anchored back to the main hearth slab

 

[sykocus]

Is the fireplace hearth now 10" above the floor level? If so that is the exception. Can you post a picture so that we see what you are working with?

Here's the original and what it's like now. This is what is looks like now sorry there's a tv in the way. About 5-6" below the current floor is the old subfloor. When we did the demo there was a gap of of about 6" in front of the fireplace opening that was outlined in masonry (cement block?) and the old extension must have been built out from that.

 

[begreen]

So the floor was built back up about 4" after the demolition so that now the hearth is about 6" above the floor? What is below the sub-floor?

 

[sykocus]

Basically what you said. The current floor is about 5" above the original and the hearth is about 5" above the current floor.

The whole stratum would look something like:
Flooring
OSB Subfloor
Floor Joist
Original Particle Board Subfloor
2x4 tongue and groove treds <--I'm guessing about this based on other parts of the house
Crawspace

I just found a picture from when we demoed the old extension

 

[EbS-P]

What is your goal? To use as fireplace again? Ambiance or heat?

If so I would be looking at a hearth installed stove or insert and then build extension to meet install requirements of the stove/insert???? Not even sure that wood be up to code since the original hearth has been removed. Other experts can offer info there.

but let’s say stove just needs ember protection, that’s easy then you can just trim the exposed unfinished areas with non combustible material say the same tile as you ember protection(hearth extension) ??? Some inserts need a specific R value for the extension but that’s still doable keeping the total thickness under 2”. Really different than you were originally thinking but it seems like less work. But making it a functioning fireplace is lots of work. Have you had the flue clean and inspected?
Evan

 

[Rearscreen]

If you can get over the combustible hurtles underneath, an alternative could be to pour the slab in a melamine form then flip the concrete slab over. A little diamond disk work, then carnauba wax and you have a glass like finish.

 

[sykocus]

What is your goal? To use as fireplace again? Ambiance or heat?

If so I would be looking at a hearth installed stove or insert and then build extension to meet install requirements of the stove/insert???? Not even sure that wood be up to code since the original hearth has been removed. Other experts can offer info there.

but let’s say stove just needs ember protection, that’s easy then you can just trim the exposed unfinished areas with non combustible material say the same tile as you ember protection(hearth extension) ??? Some inserts need a specific R value for the extension but that’s still doable keeping the total thickness under 2”. Really different than you were originally thinking but it seems like less work. But making it a functioning fireplace is lots of work. Have you had the flue clean and inspected?
Evan

We want a functional fireplace. My wife wants a fireplace for the ambiance and the the whole activity. By that I mean she enjoys building the fire tending to it, etc. We're getting a cleaning and inspection in a few weeks.

 

[begreen]

To do this correctly for a functioning fireplace, follow the guidance that bholler provided in his posting.

 

[TomasB]

We want a functional fireplace. My wife wants a fireplace for the ambiance and the the whole activity. By that I mean she enjoys building the fire tending to it, etc. We're getting a cleaning and inspection in a few weeks.

So, what did you end up doing, did it pass inspection, etc?