Hearth Construction

[Iwagner]

I'm going to be installing a Jotul C450 insert in our existing masonry fireplace. I know that the Jotul requires a hearth with an R value of 2.92. My current hearth is a plywood box with a durorock top and a layer of ceramic tile which is an R value of .027. Without sandwiching 11 sheets of durorock on top of my plywood frame, would it just be easier to build a metal stud frame and cover it with durorock and tile and use the air gap for R value? I also need to extend the hearth further out from the mouth of the fireplace. Thanks for suggestions!

 

[bholler]

Are you sure it is a masonry fireplace? I could definatly be wrong but it looks more like a zero clearancevunit to me. But if it is a masonry fireplace your hearth extension needs to meet code requirements for a fireplace. Which steels studs snd durarock do not

 

[Iwagner]

It's a floor to ceiling brick fireplace. About 15 years ago I attached furring strips to the area above the mantel and dry walled. It had a floating concrete hearth which was removed and then I built the boxes hearth. The area around the fireplace opening has Hardie backer screwed into the brick and then tiled. Attached is a picture of an identical fireplace to illustrate what it used to look like.

 

[Iwagner]

Here's a full size pic of the current design. I'm not opposed to completely tearing everything back down to the brick and starting over. Any suggestions?

 

[bholler]

It's a floor to ceiling brick fireplace. About 15 years ago I attached furring strips to the area above the mantel and dry walled. It had a floating concrete hearth which was removed and then I built the boxes hearth. The area around the fireplace opening has Hardie backer screwed into the brick and then tiled. Attached is a picture of an identical fireplace to illustrate what it used to look like.

Ok then it is a masonry fireplace. The hearth extension you constructed was absolutly nowhere near adequate and one out of steel studs and cement board is not either

R1003.9 Hearth and hearth extension. Masonry fireplace hearths and hearth extensions shall be constructed of concrete or masonry, supported by noncombustible materials, and reinforced to carry their own weight and all imposed loads. No combustible material shall remain against the underside of hearths and hearth extensions after construction.

R1003.9.2 Hearth extension thickness. The minimum thickness of hearth extensions shall be 2 inches (51 mm).

The only way you could just use cement board would be if the hearth extension was 8" or more below the hearth

Exception: When the bottom of the firebox opening is raised at least 8 inches (203 mm) above the top of the hearth extension, a hearth extension of not less than 3/8 -inch-thick (9.5 mm) brick, concrete, stone, tile or other approved noncombustible material is permitted.

 

[Iwagner]

Thanks for the info. So to summarize, if I remove my current hearth extension and measure from the floor to the hearth I have 15". I could then build a hearth extension on the floor 3/8" thick and to the appropriate dimensions and be within code? Or as an alternate I could make a flush hearth extension as long as it was 2" thick and concrete or brick. Could the flush extension be built on a steel stud box? Thanks!

 

[bholler]

So to summarize, if I remove my current hearth extension and measure from the floor to the hearth I have 15". I could then build a hearth extension on the floor 3/8" thick and to the appropriate dimensions and be within code?

As long as you insert is also ok with that yes

Or as an alternate I could make a flush hearth extension as long as it was 2" thick and concrete or brick. Could the flush extension be built on a steel stud box? Thanks!

Yes you could but it still needs to be a slab that is tied into the slab of the main hearth. We typically do this by drilling in rebar at opposing angles

 

[begreen]

Could be built on top of cement blocks too.

 

[bholler]

Could be built on top of cement blocks too.

Yes as long as it is properly anchored to the slab of the hearth

 

[Iwagner]

Thanks for the wealth of information. Think I'll build a steel stud box and pour a 2" concrete flush hearth extension slab tied into the original hearth with rebar and then face the entire fireplace with el dorado stone. Thanks again!

 

[bholler]

Thanks for the wealth of information. Think I'll build a steel stud box and pour a 2" concrete flush hearth extension slab tied into the original hearth with rebar and then face the entire fireplace with el dorado stone. Thanks again!

I personally would go to 4". 2" is pretty thin