Masonry Fireplace Remodel Questions

[SycamoreCA]

Hey folks,

I've read around a bit on the forums and I've found a few answers related to these questions but thought I'd try to find specifics. I'm a professional woodworker doing a fireplace remodel in my own home (built 1950, fireplace part of a remodel done in the 70s, I think) and I'm comfortable with the mantel/built-ins I'm installing around the fireplace but I'm still trying to get my head around the masonry part of this project. Anyway, I've attached a few pictures, the first is of what the fireplace currently looks like, the second is my proposed plan for rebuilding the surround, the third is my finished plan.

Here's my two big questions at this point:

(1) I need to rebuild the raised hearth. It sits about 12" off the floor and extends about 18". I've heard I can simply frame out a box using 2x4s on the existing concrete slab (buried under the carpet in this picture) and then face the box with hardiebacker, thus creating a raised hearth that I will then cover with granite or quartz. I thought, though, that the wood 2x4s under the hardiebacker would be considered combustible. Is it okay, though, to use wood framing because it would be covered with a non-combustible surface? I've also thought about using steel framing to build the box to get around this problem . . . but wood would be way easier for me. Anyone know the answer to this question or the best way to go about rebuilding the hearth?


(2) I intend to cover the existing (thrashed) brick surround with a layer of hardiebacker using thinset to attach it to the existing brick (see second picture). There are some pretty big voids in the existing brick from the demo process, though. I'm wondering what the best methods to fill these voids might be. Should I simply butter on a bunch of extra thinset or use spec-mix or another product to fill in the voids?

Thanks for the help!

 

[bholler]

(1) I need to rebuild the raised hearth. It sits about 12" off the floor and extends about 18". I've heard I can simply frame out a box using 2x4s on the existing concrete slab (buried under the carpet in this picture) and then face the box with hardiebacker, thus creating a raised hearth that I will then cover with granite or quartz. I thought, though, that the wood 2x4s under the hardiebacker would be considered combustible. Is it okay, though, to use wood framing because it would be covered with a non-combustible surface? I've also thought about using steel framing to build the box to get around this problem . . . but wood would be way easier for me. Anyone know the answer to this question or the best way to go about rebuilding the hearth?

no you cannot have combustibles in your hearth extension it needs to be all masonry

(2) I intend to cover the existing (thrashed) brick surround with a layer of hardiebacker using thinset to attach it to the existing brick (see second picture). There are some pretty big voids in the existing brick from the demo process, though. I'm wondering what the best methods to fill these voids might be. Should I simply butter on a bunch of extra thinset or use spec-mix or another product to fill in the voids?

You really dont have to fill all of the voids and I would use tapered head tapcons so you can countersink them into the backer board and leave them there.

 

[rwhite]

no you cannot have combustibles in your hearth extension it needs to be all masonry

Curious as to why. Is this code? Not doubting it, but how does a fireplace hearth differ from a hearth pad that is made from combustibles as long as it's covered with the proper r-value non-combustible material?

 

[bholler]

Is this code?

yes it is

(broken link removed to http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%2010.html)

Read R1001.9

The difference is that a hearth for an open fireplace is meant to have fire directly on it. A hearth pad is meant to have a stove on it which has been tested to find out what r value is needed.

 

[rwhite]

yes it is

(broken link removed to http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%2010.html)

Read R1001.9

The difference is that a hearth for an open fireplace is meant to have fire directly on it. A hearth pad is meant to have a stove on it which has been tested to find out what r value is needed.

Makes sense. Thanks!