Building a hearth extension..

[Jdsmitty10]

Hello,
I had a wood burning fireplace built in my new home. The masons built the firebox and chimney but left the hearth extension for me to do. I have been researching this and found it needs to be build out of non combustible materials(the framing). My plan was to frame it out of 2x4's and then put a 3/4 plywood down for deflection strength. Then I was going to put a layer of cement board. Then I will put 2" thick hearth stones on as the finished product. The hearth is raised off the floor. Does my plan sound safe? Would heat from the fireplace affect the wood framing with the cement board and 2" thick hearth stone? If so, What other options should I look into?
Thank you,
Jon

 

[mol1jb]

Well first I would call your local building inspector and get his take on it. Usually they are knowledgeable enough to give an opinion over the phone with a basic description. Always better to get the inspector involved early before you build anything.

 

[pvfjr]

Metal wall studs are sometimes a good option where wood can't be used.

 

[bholler]

here is the code for hearths and hearth extensions

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.

R1003.9.1 Hearth thickness. The minimum thickness of fireplace hearths shall be 4 inches (102 mm).

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

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.

It cannot be sitting on any combustible material. It should have been formed and poured as part of the slab under the hearth floor. It is going to be very hard to do properly now