Confused Hearth Help: Millboard, Cement Board, Durock, oh my

  • Active since 1995, is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.


New Member
Sep 15, 2017
Pacific Northwest
I'm replacing a hearth and pulled up what (I assume) was an old 1/2" millboard pad with tiles glued to it with adhesive.

I was able to find installation instructions for my original 1980s model and it mentions that the hearth extension should be:

1/2" thickness of millboard (or equivalent) or 2" thickness of brick to protect combustible floors…height not to exceed 2".

It was the 80s, so that's all the information provided.

I decided to look up one of their modern installations to see what is currently recommended. This model looked like it had similar requirements for the hearth extension.

A hearth extension projecting a minimum of 20" in front of and a minimum of 12" beyond each side of fireplace opening is required to protect combustible floor construction in front of fireplace. Fabricate a hearth extension using a material which meets the following specifications: a layer of noncombustible, inorganic material having a thermal conductivity of k = .84 BTU IN/FT2 HR °F (or less) at 1" thick.

I've got 3/8" porcelain tiles. I just need to figure out what goes underneath it. I've stumbled on several discussions but get rather lost. Can anyone help out?

Note: If important, my subfloor is 2x6 tongue and groove. There is currently an ember plate installed under the front of the fireplace: half under the fireplace, half over the subfloor.