Hearth Expansion Underlayment

[agramovot]

Good morning everyone.

I am re-tiling my hearth and have a question about underlayment.

House is 100 years old and the old hearth has a massive 6 inch thick poured concrete slab under it. They built a "box" that is visible from the basement that holds the concrete. The old tiles were 6x6, new tiles are 8x8, so the footprint has expanded past the edges of the existing concrete slab. I have already removed the wood flooring and exposed the tongue and groove subfloor, all in great shape.

My question is what to do for underlayment? Should I just trim cement board to the dimensions of the wood subfloor, and lay a bed of mortar on the existing concrete pad to level it all out? Cement board the whole thing, bed it with mortar, and screw it into the joists?

Thank you in advance, I have read a lot about underlayment over wood or concrete, but never both.

 

[bholler]

Good morning everyone.

I am re-tiling my hearth and have a question about underlayment.

House is 100 years old and the old hearth has a massive 6 inch thick poured concrete slab under it. They built a "box" that is visible from the basement that holds the concrete. The old tiles were 6x6, new tiles are 8x8, so the footprint has expanded past the edges of the existing concrete slab. I have already removed the wood flooring and exposed the tongue and groove subfloor, all in great shape.

My question is what to do for underlayment? Should I just trim cement board to the dimensions of the wood subfloor, and lay a bed of mortar on the existing concrete pad to level it all out? Cement board the whole thing, bed it with mortar, and screw it into the joists?

Thank you in advance, I have read a lot about underlayment over wood or concrete, but never both.

In order to meet code the underlayment needs to be that concrete slab. There can be no combustible materials touching the bottom of that hearth extension

 

[Ashful]

I've seen the hearth extension boxes you describe, but never on anything as new as your house. I have seen them on houses form the 1750's, but not houses I have owned myself, and I always wondered if they were later additions.

To elaborate on what bholler is saying, while a stove may only require ember protection, any hearth extension to a fireplace must meet the code for a fireplace. That means no combustible support structure. I believe the reason for this is that you do not know what future owner may remove your stove and use that hearth as an open fireplace, again.

Of course, you could argue that the existing extension is already combustible, and clearly grandfathered. If it were to be extended farther, you could also argue with yourself that it's no less safe than what you have now, in fact you coul say it is more safe for your intended "ember only" purpose. Again, you could argue it with yourself, just don't try arguing it with the code enforcement officer.

 

[bholler]

I've seen the hearth extension boxes you describe, but never on anything as new as your house. I have seen them on houses form the 1750's, but not houses I have owned myself, and I always wondered if they were later additions.

To elaborate on what bholler is saying, while a stove may only require ember protection, any hearth extension to a fireplace must meet the code for a fireplace. That means no combustible support structure. I believe the reason for this is that you do not know what future owner may remove your stove and use that hearth as an open fireplace, again.

Of course, you could argue that the existing extension is already combustible, and clearly grandfathered. If it were to be extended farther, you could also argue with yourself that it's no less safe than what you have now, in fact you coul say it is more safe for your intended "ember only" purpose. Again, you could argue it with yourself, just don't try arguing it with the code enforcement officer.

What they have isn't a hearth extension box. It it a form for the concrete hearth extension slab which should have been and still can be removed. The old hearth extension boxes were typically sand filled. And once you modify it it is no longer grandfathered in. You need to bring it up to code.

We also don't know that this is for installation of an ember protection only stove. Nothing indicated that

 

[Ashful]

We also don't know that this is for installation of an ember protection only stove. Nothing indicated that

Yep, that’s why I said “may”, not “does”.