# Pouring concrete for new hearth



## garimh (Jun 10, 2017)

I'm a newbie and took on the project of replacing the tiles on my fireplace hearth. As things do, it's gotten a bit complicated.

The tiles were brick inlay that were embedded in the concrete or mud base and were not coming off so I ended up removing everything expecting to hit a flat wood or concrete base. Instead I reached this concrete hump





and this is what it looks like underneath from a closet in my basement. 





My plan was to fill the heath in with concrete and then tile on top but having never poured concrete before and knowing that if I end up with a slab that it not level or the exact height I need, I'll need to break the whole thing out again, I have the following concerns:

 - should I be concerned about the weight of the concrete?  Can I assume that since the previous concrete and brick held for the last 50 years that it can support the weight of the new concrete?

- the wood frame in the hearth looks solid but should I be concerned about possible pressure from the concrete buckling the wood?

 - because I want the new tile to be flush with the wood frame on the floor, the height of the slab will need to sit below the surface to account for height of tile and thin set. How do I screed below the surface? I thought about cutting notches in the ends of a 2x4 so the middle portion sits below the surface. Is that the right approach? 

Watching some slab pouring videos online, it essentially seems to be pour, distribute the concrete, screed, smooth out and float. Am I missing anything here?  Anything else I should in keep in mind so I don't screw this up?

Lastly, my other option is hire a pro to come in and lay the slab. Given my concerns and relative inexperience here, should I consider that or is this not that complicated a job?


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## WoodyIsGoody (Jun 10, 2017)

garimh said:


> I'm a newbie and took on the project of replacing the tiles on my fireplace hearth. As things do, it's gotten a bit complicated.
> 
> The tiles were brick inlay that were embedded in the concrete or mud base and were not coming off so I ended up removing everything expecting to hit a flat wood or concrete base. Instead I reached this concrete hump
> 
> ...



It's not that complicated of a job so I would do it myself. But I wouldn't pour concrete in there, way over-kill. Are you sure what you took out was actually concrete? Concrete has different sizes of aggregate in it. It's hard to see exactly what you have going on there but, if it's what it looks like, I would fill it with dry pack which is nothing more than damp coarse sand with a little portland cement powder mixed in. It looks like the floor on the far end of the hearth needs some added framing (joist) to support it.

The great thing about dry pack is it has excellent compressive strength but, if you ever need to remove it, it comes out a whole lot easier than concrete. And it's cheap to make. Use a good angular sand (sometimes called torpedo sand or concrete sand) 5 parts sand/1 part portland cement. This video shows how easy it is to mix and apply:



The video recommends 4:1 mix but I would go for 5:1 or 6:1. It will be easier to work and will be plenty strong. Make sure you don't get it too wet. You can always spray it down once it's hard if you want to make sure all the cement has been activated. After it's just set up but before 24 hours or so, it's easy to scrape off any high spots you may have missed using a regular scraper (like a spatula but stronger). When it has set you can tile right to it.


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## bholler (Jun 10, 2017)

WoodyIsGoody said:


> It's not that complicated of a job so I would do it myself. But I wouldn't pour concrete in there, way over-kill. Are you sure what you took out was actually concrete? Concrete has different sizes of aggregate in it. It's hard to see exactly what you have going on there but, if it's what it looks like, I would fill it with dry pack which is nothing more than damp coarse sand with a little portland cement powder mixed in. It looks like the floor on the far end of the hearth needs some added framing (joist) to support it.
> 
> The great thing about dry pack is it has excellent compressive strength but, if you ever need to remove it, it comes out a whole lot easier than concrete. And it's cheap to make. Use a good angular sand (sometimes called torpedo sand or concrete sand) 5 parts sand/1 part portland cement. This video shows how easy it is to mix and apply:
> 
> ...



The hearth extension should be a concrete slab that is tied into the hearth.  Actually it is supposed to all be poured as one slab but it rarely is.  Dry pack was done for a long time but a solid slab is much better and is actually required by code now.


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## bholler (Jun 10, 2017)

garimh said:


> I'm a newbie and took on the project of replacing the tiles on my fireplace hearth. As things do, it's gotten a bit complicated.
> 
> The tiles were brick inlay that were embedded in the concrete or mud base and were not coming off so I ended up removing everything expecting to hit a flat wood or concrete base. Instead I reached this concrete hump
> 
> ...


No it is not that complicated but you should drill holes into the existing hearth floor at varying angles to tie your new slab. And your old slab together.  It really does not have. To be perfect you can make up some inconsistencies with your thinset.


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## garimh (Jun 10, 2017)

Thanks a lot for the reply.  Below is a photo of what I pulled out of the hearth. That tile bit at top was definitely not going to separate but I regret now not just busting off the top of the tile bit and they laying down the dry pack you described over the rough remaining part. 

Regarding the dry pack, I've done a bit of googling and from what I can see, dry pack is not meant to be used over a thickness of 2 or sometimes 3 inches. Is that right?  I need about 4-5 inches of filling and as much as 7 inches on the right side (side away from the firebox).


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## WoodyIsGoody (Jun 10, 2017)

bholler said:


> The hearth extension should be a concrete slab that is tied into the hearth.  Actually it is supposed to all be poured as one slab but it rarely is.  Dry pack was done for a long time but a solid slab is much better and is actually required by code now.



Ha! I didn't even consider that it was going to be used as an open fireplace. Maybe because I dislike open fireplaces with a passion.

If he want's to meet current code, won't he have to extend it?


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## bholler (Jun 10, 2017)

WoodyIsGoody said:


> Ha! I didn't even consider that it was going to be used as an open fireplace. Maybe because I dislike open fireplaces with a passion.


It doesnt matter how it will be used it is part of a fireplace and needs to meet that code.  But i agree i am not a fan of open fireplaces.  Other than we make allot of money fixing them because masons dont know how to design or build them properly


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## garimh (Jun 10, 2017)

If by open fireplace, you mean one without glass doors, then no, it will not be an open fireplace. But sounds like code says it has to be concrete either way.


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## WoodyIsGoody (Jun 10, 2017)

bholler said:


> It doesnt matter how it will be used it is part of a fireplace and needs to meet that code.  But i agree i am not a fan of open fireplaces.  Other than we make allot of money fixing them because masons dont know how to design or build them properly



Isn't it only a fireplace until it's not a fireplace anymore? What is involved in making it not a fireplace? I would think blocking the flue and installing a  woodstove would mean fireplace code wouldn't need to be followed (seeing how it would no longer be a fireplace).

It looks to me like it doesn't meet current code for a more fundamental reason. The hearth looks awfully shallow and there is a wood floor there.


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## bholler (Jun 10, 2017)

WoodyIsGoody said:


> Isn't it only a fireplace until it's not a fireplace anymore? What is involved in making it not a fireplace? I would think blocking the flue and installing a  woodstove would mean fireplace code wouldn't need to be followed (seeing how it would no longer be a fireplace).


Nope it is still a fireplace and most inserts require a code compliant fireplace anyway.  But from the sound of it the op is going to use it as a fireplace.


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## WoodyIsGoody (Jun 10, 2017)

garimh said:


> Thanks a lot for the reply.  Below is a photo of what I pulled out of the hearth. That tile bit at top was definitely not going to separate but I regret now not just busting off the top of the tile bit and they laying down the dry pack you described over the rough remaining part.
> 
> Regarding the dry pack, I've done a bit of googling and from what I can see, dry pack is not meant to be used over a thickness of 2 or sometimes 3 inches. Is that right?  I need about 4-5 inches of filling and as much as 7 inches on the right side (side away from the firebox).
> 
> ...



I don't know much about current code but, from a practical standpoint, it should be fine as long as you pack it as you go. In other words, don't just fill it to the top and pack it from there. Dry pack has very low shrinkage (lower than concrete) so it shouldn't have cracking problems. It also has a lower thermal conductivity coefficient vs concrete so if it's an open fireplace I don't know if that would be a problem. I suspect not but...

I can't tell what substance you pulled out of there except that it's cementious. It does look like it has some aggregate larger than sand. Whatever it is, it looks like it worked just fine.


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## WoodyIsGoody (Jun 10, 2017)

bholler said:


> Nope it is still a fireplace and most inserts require a code compliant fireplace anyway.  But from the sound of it the op is going to use it as a fireplace.



It doesn't meet current code now, does it?


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## bholler (Jun 10, 2017)

WoodyIsGoody said:


> It doesn't meet current code now, does it?


With no hearth extension no


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## garimh (Jun 13, 2017)

Why does it now not meet code?  The depth on either side of the middle hump bit is about 7 inches and the hump itself is brick (see photo). The width of the hearth also extends 16 inches on either side of the firebox. Where is the code problem? 

Also, on the pouring of concrete, am I able to just pour it in as is or do I need to first level the base somehow (i.e. with sand?). I'm assuming not and that the concrete will fill in the depth differences but just want to make sure. 

Thanks


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## bholler (Jun 13, 2017)

garimh said:


> Why does it now not meet code?  The depth on either side of the middle hump bit is about 7 inches and the hump itself is brick (see photo). The width of the hearth also extends 16 inches on either side of the firebox. Where is the code problem?
> 
> Also, on the pouring of concrete, am I able to just pour it in as is or do I need to first level the base somehow (i.e. with sand?). I'm assuming not and that the concrete will fill in the depth differences but just want to make sure.
> 
> Thanks


It doesnt pass now because you have a big hole in your floor where your hearth extension should be once you pour your new slab it should be fine as long as you dont leave wood forms under it.  And it extends out far enough from the firebox

And yes you can just pour it in.  I would drill and pin your new slab to the old but otherwise just pour it.


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## Supersurvey (Jun 17, 2017)

For anyone who removed bricks from the rear of a fireplace to make an insert fit, needs to reinstall them so the fireplace is code compliant?


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## bholler (Jun 18, 2017)

Supersurvey said:


> For anyone who removed bricks from the rear of a fireplace to make an insert fit, needs to reinstall them so the fireplace is code compliant?


If you still have enough masonry between the insert and any combustible material then no you are fine.  Without fire brick you need 10" of solid masonry between the insert and any combustible material


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