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A structural question on brick fireplaces

jonrossen's picture

This is sort of a weird question and didn't know where else to post it. 

Does anyone know if the firebricks that line the inside of a firebox are considered to be structural bricks or are they merely a veneer for heat/fire safety?  Perhaps there's no hard and fast answer but I'm hoping that someone here has done some demo on a fireplace chimney at some point and may recall something from how the bricks fit together.

The fire bricks are different in length and width from the regular common bricks that make up the fireplace/chimney. At this point I don't know how thick they are since I haven't removed one yet. From what I've read they typically are 2.5" or 3" thick. However, the main concern is whether they tend to be an integral part of the structure or more likely merely a veneer put on for heat and fire safety...or maybe both.

The reason for the question is I want to install a gas insert fireplace unit and the smallest one I could find is still just a bit too's very, very close.  Based on my drawings it seems that if I removed some of the fire bricks it would be a good fit.  The guy from the fireplace shop is doing a much expected CYA routine about removing the bricks....and I frankly don't blame him.  He is the installation mgr. who was on vacation two weeks ago when I was in the shop talking to other folks there who seemed to have a more cavalier attitude towards making modifications to the inside of the firebox.  I tend to trust his opinion over his co-workers (since he manages the installation efforts) and am hoping he'll be more encouraging once he sees my really accurate CAD diagrams showing the small conflict in sizing.

Much thanks in advance for any info or ideas.  Cheers, jonR



They are a veneer and have (post #216198, reply #1 of 14)

They are a veneer and have nothing to do with the structure.

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

It depends on the (post #216198, reply #2 of 14)

It depends on the construction.  There are several different ways to build a "brick fireplace".

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Nope. Fireboxes are always (post #216198, reply #3 of 14)

Nope. Fireboxes are always constructed inside the structual envelope so they can expand and contract. 

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Thanks to both DanH and (post #216198, reply #4 of 14)

Thanks to both DanH and Florida for their feedback; much appreciated.

Florida, I hope you are correct.  However, let me give you some more info---this fireplace/chimney is the original one in a house built in 1910.  Your point about the fireboxes constructed inside the already independent structural envelope is a good once.  However, that may be what is done today or maybe started 30 to 40 years ago to allow for contraction and expansion, but what about houses built in the early 1900s?  Could that concept not have been 'in play' way back then around 1910?  Just wondering if your point about needing to have the firebox apart from the structure to allow for contraction and expansion is best practices but may not have been adopted in 1910.

A fireplace/chimney guy with whom I spoke to over the phone didn't want to commit to telling me anything either way.  But what he did say is that if the firebricks go up to and seem to be supporting the lintel that spans the firebox opening, then that means that the firebricks in the firebox are indeed structural.  However his reasoning seemed to be more circumstantial, abstract and not necessarily based on reality. The metal lintel could be imbedded into the structural bricks inside the wall and the firebricks could merely be underneath just because that arrangement is what is needed to cover the entire inside of the firebox.  Again, his theory and/or reasoning on this seems a little bit abstract and 'out to lunch' and riddled with assumptions.

I have no idea how thick fhe firebricks in an old house typically are.  I've read that they are 2.5" to 3" thick.  I can certainly see that they have a different height and width dimension as the common bricks of the chimney/fireplace assembly.  I may be able to figure out the depth once I remove an abandoned section of galvanized gas line that is sticking into the firebox.

Thanks again for the feedback, and am wondering if your info applies to very old houses such as mine built in 1910.


Okay, let me re-phrase. In 50 (post #216198, reply #5 of 14)

Okay, let me re-phrase. In 50 years of remodeling old houses I've never seen one with structural firebricks. I've taken apart and rebuilt a lot of old fireplaces but all the fireboxes were isolated from the structure. The smoke shelf is part of the structure and the firebox is built after the structural part of the fireplace is built. Expansion from the heat in the  firebox would destroy the whole thing otherwise.

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Thanks for the additional (post #216198, reply #6 of 14)

Thanks for the additional clarification.

The smoke shelf you refer to is the name that back sloping wall of the fire box that is sort of a 'bump' in the back wall of the chimney, right?  In other words if you cut a section (perpindicular to the width of the firebox opening) through the fireplace / chimney assembly, you'd see that 'blip' known as the smoke shelf.  So, I take it that any fire bricks on the surface of the smoke shelf could be removed, correct?

I did see for mself that the fire bricks are indeed as you say.  I removed the old gas line that was sticking into the fireplace and took the opportunity to examine the hole in the firebricks where it the pipe had entered.  Then I was able to quickly remove that brick without much effort.  This brick was on the side of the firebox at the bottom.  I think tapped with a smal hand held sledge on an adjacent fire brick and that came out fairly easily after about 12 hits of the sledge hammer.  There was a bit of a 'gap' behind these fire bricks and appeared they were indeed not part of the overall structure.

If you were to look down the (post #216198, reply #7 of 14)

If you were to look down the chimney from the top you'd see the smoke shelf at the bottom.  The idea is that any cold air coming down the chimney hits the smoke shelf  before it falls into the fire box. If you lay on your back and look up the fire box you'll see that it's offset from the chimney. Typically, if you have a damper, that's where it will be. If you don't now would be a good time to add one.

The air space between the fire bricks and the structure is there to stop the structure from degrading due to the heat from the fire box.

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Thanks again for this new (post #216198, reply #8 of 14)

Thanks again for this new additional info.

I haven't looked down from the top but I have looked up from the fire box and can see the shelf and that from how it's built there's an offset..  I reached my hand up in there briefly and felt something 'metallic' which I guess could be the damper.  I need to do some more investigating but first need to put down some plastic.  There's a bunch of crap/soot on top of the shelf that I"ll be knocking down while I root around up there.

There was one other question that I asked that you didn't directly address in your last post.  It's about the firebricks that are on the back of the fire box and that go up and cover the underside of the shelf structure.  By 'underside' I'm referring to of course not the actual horizontal shelf itself, but the part that starts sloping towards the firebox opening that creates the geometric possibility that there can be a shelf.  The firebricks that appear here can also be removed correct?  As any firebricks that aren't structurally important, the ones going up the back slope area of the firebox can be taken out.

Can't recall whether I stated this before, the reason why I want the option of removing firebricks is that I'm installing a gas-insert fireplace unit and may need more room to get the unit in place.  Also, because of this a damper is not something that I need or want as this unit will have intake and outflow ducts going up and out the chimney.  The other space concern for this ducting is the size of the 'throat' (where the offset is), and this is why I was asking about the fire bricks on the backside.  If those can be taken out at the top, right befire the shelf, that will give me more room for the ducting.


You can remove any of the (post #216198, reply #9 of 14)

You can remove any of the firebricks without any concerns. The actual strucre may be bricks, concrete blocks or any combination.  Most of them look like a mess but it doesn't matter.

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Thanks again for your (post #216198, reply #10 of 14)

Thanks again for your additional info.

Geez, the guy that I"m working with at this gas insert place keeps 'moving the goal posts on me'.  Now that I've tackled his structural questions/concerns, he introduces another one.  One that he *never* mentioned before.  Now, the concern is whether the manufacturer of the unit will stand by their unit in this type of installation (removal of some of the fire bricks)

The issue seems to be this:  Gas insert units are designed to go into masonry fireplaces that had been previously used / rated for fire (wood burning).  But what does that mean?  Does it mean that you merely need a masonry fireplace?  Or does it have to be fully intact with all the fire bricks as well?  It should be obvious that the fire protection needs for a wood burning (with an open flame) fireplace would be much higher than a gas insert fireplace unit.  Yet, these companies may take the easy way out and give an overly 'Cover your a**' (CYA) blanket statement that doesn't address what I need to do ( remove some of the fire bricks).  In my case there still will be LOTs of masonry around the unit, no wood or other combustibles.  There of course will be some missing fire bricks and that's the ONLY difference.

What's your positionon something like this?  Do you think it is being overly paranoid to worry over removal of fire bricks in a fireplace where there will be no open flame actually touching any of the bricks? 

Thanks for any info.

I've installed half a dozen (post #216198, reply #11 of 14)

I've installed half a dozen gas fireplaces overthe years and all of them have been    wood. Usually we lined the fireplace area with Type X drywall.

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Are the units you are (post #216198, reply #12 of 14)

Are the units you are referring to the type known as 'zero clearance'?  The general type that I'm talking about are called 'gas fireplace inserts' and are designed to go in an existing masonry fireplace, typically one that was formerly used to burn wood.  These inserts *don't burn wood* but instead use fairly realistic ceramic logs and the end result is it looks like a real fire.  They don't require the addition of any type of fire-resitant verneer such as the type x drywall you mentioned.

People use the gas inserts that I'm referring to for numerous reasons:  1) If your existing masonry fireplace and chimney over the years has become unsafe for burning, renovating it may be too expensive so a gas insert could be more affordable. 2) You can use them even on 'no burn' days (different regions in the country differ on how many of these days there are..depends on air quality and of course the local politics) 3) Related to #1: Many jurisdictions won't issue building permits to renovate an existing masonry fireplace / chimney because they don't want people to burn wood.  This includes permits for new construction....a permit won't be issued on any design that calls for a masonry wood burning fireplace.

Stop dealing with the sales (post #216198, reply #13 of 14)

Stop dealing with the sales people and call the manufacturer directly. 

Interesting that you mention (post #216198, reply #14 of 14)

Interesting that you mention that:  I have; and it was ridiculous.  I felt like the character Yossarian in the movie/novel Catch 22. :-)

The manufacturer didn't like the idea of removing bricks and didn't want to think about it rationally.  Instead they did the typical 'CYA' position and didn't even want to discuss it.  ....'We need to have a complete masonry fireplace.''

To back up a bit, the person I was dealing with at the shop at the end was the installation manager, NOT a salesperson.  He was technically savvy but too scared to want to deal with removing some of the fire-brick veneer.  It was he who suggested I call the manufacturer, and I think he did so fully knowing they would just brush me off.  The tech support guy at the manufacturer sounded as if he would have preferred to be water-boarded rather than to rationally discuss the issues around removing a few fire-bricks.

However, I found a couple other shops and they are more reasonable.  I have one of them coming out for a site visit.  They agree with Florida here that fire-bricks are never structural.  So, from that aspect, it's OK to remove them.  Secondly, since fire-bricks are for protection from an open-flame type of fire, they are not needed for fire protection in a self-contained gas insert unit.  So both conditions and issues have been addressed.  I should add that after the fire-bricks are removed, I stil have lots of masonry (common bricks) around the insert fireplace unit.