Brick fireplace construction | Episode 6: Completed firebox and smoke chamber

I finish building the firebox and set the cast iron damper before transitioning into the smoke chamber.

[@miketucker31]

Very informative. Finally someone showed setting the damper and corbaling the bricks directly over the damper. Thanks

[@restlessnative781]

I'm in the process of building my fireplace and I found this video very helpful in my pursuit of that. Your explanations and craftsmanship are truly appreciated.

[@chadvaillancourt]

Good luck with your project! You can find fireplace/chimney building codes at iccsafe.org, look for chapter 10

[@MrDusa10]

Your craftsmanship and professionalism is at a whole different level, I am impressed. Looking forward to episode 7.

[@chadvaillancourt]

Thanks for the kind words!

[@stevenbyrd4310]

But thank you for the video. Thank you for putting it out there. I'm a brick Mason. I've been amazing since 1975 I worked in the Union for decades and I also headline business. I still do buildings thanks for the video once again.

[@chrisspallina1487]

Hey thank you for the video I'm building a fireplace in a couple of weeks haven't built one In a few years.. Thank you for the refresher

[@chadvaillancourt]

My pleasure, glad you find it helpful. Not many people building real fireplaces anymore. If you want to read up on the national codes just google “fireplace codes” and look for the iccsafe.org link it has everything you need for fireplace and chimney codes. Good luck with your project!

[@sfreud2079]

That smoke chamber cut out of 13x18 flue liner is a great idea.

[@Deep_Divers]

Well done video series. Very helpful.

[@geraldjones8925]

Thanks so much for the knowledge and inspiration you've provided me to build my own. I am in a metal roof post frame with hopes of transitioning to double walled stainless 12"ID for through the roof and termination. I know level, plumb & square and with the building codes I'm going in head first. Thanks

[@chadvaillancourt]

That’s awesome! Just do a google search for fireplace codes and click the Iccsafe.org link, it will bring you straight to the digital codes. Make sure you take some time to think about how you will transition from a masonry system into a mechanical system. You need to allow for plenty of movement. Good luck!

[@richmaynor8496]

Good job, and good help. Like angling the tile

[@jonathanwalker6521]

Your an artist. Great videos...

[@frankzaino]

Just watched two videos of bricklaying and both times they mortared right up to flu pipe with no gap- looks like trouble down the road!!

[@MrCurletti]

Great video!!! My favorite quote "Is money all it's about?" Any chance you could add a picture of the interior slant that is corbelled. Sounds amazing. You are a true pro and passing on a truly useful skill. thanks

[@stevenbyrd4310]

Even when you sped the film up, I can still tell. It's excruciating to watch how slow you are period

[@chadvaillancourt]

Don't watch bud. You're fast enough for both of us.

[@stevenbyrd4310]

@@chadvaillancourt Lmao

[@Nobodylikesshuli]

Haha does the GC or homeowner pay you To take your time making these

[@grind1king]

Your an amazing mason dude! Got to say the best around!

[@halfgrassgardener5060]

Impressive

[@dekonfrost7]

Really good

[@casehunter8376]

Awesome work! I’m really enjoying watching this series. I have a question. In the future when the flue for the fire place needs cleaned, how do you get the creosote out of the smoke shelf? Would it have been possible to put a clean out door there to make that easy? You are definitely a first class brick layer and I like how you pay attention to detail to make sure it’s up to code.

[@chadvaillancourt]

Very kind of you, thanks. Cleaning the smoke chamber is fairly easy through the damper with the proper tools. Most use rotary whips on drills now. Adding a clean out door would only create problems and also would not be code compliant.

[@casehunter8376]

@@chadvaillancourt Thanks for the reply. At my old house I had a stone fire place and always wondered how I would get anything out should I ever need to clean the chimney. My plan in the back of head was to take the cast iron damper out so I could get a shop vac with an exhaust hose and suck up the creosote.

[@chadvaillancourt]

This is the way

[@eastcoastfishing-simpleliv4241]

Hi Chad, thanks for sharing. I like your way of making your own smoke chamber lining. Could you share how you achieve those perfect cuts and how do you cut those 13 x 18 flue pipes (angles and measurements) to make a perfect smoke chamber lining. Thanks

[@chadvaillancourt]

My pleasure! I’m glad you like it. It’s complicated to explain, I will get this on video at some point. If you look in the comments below, I did try my best to give an accurate description on how to make the cuts. Good luck!

[@mr.p802]

@@chadvaillancourt Thanks, a video would be great. I've been building masonry fireplaces for 25 years, I usually use 8" solid masonry and corbeling to build smoke chambers, but it takes too long. I've been thinking to buy ready components and use 4 inch masonry around it, until my friend recommend using cut flue liners like you do.

[@lenleitch5917]

I have 45 years experience as a bricklayer and have never had an opportunity to build a fireplace from scratch, till now. Could you elaborate a bit on the cuts that you made in the flue tiles to create the smoke chamber? Great workmanship by the way. Len Leitch Canada

[@chadvaillancourt]

Hey Len, This sounds very exiting! It’s a little tricky to explain, but I’ll give it a shot. Let me get back to you tonight when I have a chance to sort it out. I did try to get this on video but I accidentally had the camera on time lapse. 😕

[@chadvaillancourt]

I will assume that you’re using a 13x13 flue for the fireplace. Start by choosing two 13x18 flues that are consistent in size (clay flues can very in size and shape). Try to avoid anything from Superior clay corp, their products are really irregular. Lay them perfectly flat side by side so you can visualize how things will be. I will provide rough dimensions and you will have to change slightly to whatever size flues you receive. Starting with the left flue: - from the top left measure on the top edge of the flue over 5.75” - strike a diagonal line from the bottom right corner to the mark on the top edge - from the bottom right measure up and mark at 9.25” - strike a diagonal from the bottom left corner to the mark described above - measure along the left edge from the bottom and mark at 21” (about 3” from the top) - connect this line described above with the original 5.75” line. - make sure the top and bottom line are perfectly square with the long diagonal - trace lines to the backside of flue and start cutting. - mark everything on the right flue opposite and you will have your matching pieces. This will give you a rough idea where to start. I’m sure your dimensions will be slightly different, so you’ll have to adjust accordingly. The most important part is that the transition into the flue has to be perfect so take your dimensions from the inside of the flue you are transitioning into and make sure they match with the dimensions when the smoke chamber is mortared together. I hope this helps. Good luck!

[@lenleitch5917]

@@chadvaillancourtThanks for this Chad. I am using 12x12 flue tiles (Canadian size) but the concept will be the same. I will adjust your dimensions accordingly when I order the larger f.tiles and lay them out as you discribed. If I have further questions I will get back to you. Thanks again. Len

[@oldcityoutdoors]

@@chadvaillancourt Thanks for taking the time for these dimensions. Giving this a go myself for outside fireplace build.

[@shawnc33]

Can you share more details on how you measured and cut the two 13x18 flues to achieve the 13” opening to connect a 13x13 flue?

[@chadvaillancourt]

I’ll do a video on this someday. I left a detailed comment for someone about this, I think it’s under this video, but I’m not positive.

[@Mikala-p6f]

Hi! This is so informational! Thank you for posting! We are DIYers learning as we go, so sorry if my terminology is not exact here, but i have a question! We are refacing our fireplace currently, and I've been trying to figure out how to frame the firebox with the bricks above it since there is no brick support in the opening of the firebox. I see you used a metal 90° piece to support it. What is the white fluff you applied at each end of the support? Would love more info on how to do this.

[@chadvaillancourt]

The white stuff is ceramic wool insulation. The steel is a 3"x3" (lintel) angle iron. Thanks

[@patriciaacker6390]

Didn't notice what you put under the angle iron over the opening. Was that some kind of insulation?

[@tico5126]

Hi good video thank you I née some help . Are you using an air space behind fire brick ? And have you considered using a second lintel to keep any weight off the damper (maybe the insulation is enough in that regard )

[@chadvaillancourt]

Not using an airspace in this particular case. It was built to the exact standard as it is written in the code book. There isn't any weight on the damper, so there is no need for a second lintel.

[@jeremyjohann2609]

Always fun to watch someone else work lol. Especially a quality Mason 👌. Curious though why you parge the interior un-exposed brick work?

[@chadvaillancourt]

Thanks. It’s very difficult to get all the mortar joints full on the backside of the brickwork. Code requires 4” nominal thickness for chimney and fireplaces, I feel this is the only way to ensure I’m getting the minimum thickness. It’s also great to prevent water infiltration. It only takes a couple extra bags of mortar and a small amount of time for a much better, longer lasting product.

[@nannetteaustin2308]

Fascinating but what is the white fluffy stuff you added at 5 minutes, 1 second mark? Love watching your work!

[@chadvaillancourt]

It was probably the ceramic wool insulation. It's used to allow expansion of the steel lintel and damper.

[@michaelsumerlin5189]

Chad ive watched your video several times now.I cant find where you say at what hight you start to taper the back wall of the fire box in & at what hight the dampner is (measured from brick floor ). please advise thanks.

[@Nobodylikesshuli]

I’ll tell you, I usually start sloping in around 5/6 course depending on the depth of firebox then tilt that back row about 1/4” or draw it on side wall where you want to end up and use as reference

[@joser5787]

Great video! I learned a lot about how this brick fireplaces are built. In the process of trying to mount my TV on the wall above the fireplace I found out that it was built just like that. Any recommendations on how to mount it? I mean the studs are pretty far apart, so I should try to aim for bricks?

[@chadvaillancourt]

Thanks! I’ve seen people use a rectangular piece of wood, painted black, to span the studs and make a solid mounting surface for the tv. This way has a nice clean look to it.

[@joser5787]

that’s actually a great idea, that sounds cleaner and safer. Thank you!

[@RoCDIY]

Good video. The music is out of place. Not sure if the village is getting ready to be raided or the house party is getting ready to start.

[@chadvaillancourt]

This is so funny! I’m very new to video editing and I struggle with finding/adding music. I wish KZbin would just allow us to play the music we actually want to listen to.

[@RoCDIY]

@@chadvaillancourt Same here. It is hard to find music. I just leave music out now. Keep up the good work!

[@jamesmlavelle5891]

Could you tell me how you cut the two 12x18? Just from looking at it,i can't figure it out.I'm in mA so we are not competitors.thanks

[@msciarri]

Hi Chad- beautiful work! If possible can you explain how you come up with the firebox dimensions/geometry like the width/height/depth ratios?

[@chadvaillancourt]

Thanks! The width in this case was determined by the foundation size. The foundation was only 5’ wide, I wanted 12” of brickwork on either side of the firebox so that only leaves 3’ for the width of the opening. I do prefer them a little wider if the space permits. The depth is always 20” because that’s the minimum requirement by code for a standard fireplace. Also I need to have enough room for a flue from the basement to come up the back corner. The overall height is kinda by eye depending on where the mantle height and the overall height of the room. I usually work this out with the homeowner. Also, the flue size is a limiting factor here, you can only go as high as the flue size will permit. The inside dimension of the flue cannot be less than 1/10th of the height multiplied by the width of the firebox. A Rumford style would be different The depth could be as little as 12” The back wall will almost always be 1/3 the width of the front The height is usually the same as the width There are many other factors to consider though. Mostly people can build whatever they want so long as it’s compliant. Some people only build square deep boxes that aren’t effective, but they’re compliant.

[@msciarri]

@@chadvaillancourt Hi Chad, thanks for the response- some of the nicest brick work I've seen, especially on the internal spaces like the transition into the clay smoke chamber. I also noticed that the slanted back is done with subtle corbelling. This is a beautiful detail...Is that something that you came up with?

[@chadvaillancourt]

This is something I adopted over time. I used to lean my back wall all in one smooth plane, the problem is the brickwork is not tied together with anything other than a thin bead of mortar so you get cracking. I like to do it like they would have in colonial times with the brickwork all interlocking at the corners. If I was building a Rumford the back wall would be perfectly plumb top to bottom and the corners would also be interlocking. Thanks for your kind words! And thanks for watching!

[@CanadianMason85]

I love the corbelled back and am thinking of doing the same. Do the corbels cause any issue w smoke or soot build-up? Every plan Ive seen shows a smooth slanted back with a side note explaining that lips and inconsistencies cause air/smoke flow issues. Is this the case or do the corbels "roll" the smoke like a shelf? I really respect your craftsmanship and professionalism. Thank you for providing these videos for us and in advance for any feedback.

[@chadvaillancourt]

I don’t think the corbels create more soot build up or inhibit the draft enough to make a difference. In most case’s I think building a fireplace without a slanted back is better. Personally I would prefer a Rumford style, with Laminar air flow in mind. But not everybody wants a shallow fireplace, so we slant the back wall in order to make the throat manageable. Actually, in most cases; it’s the slanted back wall propelling the heated air forward, and mixing with the cooler room air coming from underneath the lintel that causes turbulence, and a slower draft. If the heated air can go straight up without mixing, it’s definitely better for draft and you can use a much narrower throat/damper to help retain more heat in the room.

[@CanadianMason85]

@@chadvaillancourt I'm currently building my first outdoor fireplace and I've been using your videos as sort of a tutorial (I'm a lick and stick stone veneer guy so structures aren't my wheelhouse) I decided to corbel it bc of your video I think it looks better and it feels more stable. Thanks again

[@chadvaillancourt]

Agreed, anytime you can tie the brickwork together it’s going to be much stronger. Sounds like a fun project. I’m sure you’ll do a great job! Good luck!

[@CanadianMason85]

​@@chadvaillancourt kzbin.info/www/bejne/apalmXaKhdSAhcU Thanks for your help. My first one, hopefully not my last.

[@dougacker7974]

Wondering if that is high temperature mortar you use for the fire brick. Thinking of tackling an outdoor one myself. Great job by the way.

[@scottdowney4318]

IT BETTER BE, I am dealing with one built in 1973 that used regular mortar with too much spacing, the entire firebox fell to pieces. Spacing needs to be 1/4" max between fire brick.

[@darrenbond8485]

I have a wood fireplace but instead of firebricks on the back and sides it is steel. I like the look of the brick and I was wondering if you have ever converted one of these or have an idea of how to convert one?

[@chadvaillancourt]

I have done this several times with old Heatilator style fireboxes. As long as you have a masonry chase, not wood, it can be done. If you have a wood chase, you would have to demo and rebuild everything and make sure the footing is adequate as well.

[@brikkijim]

I do have a couple of questions and please do not take it the wrong way. Rumford firebox? If so why the drop down and the use of a conventional l screw damper instead of flat damper. Rumford's need a curved throat. I will start watching series from beginning to better understand what is happening here. I've built conventional and Rumford. Detailed work. Thank you.

[@chadvaillancourt]

This is not a Rumford fireplace. I will be starting one next month, that will be built in a true 1800s style, I plan to video the entire process.

[@coltonnixon9017]

If people building chimney boxes straight I'm the back why was i taught to roll the back?

[@samuelmethvin2494]

Is the insulation not flammable?

[@micahcarle1157]

Dude you make it look so easy, one question, your mortar is just type n mortar mix right? Your mud seems to stick pretty nice without falling everywhere. Do you mix it with anything else or do you just use what’s in the bag. Or possibly just your mix is looser?

[@chadvaillancourt]

Thanks! I do use type N and I add hydrated lime to the mix until I like the feel of it. It makes a huge difference

[@Cmonnow1975]

What was the what foam looking stuff you added?

[@danolson2980]

A down draft of any kind will cause smoke-back regardless of smoke shelf of any design.

[@chadvaillancourt]

I’ve seen them built both ways. Fireplaces without a smoke shelf don’t do nearly as well. I wouldn’t build one without.

[@ryanw5178]

Hi Chad, I understand the smoke chamber walls have to be 8” thick on all sides unless firebrick is used which brings it down to 6” thick (under the IRC). Are the solid bricks you use around the chamber refractory?

[@chadvaillancourt]

I wouldn’t have used firebrick around the chamber, they are too expensive for fillers. A lining of 5/8”vitrified clay also gets you down to a 6” minimum thickness.

[@ReneeEmerick-p4e]

Question: how far can you corbel each brick inside of the smoke chamber and still retain strength?

[@chadvaillancourt]

The codes regarding corbeling are written really poorly. Personally, I wouldn’t corbel more than 1”, if I needed any more than that I’ll cut an angle on it. Just use common sense, make sure the smoke chamber doesn’t go beyond 45° and smooth parge it as you go.

[@retiredperson4054]

Hey Chad -- I have a question --- You have mentioned several times about the chimney for a wood stove --- So where is this wood stove going to be located -- and how is it to be tied into a stainless chimney liner?

[@chadvaillancourt]

The wood stove is in the basement. It will make more sense in the last episode.

[@brandontrudeau5903]

How much weight would you say is on the top of a fire box? I have a customer who wants her fire box only replaced with new fire brick. (It was built with swb solid brick) I'm thinking that tearing out one wall and replacing it at a time would work. Giving each wall a day to rest before tearing the next.

[@chadvaillancourt]

It’s tough to know the weight, it all depends on how things were built. I usually will remove the entire firebox before rebuilding. When you start removing the fire brick check to see if there are solid walls of brick or block behind them. If you find that they did back the firebrick with something solid, the remaining walls should support everything above. If the space behind the firebrick is hollow (this is rare but does happen)then you’re in trouble and will have to remove and build a little at a time.

[@billdavis3957]

To me the fire box looks tiny - not deep enough for more than 2 logs wide.

[@chadvaillancourt]

You're not alone; a lot of people share this sentiment. This is why the modern fireplace has been dubbed inefficient and ineffective.

[@5050johnsmith]

But what about the ash box ones in the basment how are those made to get to the first floor ?

[@chadvaillancourt]

I don’t know what you’re asking. You may have to watch the previous video in the series for clarification.

[@hdckdsadd]

I have a question! :) I want to make a fireplace but I don't have a classical cast iron damper and I want to use a top chimney damper instead (that closes by pulling a steel cable). After I build the fire chamber and smoke shelf, I just then continue upwards with the smoke chamber? What can I do to protect from downdraft?

[@chadvaillancourt]

I wouldn’t recommend building without a cast iron damper, I’ve yet to see one without problems. Also be careful with top mount dampers, some types will restrict the flow of air because they’re smaller than the flue size, some send wind down the chimney with a like a rudder sticking out the top. If you buy a decent one, then maybe you’ll only have to deal with broken cable connections and may get a few years before the tempered stainless springs give up. I’ve installed and troubleshooted enough to not be a fan. They do seem like a good idea in principle, but nobody makes one to last, unless you never use your fireplace.

[@hdckdsadd]

@@chadvaillancourt thank you the reply. unfortunately I cannot procure a cast iron damper so I am forced to use a top chimney damper

[@chadvaillancourt]

Probably you should build something with more of a Rumford design, the back wall will be perfect plumb all the way to the top, and only have a 4” wide maximum throat into the smoke chamber. Behind the throat you can form a concave smoke shelf that will help redirect down drafts.

[@faramarzmokri9136]

How do you build a fireplace on each floor of a three story building? Thank you

[@chadvaillancourt]

It just takes careful planning, the dimensions have to be perfect. It’s a lot of work. The two lower fireplaces would be the left flue and the right flue, the 3rd floor would be the center.

[@JackJack-ld5eo]

Cored brick for a chimney is not a good choice. I know Massachusetts code allows it, but that doesn't make it right. They are prone to failure which is why more and more cities and towns do no not allow it anymore.

[@chadvaillancourt]

The truth is I've removed and rebuilt more chimneys that were built with solid brick that were crumbling than cored brick. I've also not read any standards for this country that don't allow for cored brick. If your state has adopted NFPA211 or ICC, cored brick is perfectly fine. The mortar is what causes failure. If we were all still using quicklime mortar, we wouldn't be having these issues.

[@JackJack-ld5eo]

@@chadvaillancourt Plenty of chimneys built in the 30's and 40's with modern mortar and solid brick that are still standing today. Water will always find its way into cored brick especially above the roofline where it is exposed to constant wind driven rain. It sits in the cells and freezes, which is why you always get those hairline cracks in the bricks. Massachusetts has for the most part adopted the ICC, but as I said, some of the local municipalities do not allow cored brick in chimneys. 90% of the tear downs we do are cored brick chimneys that are less than 30 years old, with some only 9-10 years old. And all from water penetration.

[@hdckdsadd]

on top of the angle iron, why there's not also fire brick?

[@chadvaillancourt]

There is no need for firebrick there, although you could certainly do it that way, you would just have to do the entire face in firebrick. Usually people prefer a traditional looking brick or stone for the finished facade.

[@mikechellino5079]

where can i find this damper

[@chadvaillancourt]

Most masonry supply stores will sell vestal or hutch dampers. Should be easy to find.

[@cancrittercancritter9360]

gj

[@notatheory4488]

All I wanna know is if chads single.

[@chadvaillancourt]

Haha. I’m happily married to a wonderful woman that is way out of my league. I’m very flattered though. Thanks for watching.

[@emirhanc10]

Is it fireproof in the mortar between the bricks?

[@chadvaillancourt]

The Firebrick are laid with high temperature refractory mortar 2500°F +/- . The mortar on the exterior chimney is just type n, supposedly rated to 600°F

[@emirhanc10]

@@chadvaillancourt Thank you for the answer.

[@MrTucodj]

Excuse me what is the name of that tube that goes first?

[@chadvaillancourt]

I’m not sure what you’re asking. You may be talking about the clay flue liner? That’s the only thing I can think it may be.

[@danolson2980]

I have 50 yrs. as mason and fireplace builder. I think it is great that you are using a Rumford style fireplace. But you made a serious error. The placement of a steel lintel above the front of the firebox is a no-no. It should be a narrow edge with a curve upwards to the damper. The square edge usually leads to turbulence and back-smoking.

[@chadvaillancourt]

Thanks, but this isn’t a Rumford style fireplace. It’s just a modern style that I build sometimes. I can’t really agree with you on the “serious error” as almost all fireplaces dating back to the 1700s have the similar 90° bend on the back of the lintel, steel or not. This is a minor concern and is almost never a problem. I’ll be building a Rumford fireplace in the spring that I plan to video. It’ll be based exactly off Rumfords writings and will illustrate laminar airflow and a curved breast as you speak of. It’ll also have a smoke shelf to counter downdrafts as he describes as well. Stay tuned!

[@Nobodylikesshuli]

You dont take criticism well mate that explains why you’re doing it wrong

[@petemorton8403]

So much space robbed. Fireplace hearth far to small

[@chadvaillancourt]

What does this even mean?

[@jamiewatkinson4123]

Did you roll the backside of the firebox because it looks straight

[@chadvaillancourt]

The back wall corbels forward somewhere between 4-5 inches I can’t recall. It could come forward more if I used a narrow throat damper.

[@jamiewatkinson4123]

Just saying start curling the back after 3rd course.

[@blackhammertribe2364]

@@jamiewatkinson4123 Wow! We have a know it all in the chat. This guy is a professional he clearly takes the time to do it correct. Maybe you should stick to hiring the professionals to do work for you because you ain’t got a clue 🤡

[@jamiewatkinson4123]

@@blackhammertribe2364and u roll the back end of the firebox on the 3rd to fourth course u shoemaker

[@reccocon3442]

A visual headache. Better to read masonry book

[@chadvaillancourt]

I’m sorry to have offended you by sharing. Maybe you have posted some of your work, or accumulated knowledge that we could all learn from? I’m always willing to learn, especially from someone who clearly knows a better way. Please share a link.

[@stevenbyrd4310]

You are taking way too long dude speed up

[@chadvaillancourt]

No

[@Nobodylikesshuli]

Typical commercial mason trying to build a fireplace, I wouldn’t recommend this

[@chadvaillancourt]

Please recommend a better video on the internet. From anywhere in the world.

[@Nobodylikesshuli]

You’re right there aren’t any and every mason does it differently 🤷🏼‍♂️

[@Nobodylikesshuli]

You do t know anything about how to build a fireplace metal gets hot and expands your cheek wall will crack first fire on top of that you shouldn’t be using cored brick

[@chadvaillancourt]

Obviously, you didn't watch the entire video. Clearly, you have never read anything on U.S. adopted codes and standards. Maybe you should make some videos so we can all learn the proper way...

[@Nobodylikesshuli]

I’m a mason not a KZbinr

[@chadvaillancourt]

It's hard to be a youtuber if you have nothing to offer. I would still watch your videos, just to gain perspective. For now, I'll keep thinking you're just another inernet coward who likes to bully people online because you wouldn't do it in person. If it was criticism, you would have context to your messages with facts to back it up. I love constructive criticism and conversations about how we can all get better. That's why I share. I'm sure you're a much better mason, I just wish you could form words or capture it on video so we could learn from you.

[@Nobodylikesshuli]

I’m just old school and dont like KZbin videos have replaced hard earned experience I do appreciate that you care about doing a quality job. I do t have any videos just pictures. If you know a way to get I touch privately I wouldn’t mind as long as your far enough away to not be my competitor

[@johnduff5674]

whats the purpose of the small square void in the right rear of the fireplace?