Q&A / 

How to Build A Masonry Fireplace

masonry fireplace cast iron damper smoke shelf fire brick

Odds are you’ve never seen what’s above and behind the mantle of your masonry fireplace. Behold the magic of why the fireplaces in the houses I built never smoked.  Note how high the bottom of the cast-iron damper is in relationship to the bottom of the concrete lintel that forms the top of the opening of the firebox. This separation is very critical. Copyright 2022 Tim Carter

How to Build a Masonry Fireplace - It's Very Complex

The two main reasons for smoke in his home were the lack of a combustion air intake and his chimney was not the correct height. A few of his photos also showed a very dangerous situation where his flue liners were not surrounded by solid masonry, rubble, and mortar. A hot ember could make contact with wood framing with little difficulty.

Over the years I’ve visited homes that have fireplaces built by amateurs or paid masons that have no clue how to build them. You may not think it’s that hard to build a fireplace but it’s actually very technical. The width of the fireplace opening controls all the other dimensions.

The Brick Industry Association (BIA) should be your go-to source for all the information you need if a traditional masonry fireplace is in your future. I’ve used their specifications for over 40 years to construct smoke-free fireplaces in all of the homes I’ve ever built.

I urge you to go to their website and download the free technical notes publications about masonry fireplaces. They are:

  • #19 Residential Fireplace Design
  • #19A Residential Fireplaces, Details and Construction
  • #19B Residential Chimneys - Design and Construction

These publications are written so a layperson can understand them with ease. The illustrations and tables allow you to easily visualize exactly how a masonry fireplace should be constructed. If you have a smoking fireplace, you’ll no doubt discover the cause of why you’re coughing by using the three publications. I guarantee they’ll help you do an autopsy on your fireplace.

When you dive into the documents, you’ll discover that the shape of the firebox is mission-critical. The sides have to be angled and tapered to the rear of the firebox. The rear wall needs to be vertical rising up from the floor, but then at a certain height, it needs to start to gently slope towards the front of the fireplace.

The position of the damper is extremely important. Not only does it need to be at least 8 inches above the opening of your fireplace, but it also needs to be forward and close to the room. By default, this creates the smoke shelf behind it. It’s far more complex than you probably ever realized.

One of the biggest mistakes I routinely see when homeowners send me photos is the use of regular mortar to set the actual high-temperature firebrick. That’s a huge mistake. The firebrick are supposed to be set using fire clay. Regular mortar will crumble in short order. If you see sand in the mortar joints in between your firebrick and the joints are about 3/8-inch tall, you’ve got the wrong mortar.

Fire clay is a powdered clay that’s mixed with water. It’s rich in both aluminum and silica and can withstand decades of high temperatures inside the firebox. The fire clay joints are very thin, normally less than 1/8-inch in height. A mason that knows what she/he is doing carefully dips the side of the brick that will touch another one into the soupy batter.

bag of fire clay

Fire clay comes in bags like this. CLICK HERE or the image to have it delivered to your home.

Your mason may build the firebox correctly and yet your fireplace still smokes. If that’s the case, you then need to see what went wrong with the chimney. The size of the flue is directly related to the size of the firebox. All of these specifications can be found in the BIA Technical Notes.

Don’t overlook the height of the chimney. It’s very important because of downdrafts created by nearby trees or parts of your roof. All of this is covered in the documents.

Last but no least, be sure that the mason builds the chimney crown properly. Based on my lifetime experience I’d say that less than one percent of all chimney crowns are constructed properly. To see how to do it, pay close attention to the diagrams in the #19B document. Don’t forget the all-important drip kerf under the crown overhang!

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