Fireplace Fireboxes (A Complete Guide)

In Indoor Fireplaces, Wood Burning Fireplaces by James O'KellyLeave a Comment

The firebox in a fireplace can be considered as the most important part or aspect of any open wood burning fireplace.

A firebox is where all fires are built and managed and so need to be of sufficient size and made from non combustible materials.

The firebox also brings all of the other main components of a fireplace together, including the hearth, fireplace opening with surround, and the chimney, into one complete fireplace unit.

We’ve therefore put together this complete guide to explain exactly what a fireplace firebox is, what fireboxes typically look, what materials they’re typically made from and what the codes and regulations are for them.

What Is A Fireplace Firebox?

The firebox in a fireplace is the area behind a fireplace opening where a fire is built and maintained. A firebox is typically built from masonry materials and will have a hearth at the base, masonry back and sides, and an opening to the chimney at the top.

All indoor open wood burning fireplaces will have some form of firebox.

A fireplace firebox is the area located behind the fireplace opening. A fire is built and maintained within the firebox area.

The firebox of a fireplace can be defined as:

The box or chamber containing the fire’.

The labelled image below shows where the firebox is located for our own living room fireplace in comparison with the other main components of a fireplace.

Fireplace Surround Labelled
The firebox of a typical open masonry fireplace

The firebox, as the name suggests, is typically a box formation with a square or rectangular shape.

A fireplace firebox can typically be found as one of two main types:

  • Masonry
  • Prefabricated

Fireplaces are traditionally constructed on site and built using bricks and mortar, while prefabricated fireboxes can also come from the factory and installed as complete units.

The CSIA explains that a masonry firebox can typically be identified by being built of individual firebricks with a pyramid shaped chimney rising up above the damper, also built of brick, and that a prefab fireplace can typically be identified by its firebox made from cast refractory panels with metal components potentially visible, and a round metal chimney visible above the firebox.

The diagram below shows how a firebox typically looks in relation to the other components of a masonry fireplace.

Fireplace & Chimney Diagram
Where the firebox sits within the whole fireplace environment

See another one of our articles for more information and explanations on all the parts of a fireplace and chimney.

The main components of a firebox will typically include:

  • A flat base (which may form part of the hearth)
  • Sides and back (typically made from masonry materials such as brick)
  • An open front (known as the fireplace opening)
  • An open top (where the firebox meets the chimney)
Firebox Labeled Diagram

As a firebox is where the fire is built and maintained, the materials that make up the surrounding walls and floor need to be non-combustible. We’ve discussed what materials are typically found in and around a firebox further in this article.

Fireplace Firebox Base

The floor of a firebox can also be known as the hearth or the inner hearth.

The hearth typically sticks out into the room by a certain distance (in line with local regulations) to help protect any combustible materials used within the floor of the room, such as carpet or wood.

A as fire will be started on, or near to, the floor of the firebox, it will need to be made from a non-combustible material.

Fireplace grates are also commonly used with open fireplaces and a grate will be placed on the floor of the firebox. We use a grate when having fires in our living room fireplace.

Fireplace Logs
A fireplace grate sits at the bottom of the firebox

Common materials found at the base of a fireplace include concrete, brick and stone, but may also include common hearth materials such as granite and ceramic tiles.

For the five of our own fireplaces, we have:

  • Three that have concrete firebox bases.
  • One stone firebox base.
  • One tile firebox base.
The concrete floor of one of our fireboxes

In many fireplaces an ash dump can be found located beneath the firebox.

For fireplaces with ash dumps, there will typically be some sort of grate or hole located on the floor of the firebox, usually within the center or more towards the back, where excess ash can fall through.

Fireplace Firebox Back & Sides

The back and sides of a firebox in a fireplace need to be made from a non-combustible much like the floor, but the sides and back can be more commonly made from bricks.

This brick material lining the walls of a firebox can also be referred to as firebrick.

Brick is commonly used within fireplace firebox construction because of its availability, ability to cope with high temperatures and overall cost-effectiveness.

In the firebox of all of our own fireplaces, brick has been used for the sides and back walls.

Brick Fireplace Surround
Brick firebox back and sides for a disused fireplace
Multi Fuel Stove Hot Coals
Brick firebox in which we installed a multi fuel stove
Our brick firebox in which a wood burning stove was installed

We really like the traditional natural brick look for our fireplaces, but the brick firebox for one of our fireplaces had already been painted black when we bought house. We’ve since touched up the black paint with heat-resistant paint spray to match the black surround and hearth.

Fireplace Screen
One of our fireboxes is painted black

Fireplace Firebox Opening

The opening of a fireplace firebox is simply the open area at the front of the firebox where it meets the outer wall of the fireplace to the room.

Fireplace openings are usually completely open, allowing you build and maintain fires, and clean the firebox from outside the fireplace. However, glass doors can also be installed into a fireplace opening to help keep fireplaces hidden away when not in use.

A fireplace screen is commonly placed within, or just in front of, the fireplace opening to help prevent any hot embers from coming into any combustible materials such as wooden flooring or carpet.

Fireplace Screen
Our fireplace screen placed in the opening at the front of the firebox

Read more about fireplace screens in another one of our articles here.

At the base of a fireplace firebox opening will be the hearth, while the sides of the opening will form part of the firebox sides.

At the top of a firebox opening will be the lintel.

A lintel is a long piece of strong or reinforced material that helps to spread the loads from above the fireplace opening down through the sides of the firebox.

Common lintel materials include steel and concrete. Our own fireplaces have steel lintels in various forms, one of which is shown below.

Fireplace Lintel Steel
You’ll commonly find a lintel at the top of the firebox opening

This particular lintel is hidden behind a timber board for aesthetic purposes. Looking from the front of the fireplace it gives a traditional timber lintel look. For more information on lintels see our guide to fireplace lintels here.

Fireplace Firebox Top

At the top of the firebox is where the fireplace meets the chimney.

A chimney allows for smoke and waste gases from a fire to safely leave a home. The top of a fireplace therefore needs to be open where it meets the throat of the chimney.

The top of the firebox for our own living room open fireplace is shown below with the connection into the chimney above.

Looking up the chimney from the firebox of our open fireplace

Just above where the firebox meets the chimney will typically be where the damper is located (if a fireplace has one). A damper is a plate that can be opened or closed to help prevent cold drafts between fires. For more information we have a complete guide to fireplace dampers here.

The top of a fireplace firebox may also be blocked off with a closure/register plate when a form of wood burning stove is installed.

The image below shows the register plate installed at the top of the firebox when our multi fuel stove was installed.

Register Plate
If a stove is installed within the firebox then the chimney may be capped off with a register plate

This register plate helps to ensure that any air going up the chimney flue is through the stove. We have another article explaining register plates for fireplaces and stoves here.

Difference Between A Fireplace And A Firebox

A fireplace can be defined as the whole area dedicated for use as a place where a fire can be started and includes all of the components required to ensure that a fire can be maintained safely and efficiently.

A fireplace can include all of the main components such as the hearth, throat of the chimney, surround or mantel, and incorporates the firebox at the center of it all.

On the other hand, a firebox is the place where is a fire is started and maintained, and does not typically include other features outside of this area including the outer hearth, fireplace surround, chimney and damper.

Difference Between A Fireplace Insert And Firebox

While the firebox is the area located inside an open fireplace, a fireplace insert is considered to be another form of fireplace that can be inserted into the firebox. Common types of fireplace inserts include wood burning, gas and electric.

The firebox of an open fireplace is the open area where the fire is built and maintained, either on a fireplace grate or on the floor.

A fireplace insert is a type of fireplace that can be inserted into the firebox of an open fireplace.

For example, our old gas fireplace insert had been placed inside the firebox of our kitchen open fireplace.

The old gas insert for our kitchen fireplace

Open fireplaces are traditionally very inefficient at producing heat for a room and so fireplace inserts offer a way to help either improve the effectiveness of burning wood or changing the nature of the fireplace altogether to use another type of fuel.

Common types of fireplace inserts for placement in open fireplaces include:

  • Wood burning inserts
  • Gas fireplace inserts
  • Electric fireplace inserts

These types of inserts typically take up the whole opening of a fireplace firebox and may fill up the majority of the width and depth of the firebox, depending on the dimensions of the firebox and whether any clearance is required out the back of the insert.

A fireplace insert itself can also have its own firebox.

For example, if a wood burning insert is placed inside the firebox of an existing open fireplace, this new wood insert will have its own firebox in which fires can be started (as the original masonry firebox is cut-off and contains the new insert).

Wood burning stoves can also be installed within existing open fireplaces (including multi fuel stoves and pellet stoves) but aren’t typically considered ‘inserts’ because they don’t take up the whole opening of the firebox.

See our electric fireplace insert buyers guide for more information on electric fireplace inserts.

Typical Fireplace Firebox

A typical firebox in a fireplace includes an opening in the wall with a masonry chamber located behind.

Examples of our own fireplace fireboxes are shown below. Some we use for open fires, some of which we have had wood stoves installed and some we use purely for decoration purposes.

Wood Burning Stove
A typical firebox with multi fuel stove installed
A typical firebox with wood burning stove installed

Can You Paint A Fireplace Firebox?

A firebox in a fireplace can typically be painted, as long as the paint is high heat-resistant paint that is able to cope with the temperatures generated by a fire within the firebox.

Although we’ve kept the majority of our fireplaces with the traditional brick firebox look, one of our fireplaces had already been painted black when we moved into the house.

We’ve since given the firebox a deep clean and touched it up with another layer of paint so that it looked a bit nicer.

The image below shows the before and after of our living room open fireplace with firebox (and hearth and surround) painted black.

Fireplace Surround Black
The firebox of our living room fireplace was already painted black when we bought the house

We used a dedicated fireplace heat-resistant black paint spray to paint our firebox.

Fireplace Paint
Typical fireplace paint

Painting a firebox should be in line with any local codes and regulations.

Dimensions Of A Fireplace Firebox

The dimensions of a firebox will differ between every fireplace.

Although all of our own fireplace fireboxes share the same ‘box’ shape, they’re all different sizes and dimensions.

However, as an example our living room open fireplace has the following dimensions:

  • Height – 25.5 in. (65cm)
  • Width – 31.5 in. (80cm)
  • Depth – 15 in. (38cm)

If you’re looking to install an insert or a stove inside an open fireplace then consideration of the dimensions of a fireplace firebox is key to ensuring that the stove or insert fits.

Fireplace Firebox Codes

In the US, the National Fire Protection Agency (NPFA) provides Code 211 Standard for Fireplaces.

In terms of fireboxes for fireplaces, paragraph of the 2019 Edition of Code 211 states that:

The firebox of a concrete or masonry fireplace shall have a minimum depth of 20 in. (508mm)


Check with a local certified professional for guidance and advice on the codes and regulations for fireplace fireboxes for your particular area of residence.

Cracks In Fireplace Firebox

Cracks in a fireplace firebox are a sign of damage to either the firebricks or the mortar joints in between the bricks. Cracks in firebox should be investigated by a certified professional and corrected if required before any further fires are started.

Fireboxes are built from non-combustible materials, typically bricks or cement, which are designed to withstand the heat from open fires, or any wood burning, electric or gas appliance that is installed within the fireplace.

Over time and through use, cracks or gaps may start to form within the walls of the firebox.

Cracks or damage in fireplace fireboxes can be due to a number of reasons including:

  • Low quality materials and construction
  • General wear and tear
  • Subsidence
  • Water ingress
  • Regular expanding and contracting due to fires

We’ve actually had gaps start to form in our own living room fireplace, within the bricks at the back of the fireplace.

We won’t be using our fireplace until this is fixed. We’re not sure what has caused it but the firebox backs onto another firebox located in another room in the house.

Our firebox has started to crack/move at the back

If you have cracks forming in the firebox of your fireplace seek advice from a certified professional who will be able to help identify and fix the issue.

Fireplace Mantel & Surround Distance From Firebox

Any combustible materials used within the construction of a fireplace mantel or surround must be located a sufficient distance away from the opening of a firebox in line with local building codes and regulations.

Our own mantels and surrounds, made from non-combustible materials concrete and stone, line the outer wall of our fireplace fireboxes as no clearance offsets were required.

Stone Fireplace Surround
How far a surround needs to be away from a firebox will depend on whether the surround is made from a combustible material

However, our wooden surround for our gas fireplace was offset away from the firebox opening because it was made from a combustible material.

We have a complete guide to fireplace surrounds here where you’ll find more information on the required clearance distances from fireboxes for surrounds, and another guide to standalone mantels here.

Further Reading

Parts Of A Fireplace & Chimney Explained

A Guide To Fireplace Lintels

A Guide To Fireplace Hearths

A Guide To Fireplace Dampers

How A Fireplace Works

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