How to Build an indoor or outdoor wooden Fireplace: A Step-By-Step Guide

Fireplaces have been around for centuries and are still popular today. Not only do they heat the room and provide a cozy atmosphere, but they also add to your living room’s aesthetics.

You may want to have a better understanding of fireplaces if you’re looking to remodel your fireplace or build a new one.


So, how to build a fireplace? That’s what we will answer in this step-by-step article.

Step 1: Take Your Room’s Measurements

You need to know the dimensions of your room to find out how big the fireplace needs to be in order to be proportionate with your room. This way, it’ll be more aesthetically appealing and provide adequate heating.

To determine the appropriate measurements for your fireplace:

-Measure the length and width of your room in feet

-Add the two numbers together. The result will be the appropriate fireplace opening dimensions in inches

If your living room dimensions are 16 x 20 feet, the sum of the length and width would be 36, so your fireplace opening should be at least 36 inches wide

Another method to determine the most suitable fireplace dimensions is by calculating the minimum heat output—this is necessary if you’ll depend on the fireplace as your only source of heat.

A small-sized fireplace with dimensions of 24 x 24 inches will give a heat output of 10,000-18,000 BTU. Medium fireplaces with dimensions of 36 x 30 inches have a heat output of 25,000-30,000 BTU.

Lastly, the heat output of large fireplaces is 35,000-40,000 BTU, which typically measures 48 x 32 inches.

To find out the right BTU value for your room:

-Measure the length, width, and height of your living room in inches

-Calculate the volume by multiplying the three numbers together

-Multiply the volume by four to get the minimum heat output for your room

Step 2: Build the Foundation of the Fireplace

Having a sturdy base is crucial to enable it to withstand the heavy weight of the firebox, the chimney, and the hearth extension. Plus, this step is necessary if you’re going to install a masonry fireplace.

Masonry fireplaces are usually built on exterior walls. If they’re attached directly to interior walls, their heavy weight may damage your house. You should also make sure to reinforce your floor joists if you’re going to install a new fireplace during remodeling.

On the other hand, factory-built fireplaces can be installed directly against walls without worrying about building a hearth base.

Here’s what you can expect a professional to do when building a fireplace foundation.

Your contractor will usually start by digging out the foundation of the fireplace. If you’re building an outdoor fireplace, you can do this step yourself

However, you’ll most likely need a professional to do the fireplace’s foundation when remodeling your house

The depth of the foundation should be at least 12 inches, but this may vary depending on where you live

Then, they’ll pour the concrete to form the thick base slab that’ll support your fireplace

After spreading the concrete evenly, they’ll set the rebar

This method allows the rebar to be within the slab without being compressed to the bottom. However, your contractor may set the rebar before pouring the concrete

Lastly, they’ll smooth the concrete slab and leave it to set overnight

Step 3: Mark the Inner Measurements

Before laying out the base bricks of the firebox, you should take some measurements. Keep in mind that these measurements are for a fireplace opening of six square feet and will vary depending on your fireplace’s opening’s specific area.

-Draw a centerline to divide the area equally

-Measure at least 16 inches deep from the edge of the base slab and draw a line to mark the hearth extension (the hearth extension line should intersect with the center line)

-Measure at least eight inches from both sides of the hearth extension line to mark the fireplace opening

-Next, measure 20 inches deep from the hearth extension line and draw a line to mark the depth of the fireplace

-The back line should be parallel to the hearth extension line

Step 4: Build the Firebox

After marking the fireplace’s dimensions, you can start laying the base bricks.

Use refractory mortar when laying firebricks (it has high-temperature resistance and can withstand up to 3000ºF)

Position the first firebrick on the mortar line and tap on it to ensure it sticks. A tip here is to tap on the firebrick while putting a level, so you make sure every brick is positioned evenly on the base slab

Work from the centerline and extend the bricks beyond the eight-inch layout lines. The extended bricks will integrate with the eight inches of solid masonry to complete the firebox

Keep laying firebricks from the center out until you reach the 20-inch depth. Make sure to clean any excess mortar between brick joints

Extend the combustion chamber base by two rows beyond the fireplace depth to support the outer masonry

Measure eight inches on both sides of the base bricks and 20 inches back to build the side and back walls

Lay the back wall firebricks from the center out. The joints between bricks should be 1/16 an inch

After completing the first row of the back wall, work from the front edge to the back to build the side walls. Angle the side walls slightly, so that they’re wider at the front

Install the air intake vent in the third row. Cut the center brick of the back wall or the sides and lay the intake vent in it. That’ll help keep the same brick pattern

Repeat the same process until you reach six rows. Then you should angle the back wall bricks inward to meet the back of the damper

Once you reach the desired height on the back wall, set the lintel on the front edge of the firebox using refractory mortar

Put an expansion joint between the lintel and bottom bricks to avoid restraint on expansion

Lastly, fix the damper by using a layer of refractory mortar and lay the final front row of bricks—that’ll be the face of your fireplace

Step 5: Fill Around the Firebox

Once the firebox is finished, you can start laying whatever type of masonry to fill the rest of the fireplace structure. If you’re on a budget, you can use affordable brick and cover it up with stone veneer.

Make sure to leave a space between the firebox and outer masonry to avoid cracks as the firebox expands.


Knowing how to build a fireplace can make it easier to determine which steps you can DIY and which steps require a professional.

Whichever option you choose, installing a fireplace guarantees you’ll end up enjoying a cozy atmosphere that brings the whole family together!

Maintenance is important when you have a fireplace. We have outlined how to clean the fireplace glass here.