How to Build a Brick and Block Fireplace

Determine where your fireplace will sit. It needs to be on a flat concrete surface to avoid undermining its integrity. A patio is ideal.

Mark out the dimension of your fireplace with chalk. You can make the fireplace as small or large as you like. Basing your dimensions on the size of your blocks means you don’t have to cut the concrete.

Place a concrete block so its outside edge is flush with your fireplace chalk outline. Draw a chalk line along the inner edge of the block on the ground. Repeat around the perimeter of the fireplace area.

Sweep clean the area of the fireplace removing any debris. This ensures your blocks sit true and creates a sturdy, level fireplace.

Mix mortar according to the manufacturer’s instructions. Place a layer of mortar on the concrete floor between the two chalk lines you made. Use a trowel to make an even layer.

Place concrete blocks -- with the holes oriented vertically -- around this perimeter of mortar. Use a spirit level to ensure the blocks are level.

Place mortar inside this perimeter of blocks and fit concrete blocks over, using the spirit level to check they are true. Repeat until the inside area is full of blocks.

Mix concrete according to manufacturer instructions. Add concrete into the holes in the blocks and level off so it is flush with the top edge of the block. Allow to dry. This is the base of your fireplace

Place a steel sheet across the top of the concrete blocks. This protects them from the heat of the fire, which they may not be able to fully withstand.

Spray your firebricks with water 24 hours prior to use.

Place mortar around the perimeter of the fireplace around the back and sides. Place firebricks along this mortar line. Tap each into place to ensure they are true and abut the neighboring bricks to ensure a solid wall.

Place mortar across the inside of this line and fill in with firebricks, giving you a single layer of firebricks over the sheet.

Repeat, building up just the back and sides of the fireplace to the desired height. Add layers of firebricks inside until you leave a space big enough to comfortably house your fireplace insert. Stagger the joints of the bricks for added stability.

Add a steel lintel across the top of your fireplace. Add mortar and a layer of firebricks over the top.

Place your cast-iron fireplace insert into the square cavity created at the front of the fireplace.

Let the fireplace cure for five to seven days before use.