How to Replace Broken Firebricks in the Backs of Chimneys

Remove the mortar surrounding the broken bricks using a hammer and chisel. With a hammer, gently pound a chisel into the mortar around broken bricks to loosen it. Pry off loosened pieces of mortar with a the flat end of the chisel. Carefully remove all of the pieces of broken bricks.

Clean out the dust and debris in the empty brick cavities. Use a dry paintbrush to brush off smaller particles. Carefully chisel off any mortar sticking to unbroken bricks to create a smooth, clean surface for setting the new bricks.

Prepare special firebox mortar, following the manufacturer's instructions. Premixed firebox mortar is already mixed together, but requires stirring prior to use. Powdered mortar is mixed with water or liquid acrylic before use. Each manufacturer has a different formulation; follow the label directions to ensure the mortar retains its fireproof properties.

Moisten each new brick by placing it in a bucket of water for a few minutes before installing it. Spray the surfaces of the hole where the brick will be installed with water so all surfaces that will be in contact with the mortar are wet. Wetting these surfaces will help the mortar adhere to them better.

Butter one short side and one long side of the brick with the prepared mortar. Butter the two opposite short and long sides of the hole in the chimney wall where the brick will be installed. Use a mason's trowel and scoop up an egg-sized clump of mortar. Spread the mortar on the surfaces of the new brick and the sides of the hole just like you are buttering a slice of bread.

Insert the brick into the prepared space. Position the brick so its front is flush with the rest of the bricks in the chimney wall. The mortar should completely fill the space between the bricks and ooze out slightly. Use the edge of the mason's trowel to scrape off excess mortar. Repeat with the remaining bricks.

Tuck-point the mortar between the newly installed bricks after the mortar has hardened slightly by rubbing the mortar joints with a jointing tool until the joints are flush with the bricks. Do the vertical joints first, then the horizontal joints.