Extend A Chimney Flue In 5 Steps

If smoke has a tendency to build up inside your home when using the fireplace, it might mean that your chimney flue needs to be extended to create a better draft of air. The chimney flue is the open shaft that runs from the firebox up through the chimney, creating the passage for the fire smoke. The flue should extend at least 3 feet above the roof. If it doesn’t, follow these steps to extend it.

Step 1: Remove Old Chimney Crown

With your hammer and chisel, carefully chip away at the top of the chimney to remove the existing crown. The crown is the headpiece of the chimney and is probably made of mortar. If there is a stainless steel or terra cotta chimney cap, make sure you do not damage it, and make sure not to damage the flue liner that is already in place.

Step 2: Attach the Flue Liner Extension

With a portion of the old liner now exposed, clean off the very top of it with a wire brush, clearing it of all debris. Your flue liner extension will have to be the same size as the existing one. Load the caulking gun with fireplace mortar and place an even bead of adhesive around the top of the liner, then place the liner extension directly on top of the old one.

Step 3: Build up the Brick Around the New Liner

After the mortar has set and the new liner is firmly in place, build up the sides of the liner extension with brick and mortar following the instructions for the proper use of the mortar. Build up the walls, leaving about 12 inches of new flue liner extended above the new bricks.

Step 4: Build a Frame for the New Crown

Using the 2x6-inch lumber, construct a square frame around the 4 sides of the brick chimney, making sure it fits quite snugly around the outside edges of the walls. Once this is in place, mix up additional mortar and pour it inside the frame until it fills the space between the frame and new liner. As it begins to harden, using your trowel, shape the mortar at an upward angle towards the flue liner. All said and done, there should be about 4 inches of exposed flue liner on top.

Step 5: Let the Mortar Settle

Moisten the mortar and cover the whole area with the plastic sheeting, but do not remove the frame just yet. You will want to make sure it stays damp over a period of days while the mortar hardens properly. Once it has, remove the frame.

Note: Chimney Cap

If you removed a chimney cap to extend the flue liner, replace it on top of the chimney once the extension is complete. You can either affix it in place while you mortar the crown or avoid the crown all together if the chimney cap is a cap and crown in one.