How to Repair a Chimney Flue Crack


A chimney flue vents gases and smoke from the hearth into the air. The flue serves as a ventilation and exhaust system for your fireplace. When the flue cracks, repairs are imperative due to the nature of the gases escaping through the crack. Access to the flue is limited by the chimney. In order to seal the crack, the flue tile must be removed and replaced.

Things You'll Need

  • Masonry hammer
  • Chimney flue tile
  • Trowel
  • Masonry mortar
  • Remove the flue tile from the chimney with a masonry hammer. Crack the tile by striking it on the side. Remove the tile pieces by hand until the entire tile section is removed.

  • Mix masonry grade cement mortar in a five gallon bucket using a hand trowel.

  • Apply the cement mortar to the top of the flue section below the flue piece you just removed. About an inch of mortar is necessary to set the new flue tile in place.

  • Slide the new flue tile (which is a rectangular, tube-shaped section of clay flue) into the chimney and set it so the bottom edge is on top of the mortared flue tile. Tap the flue tile into place with the masonry hammer to settle the tile into the mortar.

  • Scrape the remaining overflow cement mortar from the sides of the tile with the trowel. Allow the mortar to dry for six to 10 hours. Check the flue tile again to ensure it is setting properly.

Tips & Warnings

  • A flue liner is another solution to fixing a broken flue tile. The complete installation of a new flue liner will bypass gas and smoke past the cracked flue tile. Consult a professional to install a new flue liner, as it is a complicated, involved process.

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  • Photo Credit chimney image by hazel proudlove from
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