How to Repair Chimney Cracks

Written by tara dooley
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How to Repair Chimney Cracks
Chimneys normally have a pipe running through them, making crack repairs not as urgent. (Jupiterimages/ Images)

Fireplaces can be wood or gas-burning, but their purpose is the same. If you have a crack in the chimney, this is generally not a major concern for smoke issues, due to the fact that most chimneys have a pipe running through them to route the smoke up and out of the house. The chimney is generally just for looks. However, if you have an older home or a chimney with a tile flue, you probably don't have a pipe. In this case, the chimney itself is used to remove the smoke. Either way, the chimney should be fixed as soon as possible to avoid water damage to its interior.

Skill level:

Things you need

  • Crown sealer
  • Paintbrush
  • Silicone sealer (brush-on or caulk)
  • Prehydrated mortar
  • Water
  • Trowel
  • Bucket

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  1. 1

    Brush on a crown sealer if the crack is located at the top of the chimney around the crown. The crown sealer applies a waterproof membrane over the surface and won't allow any more damaging water in.

  2. 2

    Seal hairline cracks with a silicone sealant to keep water from leaking in and increasing the size of the crack. You can use a brush-on sealant or caulk.

  3. 3

    Fill large gaps in the chimney where mortar has fallen out, using new mortar. Chisel out the old mortar, if any is still there, to a depth of approximately 1/2 inch. Use a chisel and hammer to knock it loose. Mix new mortar and fill in the gap. Make sure you push it all the way back into the open area. Run the corner of a trowel along the edge to create the concave indentation that most mortars have on a brick or block surface. Use a prehydrated mortar mix for this procedure.

Tips and warnings

  • Always have a professional inspect the chimney before you perform any repairs. This way, if the damage is extensive, a professional can let you know. Don't waste your time on minor repairs if the chimney needs to taken apart for major repairs or rebuilt entirely.

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