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How to a seal chimney stack: Stay warm and safe

Do you want to learn how to seal a chimney to protect your home? Then you’ve come to the right place. This quick guide tells you how to find and apply the best product for sealing chimney bricks and other relevant materials.

Learning how to seal chimneys starts with the right sealant

Brushing up on your knowledge of specialty sealants is a key step in learning how to seal chimneys. After all, the sealant must withstand different weather conditions, as well as the expansion and contraction of construction materials. That is why you need a professional-grade sealant that is both durable and flexible – like Loctite PL Roof and Flashing Polyurethane Sealant. It stands up to harsh exterior conditions, will not soften in the sun, and is waterproof. While this product bonds with metal and other materials found in and near your chimney, you should use it only for applications on surfaces that don’t exceed 180°F.

If you want to seal a chimney flue, you need a sealant that can withstand the high temperatures the flue is subject to and bond with the materials in your project.

Do you need to seal around your hearth, too? Another one of our guides has useful tips on choosing and using fireplace sealants!

How to seal a chimney step by step

Once you have chosen your sealant, you can start sealing chimney bricks. Keep safety in mind when working: wear gloves and make sure you cannot fall from the roof. If in doubt, secure yourself to the roof or work with a partner.

  1. Gather materials. You need gloves, chimney sealant, primer (optional: see the product’s instructions), a utility knife, a caulking gun, a tool to puncture the seal, and the proper solvent for clean-up.
  2. Prepare surfaces. Your chimney and surrounding surfaces must be structurally sound and free of all debris, dirt, and residue. You may need to grind or wire-brush the masonry to expose a clean, sound surface. Remove any rot from wood and any rust or coatings from metal to be sealed.
  3. Prepare joints. The depth of the joint should be half its width. Your product’s instructions tell you about maximum joint width and depth and when you need to use a backer rod or filler. They also contain valuable information about whether you must prime your joints. The joints must be dry before you apply sealant.
  4. Prepare sealant. The product’s instructions tell you if you need to store your chimney sealant at a certain temperature before application. Load the sealant into a caulking gun, cut the cartridge tip to the proper size, and puncture the inner seal.
  5. Seal the chimney. Apply sealant with a steady pressure, forcing the bead into the joints. Fill joints from the bottom and avoid bridging them to prevent bubbles.
  6. Perform dry tooling. Perform any dry tooling required to create the correct bead shape within the period specified by the product’s instructions. If you need to use a solvent, consult the product’s instructions for appropriate options.
  7. Clean-up. Use the solvent recommended in the product’s instructions to clean your tools and any uncured sealant immediately in a well-ventilated area. Use a sharp-edged tool to cut away cured sealant.