How to seal a chimney: Essential maintenance for your home

Protect your home from weather and pests by knowing how to seal a chimney. We provide easy-to-follow directions and product selection tips.

• How to seal a chimney: Tools and product selection
• How to seal a chimney stack
• How to seal a flue pipe to a chimney

How to seal a chimney: Tools and product selection

The best way to thoroughly seal a chimney is to fill any gaps, cracks, or open joints between materials using mortar, roofing tar, or a high-quality roof and flashing sealant. If the chimney exterior is brick or stone, the entire surface can then be sprayed with a clear waterproof masonry coating for further protection.

If you need to seal or repair any areas inside the chimney or firebox, make sure to use a product specifically rated for the high temperatures that it will be subjected to.

How to seal a chimney stack

Once you’re ready to begin, follow these steps:

1. Clean and prepare all surfaces. The chimney must be structurally sound and all surfaces should be free of dust, soot, rust, or debris. If necessary, use a wire brush to remove stubborn dirt. Repair or replace any rotted wood.
2. Prefill joints if necessary. If the depth of a joint between two surfaces is deeper than its width, prefill it with mortar or a backer rod. A backer rod is a foam rubber cylinder that can be inserted into a joint to take up extra space. Sealant will be applied over top of the rod.
3. Load your sealant into a caulking gun (if applicable) or apply it with a trowel. If using a cartridge, cut the tip to size and break the inner seal. Some products are required to be kept at a specific temperature range before use. Read and follow the manufacturer’s directions.
4. Seal all joints and areas that may potentially leak air or water. Ensure there are no gaps or bubbles in the sealant.
5. Dry tool the sealant. Use a trowel or putty knife to smooth and shape the sealant as needed. Try to feather the edges to allow water to run off the exterior.
6. Clean up: Clean all tools immediately and remove any excess sealant with the recommended solvent.

How to seal around a chimney

If you have access to the attic, it’s a good idea to inspect around the chimney. If there are any signs of leakage around the chimney base, chances are the flashing is failing. This can usually be repaired with some roofing tar or sealant. Check each piece of flashing and add sealant anywhere the old sealant is loose or missing.

How to seal chimney bricks

The exterior bricks can be sprayed with waterproofer as mentioned above or painted with a waterproof paint.

If you notice any missing mortar between bricks or stones, be sure to replace it, and allow it to dry before waterproofing the surface.

Have a gap between surfaces that is not suitable for mortar? Loctite Epoxy Metal/Concrete may be the answer. Having both adhesive and filling/rebuilding qualities, it’s a quick solution in many situations. This epoxy formula hardens to a sandable grey finish in just 5 to 12 minutes and bonds to metal, concrete, ceramic, and wood.

How to seal a flue pipe to a chimney

Is your chimney a metal flue pipe, or do you have a flue pipe insert inside of an older chimney? If so, use a metal cap that integrates with your roof to ensure there are no leaks down the sides of the pipe.

Need to seal between sections of metal flue pipe? There are special high-temperature cements made for this purpose. Put some tape above and below the seam and cover the top of your stove if needed to protect these surfaces. Then press this thick, premixed putty into the joint, smooth it out, and allow it to dry.