How to seal a chimney: A guide for homeowners

Every DIYer should know how to seal a chimney. It’s an important maintenance job and not that difficult. Get the details below.

Sealing a chimney presents some unique challenges due to the temperatures involved and the various materials that may be used in any installation. You will likely encounter metal, wood, masonry, and other building materials as you complete the job.

Since a chimney usually penetrates the roof, sealing around the chimney is crucial to prevent the elements from entering your home. So, your sealant must be waterproof as well as weatherproof. Of course, some parts of the chimney stack will get very hot. Choose a sealant that can handle high temperatures.

You may also need to seal a flue pipe made of metal, or chimney bricks. Whatever the case, always opt for a premium-quality sealant that is compatible with the materials involved. 

The first step in this project is to inspect the entire chimney system – from the fireplace, stove, or furnace all the way through the roof. Look for any gaps, flaws, or signs of moisture coming through. Take note of which areas need to be sealed and the materials involved. Also, note the temperature ranges that area will have to withstand.

Once you have your list, you can purchase a suitable sealant for the job.

Before beginning, collect the necessary materials and tools:

• nitrile or other work gloves and safety glasses
• chimney sealant
• caulking gun
• utility knife and putty knife
• dust brush, wire brush, vacuum
• solvent for clean-up (mineral spirits)

 Now you can begin sealing your chimney.

1. Clean the areas to be sealed. Make sure all surfaces are free of dirt, debris, and soot. Use a putty knife or utility knife to remove old sealant. Then use a dust brush or wire brush and vacuum the area. 
2. Replace any rotted wood or missing mortar. 
3. Use a backer rod or filler for extra deep or wide joints. 
4. Load the sealant tube into a caulking gun and cut the tip to the appropriate size. Puncture the foil seal if necessary.
5. Apply the sealant. Use steady pressure, starting at one corner and working outward. Force the bead into the joints. 
6. Spread the sealant with a tool or a gloved finger. Create a smooth bead without any gaps.
7. Clean-up. Use the recommended solvent to clean your tools and any uncured sealant immediately. 

Here are some highly rated sealants that may be the right choice for you – whether you are sealing stove pipe joints or sealing chimney bricks.

If you have a masonry or brick chimney, try LePage PL Polyurethane Concrete Crack & Masonry Sealant for quick fixes and touch-ups that will not be exposed to direct flames or excessive temperatures. It forms a weatherproof seal and bonds to brick, stone, stucco, or any masonry surface. This sealant remains flexible for a long-lasting bond and is paintable. It can stand up to temperatures ranging to 93°C (200°F).

LePage Quad Fire Block Foam is a top-of-the-line fire-rated sealant that will prevent fire, smoke, and noxious fumes from backing up into your home. It’s perfect for sealing gaps around ductwork, ventilation passages, and flue pipes. The sealant can be applied in temperatures from -10°C to 30°C (14°F to 86°F) and bonds to most common building materials such as wood, concrete, brick, metals, and more. It dispenses easily, sets in 10 minutes, and will not dry out or crack. Please be aware this product is not to be tooled!