How to Replace a Thimble in a Chimney

Measure the width of the hole through which the chimney passes to determine the size of thimble you will need. Manufacturers make thimbles in a variety of sizes. The wall hole should be four inches wider than the thimble diameter. The thimble face plate attached to the wall should be at least three times the diameter of the chimney pipe.

Prepare a mix of high-temperature cement, which can insulate your wall and house from the heat generated in your chimney. Use a large, durable bucket or trough. Follow the instructions on the bag of cement mix; you will need the right proportion of water to mix.

Layer a coat of the cement around the outside of the thimble. This cement will contact the wall; high-temperature ready-mix will protect the wall from the heat. Let the cement dry on the thimble for the recommended time, usually 24 hours or more.

Run a thick layer of silicon-based caulk on the surfaces of the wall that will adjoin the thimble. The caulk provides a better seal between the interior and exterior. Place the thimble into the wall and lay more cement into any gaps between the thimble and the wall material.

Lead the chimney pipe through the thimble, and then place the assembly into the wall. If your thimble came with masonry screws, drive them through the preset openings in the thimble face plate to secure the thimble to the inside wall.

Reattach the flue to the chimney inside the home, and install any extensions you had to the outside chimney. Your local fire code will require a minimum clearance between the chimney pipe and flue and the inside wall.