What Do You Use to Seal the Outside Top on a Fireplace Chimney?

Flashing

• Flashing, which is sheet metal wrapped around the base of the chimney, is your first line of defense against roof leaks. If your roof develops a leak, there's a good possibility that the problem's with the flashing. Youe chimney should have two types of of flashing: cap flashing, which goes around the bricks, and step flashing, which goes over the shingles.

Caulk

• A masonry flue, the conduit for smoke and gasses from your fireplace to leave your home, requires a liner. Many newer chimney tops have a gap between the masonry and the flue liner. About a fourth-inch wide, the gap allows the clay flue liner to expand and contract without coming into contact with the masonry and cracking. If it's not properly sealed at the top, water can enter. Polyurethane caulk can be used to seal this gap and keep moisture out of the chimney, according to the Family Handyman website.

Mortar

• A chimney requires sufficient mortar -- or pointing -- to remain structurally sound. If it's not sufficiently pointed, the chimney can develop leaks and even begin to fall apart. A repointing seals the gaps in the joints of the masonry. The type of mortar used can determine the success or failure of the job. A soft mortar with a higher lime content works better for repointing than a hard mortar that crack easily. The high lime content makes the mortar more waterproof.

Cap

• A properly fitted cap keeps rain from getting into the chimney. Caps are often made of metal, such as stainless steel.. You can also purchase a wire mesh cap, which keeps birds and other animals from getting into the chimney. A cap needs to be checked regularly, as any holes can allow water to enter your chimney.