Leaky chimney culprits are generally one of four primary causes. Unless fixed, they all result in frustration and moisture damage.

We’ll give you a list of what to look for if your chimney is causing you grief. If one of these issues is not your problem, chances are the chimney isn’t the offender.

Broken Chimney Cover

Your chimney cover doesn’t just keep rain out, but also birds, debris from overhanging trees, and animals such as squirrels and raccoons. If your chimney cover is broken, or you just don’t have one, then you’re risking all of these finding their way inside. This is no small issue, either; a blocked up chimney can lead to CO2 poisoning. When the smoke from your fireplace can’t escape through the chimney, it backs up and comes right into your home.

Fixing this just means having a replacement cover installed.

Damaged Chimney Crowns

The chimney crown is the sloped or beveled edge found on the top surface of a chimney, and is designed to guide water away from the inside of the chimney. Cracks in your chimney crown can come from general wear and tear (as the crown takes the brunt of the rain and snow), shrinkage in the masonry due to age, or shifting as the building sinks and settles into the ground over time. Whatever causes these cracks in your crown, water passes right through them.

How you fix these cracks depends on how bad they are. Most crowns have them, and they all need to be fixed, even the small ones. This is because every big crack starts out small. When water freezes inside the crack, it expands, exacerbating the damage. Crown coating materials can seal small cracks to prevent water from getting inside, preventing further damage for a time.

If the damage extends beyond just small cracks, you have no choice but to replace the damaged masonry altogether. Overlooking serious damage to the crown can result in significant structural water damage to your home in the immediate vicinity of the chimney.

Leaking Bricks

Brick and mortar are both porous enough to allow water to pass through. An inevitable problem that can arise from having an unlined or unsealed chimney crown is that the bricks absorb water that eventually sinks down to the base of the chimney. Once this absorbed water freezes, it expands and breaks apart the bricks that it is inside.

Waterproofing is essential to prevent this, but must be done carefully. The seal on the crown will eventually wear out and water will begin working into the bricks from top to bottom. If the bricks are sealed on the sides while allowing water in from the top, they will become soggy and/or break apart far worse when the water inside freezes.

This means that how the waterproofing seal is applied matters just as much as the type of waterproofing that is applied. This is a job you need a professional for. A Masonry Absorption Test is necessary to check how long it takes for water to be absorbed into your wall, and whether you need it sealed or not.

Old Chimneys With Modern Furnaces

Older chimneys built to handle coal or wood smoke are built to exclusively handle wood or coal smoke. Gasoline furnaces require a liner to protect the brickwork. Gasoline fumes are low-temperature and have lots of moisture in them. This causes condensation inside of the chimney. The bricks absorb all of that moisture released by the gas. Bricks allow water to pass through them, so the absorbed moisture will just be released on the other side of the chimney’s brick wall, causing water damage.

To fix this, you need to have a chimney liner installed in your chimney to either prevent the moisture from reaching the bricks, or to prevent the moisture from escaping through the outside of the chimney.

Chimney Repair: The Definitive Guide to Chimney Inspection, Chimney Sweeps, and Repairs