4 Common Sources of Chimney Leaks

chimney under construction

Leaks around a chimney contribute to a number of serious problems, and if left untreated, they can become quite costly. Roof leaks hardly stay in one place, oftentimes spreading throughout the ceiling and walls. Under the worst circumstances, moisture damage can even affect your health, as certain types of mold are serious health concerns.

Mold grows in constantly wet and dark environments and can damage nearly any type of wall finish, including plaster, drywall, wallpaper, and paint. Moisture leads to rotten wood boards, which will need to be replaced, as they attract termites, rodents, and carpenter ants.

Chimney leaks are difficult to address because there are multiple areas around the chimney where the water could be coming from. Knowing which areas are most susceptible to leaks helps make pinpointing the compromised area quickly easier. We’ll outline four common sources of chimney leaks and what to do about them.


Mud Cap

chimney cap

Chimneys have a mortared area around the top—sometimes referred to as the mud cap—that is a common source of roof leaks. Cracked mortar allows moisture to drip into the fireplace or the interior of the ceiling. A sure-fire sign of a compromised mud cap is water inside the fireplace.

How to repair mud caps:

Replace the mortar cap and insert a flu cap. Another option is to cover the entire chimney top with a cap to greatly reduce the chance of any rainwater getting inside the chimney.


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Mortar Joints

Directly underneath the mortared area are mortar joints. These joints can deteriorate over time, leaving the chimney vulnerable to water damage. It’s possible the joints are missing large chunks of mortar which will allow water to come through during rain and wind storms.

The issue with damaged mortar joints is it allows water to travel far inside the roof, depositing water above ceilings, on rafters, or other areas. This makes it particularly challenging to diagnose.

How to repair mortar joints:

Thankfully, once you’ve pinpointed the mortar joint as the culprit, simply repair them with new mortar.

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Damaged Flashing

chimney flashing

Damaged chimney flashing is another material that allows for serious water penetration.

Flashing is applied around the chimney in a few key areas: continuous flashing along the bottom, step flashing up the sides, and saddle flashing along the top. Caulk or cap flashing laps over the edges to prevent water from running behind and causing leaks.

If even a tiny hole exists, large volumes of water can get inside the roof and home.

How to repair damaged flashing:

It’s best to not attempt to reseal the metal flashing with a sealant or with caulk. Doing so will only momentarily patch the problem, but it won’t last long. It’s best to call a professional roofer and pay to have the flashing replaced. That way you know the repair is addressed properly.

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Leaking Shingles

While uncommon around chimneys, it is possible the leak is coming from a damaged shingle. High winds, a heavy tree branch, or hail may have caused the shingle to fail and allow water to get underneath it.

Typical asphalt shingles are susceptible to wind, hail, ice, and sun damage. A properly installed shingle roof can withstand the elements for 20 years or longer, but the average lifespan of a traditional asphalt shingle roof is about 15 - 18 years.

Aside from weather-related damage, shingle roofs can also be damaged by ice damming, excessive snow, fallen debris (e.g., tree limbs), and bird droppings.

How to repair leaking shingles:

It’s best to replace worn or damaged shingles. Attempting to reseal them won’t adequately address the problem, and you’ll be forced to replace them in the near future anyway.


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