What is Causing the Leak in your Chimney?

Written by Emily Martin of The Durable Slate Company
Updated July 10, 2013
Stone-finished home and chimney
If the metal flashing around your chimney is improperly installed or nearing the end of its service life, it could result in a leak. (Angie's List/Summer Galyan)

One highly rated provider explains the commonly overlooked causes for chimney leaks and how to diagnose them.

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It is important to know that there are many reasons for a chimney leak. Even when an estimator sees an obvious reason for a leak in your chimney, there can be other contributing factors that aren’t so obvious.

The most common reason for a leak is a flaw in the roofing materials above or adjacent to the chimney. Broken slate or tile could let water onto the roof deck. The water will run until it finds its way to the chimney or between roof boards and then into the house. 

Another obvious reason for a leak is the flashing. The metal may have been installed improperly or is simply reaching the end of its service life. 

Masonry issues may be a bit more difficult to diagnose because they often do not look any different than a roof leak. Missing and cracked mortar in the joints of the chimney, as well as faults in the crown (on top of the chimney), can lead to huge water problems inside your home. Even porous bricks themselves have been known to create very bad leaks, as they act like a sponge soaking up water.

Condensation from a modern furnace can create leaks as well. Some HVAC contractors vent these systems directly into the existing flue from the basement with no pipe or liner to carry out the moist, acidic fumes. 

When this hot air reaches a point in the flue where colder air exists, condensation occurs. This condensation is very acidic and will prematurely deteriorate the mortar and bricks or stones. The leak will appear around the chimney on ceilings and walls and is often misdiagnosed as a roof leak.

Capillary action and gutter leaks may be the most difficult leaks to diagnose. Capillary action is the tendency of a liquid to adhere to a solid surface and, at times, will cause water to go uphill against the force of gravity and leak into the flashing. 

Chimney leaks can go unsolved for years if contractors don’t realize this is occurring or don’t have the experience to see the existing conditions that could lead to it. Make sure to hire a repultable chimney inspector who is aware of these issues and knows what to look for.

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About this Angie’s List Expert: Emily Martin is the office manager at the Cincinnati branch of The Durable Slate Company, providing roofing services in Cincinnati, Ohio. Established in 1986, The Durable Slate Company specializes in slate, tile and copper roof and gutter services through the eastern United States and offers expertise in historic restoration.

As of July 9, 2013, this service provider was highly rated on Angie's List. Ratings are subject to change based on consumer feedback, so check Angie's List for the most up-to-date reviews. The views expressed by this author do not necessarily reflect those of Angie's List.

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