Diagnosing Chimney Leaks

This guest post comes to you from Mark Sherby, Director of Quality Control at The Durable Slate Company:

Chimney leaks are some of the most difficult leaks to diagnose. There are many reasons for chimney leaks, and it is not always possible to identify the specific reason for the leak. Even when an estimator sees an obvious reason for a leak, there may also be hidden factors or causes.

Therefore, it may take several different remedies to completely repair the leak. The following are the most common reasons for leaks around chimneys.

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Flaws in the roofing material above or adjacent to the chimney

There may be a broken slate or tile nearby that is causing a leak. For tile roofs, the underlayment, which keeps water out, may be deteriorated.

missing or broken slates causing a leaking chimney

Flashing problems

The metal flashings around chimneys deteriorate and cause leaks. The old metal must be removed and replaced with hand-soldered flashings.

hole in flashing cricket causing leaky chimney

Poor joint between the masonry and counterflashing

(the part of the flashing that is cut into the masonry joint)

poor joints between masonry and counterflashing

Condensation from a modern furnace

Newer, energy-efficient furnaces may cause problems with the chimneys in old houses. The warm exhaust air cools quickly as it enters the wide chimney flue and some of it condenses into water and corrosive acid. Even though a chimney looks intact from the outside, it’s possible the water may cause leaks, and the acid will erode the mortar joints inside the chimney letting additional moisture in.

repaired chimney flashing to prevent leaks

Missing mortar

Cracks in the crown, voids in the mortar joints and spalling (or crumbing) bricks can all contribute to a chimney leak. Masonry restoration with appropriate lime mortars will handle this problem.

missing or cracked mortar causing chimney leak

Faults in the crown of the chimney

Gaps between the flue liner and the chimney

Very porous bricks

Chimneys in some newer homes are built with bricks which are so porous rainwater seeps through very quickly, causing leaks. Through-wall flashings should be installed to line the inside of the chimney and direct water out onto the roof.

Problems with an adjacent gutter

If a section of gutter near the chimney is not draining properly, the water may find its way inside and can appear to be a chimney leak. Unclogging the gutters and downspouts, relining box gutters or adjusting hanging gutters is required to fix this problem.

Capillary action

(the tendency of a liquid to adhere to a solid surface)

“Chimney leaks are some of the most difficult leaks to diagnose. There are many reasons for chimney leaks, and it is not always possible to identify the specific reason for the leak. Even when an estimator sees an obvious reason for a leak, there may also be hidden factors or causes.” - Mark Sherby, Quality Control

Choose Durable Slate

It takes an experienced professional to correctly identify each faulty component that causes a chimney leak. Whether the problem is roof repair, flashing replacement or chimney repointing, The Durable Slate Company has you covered. That’s just one benefit of working with a contractor with such a diverse group of in-house tradesmen.

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Mark Sherby works out of The Durable Slate Company’s Columbus branch. Working in the Quality Control Division, Mark has more than 15 years of experience on slate and tile roofs.

Repairing Chimney Leaks