Why is My Chimney Leaking?


There are few things more aggravating to a homeowner than having water leaking into their home. Finding the exact source of a leak around a chimney can be particularly frustrating.  The inherent complexity of having a masonry structure pass through a framed roof can turn the proper diagnosis of a leaking chimney into a time-consuming headache.

Annual chimney maintenance is helpful because it alerts you to problems early, before they become catastrophic.

Annual chimney maintenance is helpful because it alerts you to problems early, before they become catastrophic.

To understand the fundamentals of why a masonry chimney will or won’t leak, let’s start at the top of the flue and work down to the roofline.  First, if you don’t have a chimney cap, get one.  An uncovered chimney allows whatever rain and snow are falling to enter right into the fireplace flue.  Over time, that water erodes the mortar joints between the flue tiles and it also damages the chimney as the water is heated into steam during operation of the fireplace.

The chimney crown is the ‘lid’ over your chimney.  It’s useful to think of a chimney as a hollow box built out of cinder blocks or bricks.  To keep water from entering the chimney from above, a crown is poured.  The crown should be concrete that is 4” – 5” thick and contoured to drain moisture.  Lots of chimney crowns in our area were just built with mortar instead of concrete or they lack an expansion joint to allow the top flue tile to expand when the fireplace is in use.  The result is cracks and erosion that allow water to penetrate and cause increasingly larger cracks as the freeze-thaw cycle occurs.

The sides of a masonry chimney are often the least understood culprit when making a leaky chimney diagnosis.  Like all masonry materials, mortar joints will allow water penetration when saturated.  When the mortar joints are cracked or damaged, water easily flows through them and into the space between the cinder block structure and the stone veneer.  It flows downward until it reaches the framing of the roof and into your home.  Even in our dry climate, when a chimney is exposed to excessive rain and snow, it will absorb moisture through the rocks or bricks on the sides. The solution is to locate the weakened joints and cracked rocks and seal or replace them as necessary.

Where the chimney meets the roofline can be the most complicated region of a leaking chimney to diagnose.  Every roof-chimney interface should have an overlapping system that includes both flashing and counter-flashing.  The flashing starts under the roofing materials and runs up alongside the chimney.  The counter-flashing starts out with a ‘cut’ into the side of the chimney and then overlaps the flashing to create a barrier to water intrusion.  Unfortunately, a huge percentage of homes in our community have chimneys that are flashed improperly.  We often see chimneys where the counter-flashing is just placed along the chimney and ‘sealed’ with some type of goop.  WRONG!  Chimneys like these need significant repairs to keep the water out of your home.

To correctly diagnose and fix a leaking chimney, we evaluate the entire chimney as a system: top (cap and crown), middle (stonework or bricks) and bottom (flashing and counter-flashing).  If any of the three sections is leaking, then the chimney has a potential for water to enter the home.  Next month we’ll explain how to find the leaks on a framed chase with a factory-built chimney.

To schedule a fireplace/chimney sweeping and/or inspection – call us today at Mountain Man Fireplace and Chimney, Inc – (303) 679.1601 / 838.3882 or electronically at office@MtnManChimney.com.  Semper Fi!