When a chimney and/or its framing, plaster, or drywall are wet, many mistakenly believe it's the result of a roof leak. It's not uncommon for chimneys to leak, but sometimes the wetness is caused by something else.

A roof leak usually shows up as a pattern of wetness or water damage, such as staining, rust, or decay. If the pattern of wetness or decay does not emanate from one or a few specific locations and some type of breach in the roof system cannot be found, the wetness and subsequent damage are probably not due to a roof leak.

Sometimes, we have found that the wetness around a chimney is the result of airborne moisture condensing from the exhaust gases on the cold chimney walls. When the condensate migrates through the mortar, it leaves the outside faces of the chimney wet. Condensation of exhaust gases can be observed or accompanied by any of the following:

  • There is a fairly even pattern of wetness on all sides of the exposed chimney in the attic.
  • The basement or first floor tends to be wetter than the second floor or roof.
  • The surface of the chimney, plaster, or drywall has white and tan mineral deposits.
  • Damage caused by acidic condensate to mortar, plaster, drywall, etc.
  • A cracked, missing, or separated flue liner.
  • Exhaust gases in the chimney are from a non-condensing furnace (usually over 85% efficiency).
  • Corrosion of metals embedded in or in contact with the chimney.
  • The relative humidity in the dwelling is higher than normal, especially during the winter.
  • Only during the cold weather seasons does the chimney "leak".


In the case of several of these characteristics, consider that the wetness and subsequent damage on and around the chimney might be from the condensation of the exhaust gases and condensate migrating through the mortar.

It may be necessary to replace the furnace with one that exhausts through a wall, thus preventing the introduction of exhaust gases into the chimney. installation of a non-corrosive flue liner. Contact a qualified and experienced heating contractor for more ideas.

PatchMaster can help you if you have water damage to your drywall from your chimney. Most issues like this can be diagnosed and repaired in one day. Please contact us at 1-844-PATCH-MAN (US) or 1-833-WE-PATCH (Canada) for a no-obligation, free estimate.



Photo by Sandul1234 - Own work, CC BY-SA 4.0