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How to diagnose indoor stains at chimneys or caused by chimney problems.
What causes stains in, on or around chimneys and what do these stains mean? Unsafe chimney conditions may be indicated by leaks, stains, or crud seeping out of a chimney connection or a chimney crack. We point out that stains at chimneys are usually more than cosmetic and that they may indicate serious safety hazards as well as chimney functional problems.
This article series describes procedures for inspecting and repairing chimney flues - focused on stains that appear on the exterior surfaces or visible surfaces of chimney walls inside buildings.
The pair of photos above and below teach several important chimney inspection lessons:
These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.
Leaks originating at a chimney top, sides, even at ground level as well as chimney interior moisture sources (condensation) can all show up as stains on building interior walls at or near surfaces where the chimney passes through the building.
Our photos above and below show a rather obvious water leak from the inside of a chimney flue onto the wall and floor below. But other chimney leak clues that are less obvious may be present indoors too.
Illustrated in our photos below is the importance of looking closely at walls above a fireplace and where a chimney passes through a building. Especially in poorly-lit rooms, the importance of proper use and aiming of a good flashlight become quite evident. Our photographs show the discovery of water damage and efflorescence traced to a leaky chimney on a historic home in Newburgh, NY.
and this close-up below showing water damage along the passage of an interior chimney - from the room side (below):
At above left our fireplace photograph illustrates another spot to detect water leakage down the interior of a chimney structure or chimney flue.
Note the white efflorescence and stains on the back and sides of the fire box.
Without further investigation we can't be sure if the problem is a rain cap or a chimney cap seal or perhaps (less often) even a chimney to roof flashing problem.
The flue needs to be inspected for safety and the chimney leak found and repaired.
At above left the odd installation of chimney flue tiles run up through the middle of a fireplace ought to be a red flag to inspect closely for amateur and probably unsafe workmanship.
Watch out for
Aove we show white stains and rust atop a zero-clearance fireplace insert and more rust down the side of the metal flue itself.
You won't normally have access to inspect the interior of a finished fireplace insert.
But you should always be very alert for signs of leaks and rust damage to the unit, including inspecting inside the hearth as well as from the basement or crawl space below.
Rust damaged heating equipment may be unsafe.
Brown or black oozing stains may appear on both masonry chimney and metal chimney exteriors, though if the metal chimney was properly assembled such leaks onto the metal chimney exterior surface are unlikely.
The brick chimney at left has both white and black stains as well as what looks like a vertical crack along its right side - this chimney maybe unsafe and needs prompt investigation.
The brick chimney with brown-black stains oozing from between the next chimney's mortar joints (photo at below left) tells us that water has been entering the chimney flue (missing rain cap or improper chimney top cap/crown seal or other chimney leaks).
These brown chimney stains may be still more significant: they may indicate that the chimney flue is unlined and possibly unsound and unsafe.
The pair of chimney damage photos above teach several additional important chimney inspection lessons:
These black stains on a masonry or metal chimney are not mold. (Mold prefers to grow on organic materials.
The stains down the side of the white-painted concrete block chimney shown at left were traced to foundation leaks - we found horizontal cracking in the concrete block foundation wall and more water stains in this same area.
The stains also led to observation of the cracking, damaged chimney base also visible in our photo at left.
Watch out: cracks in a masonry chimney mean movement, risk of hidden internal damage to the chimney flue, and potentially fatal flue gas leaks or to a building fire.
Brown or black oozing stains may appear near the bottom of a chimney below a thimble (where the flue vent connector inserts into the chimney) or around or below a chimney cleanout door (photo at left).
If you see marks such as those shown in our photo you will want to find the source of leaks into the chimney, such as a bad rain cap, chimney cap, roof flashing, or even ground water entering the chimney base - all problems that need be corrected.
See CHIMNEY CLEANOUT DOORS for more examples of signs of leakage into the chimney.
Watch out: leaks into a chimney risk having damaged the chimney, flue vent connector, or heating appliance - making any of these dangerous, risking chimney damage, flue gas leaks, or a building fire.
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(Feb 8, 2014) hope said:
I was in the attic where I noticed an area on the chimney that was darkest and looked like it was rotting an the chimney shoul I be alarmed for possible chimney fire?
Without knowing what the chimney is made of, what it is venting, and what you saw, I could only GUESS that you may have seen brown stains on a masonry chimney that may be a creosote leak - indications of several unsafe conditions if that's the case.
To be safe you should call a certified chimney sweep and have the chimney interior and exterior inspected for safety and function. Let us know what you are told, and send along photos if you can and we may be able to comment further. Use the CONTACT link at page bottom or top to email photos.
12/17/2014 Karen said:
Our fireplace is a Heatilator TD42B. We recently installed a heavy scupture on the wall above the fireplace. One or more of the screws holding the sculpture were inserted through the sheetmetal in the flue. We thought this would be okay since the screw would plug up the hole that was made. The next time it rained, water started leaking through the wall and through the top of the fireplace above the doors. How could water be getting in there? We had a roofer inspect up on top, but he couldn't find a problem.
Watch out: In my OPINION, the hole made in your chimney is a fire and safety hazard, even though it may, at initial puncture be small and partly sealed by the puncturing screw.
My understanding is that the Heatailator TD42B can burn wood logs. Log burning can, depending on how dry the logs are, produce both creosote and other moisture in the flue.
If you were burning gas logs the probability of condensation in the flue is even greater.
Water might also enter a flue if a chimney cap is an improper design or is improperly installed or if it has been damaged.
Water might also enter a flue from any other damage anywhere along its length, or from a poorly constructed or poorly-sealed steel top flashing around a wood chimney chase - I can only guess as we don't know how your chimney is installed.
I understand that it could seem like an absurd amount of trouble to fix a screw hole leak, but for safety my advice is that you ask for help from a certified chimney sweep in assessing and repairing the flue as well as checking the cap and the condition and safety of the flue interior.
It MIGHT be possible to seal the opening with solder or another high-temperature sealant but unfortunately in my experience, everyone around such a project is so afraid of liability that no one wants to touch a "non-standard" repair. This is so even if on their own home they'd take that risk.
As it's rain-related, before just sealing the opening or replacing that flue section (as required), I'd get a more expert inspection of the chimney cap, chimney top seal or flashing, and I'd look for leaks along the chimney run.
More advice is available from the Heatilator company's technical services department. That contact information along with a link to the installation manual for your Heatilator fireplace is just below.
(Feb 1, 2015) Debi said:
I have a stone mason fireplace with a gas starter and logs. The fireplace anchors the end of a vaulted ceiling where squirrels have been lifting the roof ridge next to the fp when it gets cold and getting in the small crawl space since there is no real attic. There is a black slow oozing stain on the mortar about a foot down from the ceiling where this is occurring. I called someone with experience with Chimneys and he said he thought it smelled like excrement from the squirrels.
I then called a roofer who told me my chimney top was flat when it was supposed to be rounded and he thought water was getting in-between the brick and the flu and it could be black mold because it couldn't be creosote since it was gas and that the squirrels were relieving themselves on the roof. I have a call in to someone to come test for black mold, but after reading your article, it seems that would be unlikely. I can get the fireplace fixed and the roof line fixed, but I don't know what the black stuff is, if this will take care of the problem, much less how to clean it off the mortar. Just in the last couple of days, a new spot has appeared adjacent to the original 8 inch drip. H E L P~
Watch out: The black stuff is typically creosote or creosote mixed with condensate since you've changed the fireplace ( I suspect ) from wood to gas - which itself is an unsafe thing to do without expert chimney inspection, cleaning, and if necessary, repair or re-lining.
So just cleaning the stuff off comes after the safety worries.
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