How to diagnose & evaluate stains on chimneys outdoors or indoors & where to look for signs of chimney damage.
This article describes stains that appear on chimney surfaces both indoors and outside. We identify common dark brown or black stains as well as white or light-colored stains or white powdery material that appears on masonry chimney surfaces.
We explain what these chimney stains might mean, why safety hazards may be present, and how to proceed to inspect and repair the chimney.
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Diagnosing White Efflorescence Stains on Chimney Exteriors
White powdery or crystalline stains on masonry chimneys is often efflorescence, a mineral salt left on the surface as water evaporates. Efflorescence may appear on the outside surfaces of a chimney both outdoors and inside as well as inside the flue itself.
(You are not likely to see these stains on a metal chimney unless the metal chimney and flue are damaged and leaking insulation material.)
If you are not sure what efflorescence is or how to recognize it, see EFFLORESCENCE SALTS & WHITE DEPOSITS.
Efflorescence might be just cosmetic - as wind-blown rain wets the chimney sides and leaches out mineral salts which remain white on the masonry surface of the chimney.
But efflorescence might also mean that water is running inside of the chimney flue or chimney structure. Further diagnostic inspection is needed.
Left unattended, water entering a masonry chimney risks damage to the flue (unsafe) and to the chimney structure (unsound and unsafe).
At SLATE ROOF DEFECTS Questions & Answers we provide additional discussion about sources of white stains that may be found on chimneys or on roofs below or near chimneys.
How to Evaluate & Diagnose Black or Brown Stains on Chimney Exteriors
This chimney is suffering water damage from at least two locations. The white efflorescence along the upper side of the chimney (at left) may originate at a bad chimney cap or rain cap.
But the chimney also seems to be suffering from roof runoff/spillage, perhaps due to a clogged roof gutter - as you can infer from the additional brown and black stains on the right side of this chimney from below the roof eaves.
Inside the building it is important to inspect building walls for signs of chimney leakage and it may be appropriate to have a professional inspect the safety and condition of the entire chimney flue.
Notice the black stains originating at the chimney shoulder on its left side, and the cracking stucco on the chimney's right side in this photograph.If your chimney inspection began at ground level, both of these clues should raise a red flag and call for close attention to the chimney condition from both outdoors and inside the building.
This poor chimney, discussed in more detail at CHIMNEY MOVEMENT, ONGOING vs STATIC, was leaning well away from the building - not so easy to notice when inspecting the chimney base shown at left.
In our chimney photo at below left, notice the black and brown stains (and that odd rain cap) at the upper section of this masonry block chimney.
Now without looking more closely we're not sure if we are seeing a history of creosote wash-down the chimney sides from a woodstove or other heater (the flue is too small for a fireplace), or alternatively, we might discover that the chimney interior flue is damaged, cracked, leaky, sending rain-washed creosote or condensate out thorough the chimney at cracks and mortar joints.
This chimney needs expert inspection for safety and condition.
Oil Burner Soot Stains at & Below a Chimney
At above right the black stains on the sides of this chimney near its top are probably soot or creosote wash-down.
The chimney flue is short, the chimney cap is thin and cracked, and of course there is no rain cap (see CHIMNEY CAP & CROWN) . Look closely for frost damage at both the exterior and interior of this chimney. Photos of soot at a metal chimney cap and on the roof below were provided courtesy of ASHI inspector Roger Hankey.
Soot at the chimney top (photo at left & below left) , particularly on an oil or gas fired heating equipment flue, is a good indicator of improper equipment adjustment or a related chimney and draft operating problem.
Watch out: sooting gas fired heating equipment is very dangerous and risks production of fatal carbon monoxide gases in the building.
Our second example of a roof stain traced to a chimney is shown in our photo at below right - rusting metal chimney components. See ROOF STAINS from CHIMNEYS for details and for more examples of both of these chimney related stain topics.
Dark Algae Stains on Masonry Chimneys
The dark stains at the upper center of this brick masonry chimney are probably algae growing in higher concentration on the masonry surface that is most-wetted by runoff from the chimney top.
But we'd want to make an on-roof inspection from much closer to be sure we're not looking at soot deposition.
Roof stains due to chimney defects: this section on roof stains at or around chimneys describes how to identify and diagnosis of black roof stains caused by soot or creosote washing off of building chimneys and we discuss possible safety and fire hazards associated with this condition.
Our chimney inspection, cleaning & repair articles discuss all aspects of chimney inspection & troubleshooting, while our building stain diagnosis articles articles tell readers how to identify & explain the most-common causes of black, brown, red, gray, green, or white stains appearing on roof shingles and on other building surfaces. This article focuses on stains on roofs caused by chimney problems.
Soot from fireplace flues washing down onto the roof - characterized by staining appearing below and in line with the chimney
Similarly, soot from oil-fired heating flues washing down onto the roof, characterized by the same pattern of staining around and below the chimney; also probably an indicator of an operating problem with the heating system.
That was clearly the case in the photograph shown at page top.
Gas Flue Safety Warning: If this same sooting appears on and around a gas-fired appliance flue, there is a very unsafe condition present and risk of fatal carbon monoxide production inside the building. Immediate action is be needed.
[Photograph of a soot-stained roof top at page top was courtesy of Roger Hankey a Minnesota home inspector.
The Location of Roof Stains Can Help Diagnose Their Cause
Unlike other roof shingle stain patterns shown in our article series on roof stain diagnosis, black stains caused by soot staining from a chimney or brown stains from a rusting chimney or chimney flashing, or sometimes a combination of the two will be prominent in a wide swath along the sides and below the chimney and will be little in evidence elsewhere on the roof.
Our photo of red-brown roof stains below and around a metal-enclosed chimney (left) was taken in Wappingers Falls, NY. The stains on this roof are around and below a metal enclosure that in turn surrounds a metal chimney.
This CLOSER LOOK at the chimney top [Image] shows black sooting - the heating system in this home has not been operating properly, and we pose that the roof and chimney stains are probably a combination of rust and soot wash-down from the sooty flue gases as well.
Watch out: we often see that the top of metal box frames around older metal chimneys has rusted through, but may not be noticeable from the ground. Sending water down the chimney risks costly damage to the heating furnace or boiler. Look for water leak stains on the flue and chimney base inside the building and inspect the chimney top carefully from outside.
Rust and red stains on roofs and chimneys are discussed in detail at RUST STAINS on ROOFS.
See CAUSES of ROOF STAINS and CATALOG of ALL ROOF STAINS for the causes and types of stains that occur on building roofs. There are other sources of roof staining, from cosmetic to harmful to the roof.
Continue reading at CHIMNEY STAINS, INDOORS or select a topic from the More Reading links shown below.
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