Thread: chimney discoloration - leakage?

I'm wondering if the black areas around the edge of the chimney cap and top layer of bricks indicate a flue leak. (Keep in mind I don't go on roofs in my job, this is the best image I could get). I'm aware of the spalling problems.

Thank you!

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I don't think so Kristy, Looks more like mildew from lack of sunlight on that side of the chimney.
can also just be soot that washes off the chimney cap and pours over the sides.

The spalling and efflorescence suggest moisture problems.

If house has an oil fired burner or boiler, it may be the result or an incorrect burner setting/adjustment. Have found soot from oil burner cause similar discoloration. Partially clogged burner tip that causes a sputtering of fuel resulting in excessive soot that is driven up chimney. Hitting exterior cold air causing it to fall out on cam and wash over sides.

The chimney is for two fireplaces and an indoor BBQ, so it's not an oil-fired burner/boiler problem (we don't have those 'round here, anyway). I know there are moisture issues, but didn't think of mildew. Just seemed weird the black area is concentrated where it is.

Thanks, guys!

I don't think that's mildew, quite the opposite. The sun does shine. Older chimney chase sucking moisture and wicking it - not drying out - constant moisture source for cyano bacteria needs sun.

Bleach & water solution & scrubbrush test - mildew won't green up - the "blue-green algae" will (as you scrub and wash it down with more water).

'Blue-green algae' (actually bacteria) looks black & sooty, likes that moist surface, esp. old mortar, brick, and that old concrete cap. More there is, more moisture trapped. "Eats" the mortar, brick, concrete, photosynthisizes. Feeds fungus = yummy lichen farm, holds moisture, further supporting more bacteria growth, chimney deteriorates.

Need to cut back that tree canopy. Traps/cools exhaust, redirects rain fall, and source of moisture (respiration) too. Sap, pollen, debris, digested nitrogen, phos. more food for the farm.

Get rid of the colony. Address deteriorating chimney, correct moisture problem.

Not the best article for your topic, but is comparable...the attached explored issues with fiber cement roof shingles experiencing colonization in Hawaii.

See just before applicable "Discussion" section begining p. 4. Same is problematic colonization all over the globe even near the poles. Will also colonize old concrete patios, sidewalks, pavers, on clay, Limestone sills, sandstone, concrete walls, painted siding, etc. Similar problems with different bacteria with fiberglass composition 3-tab shingles in the East without exposed copper strips (zinc strips work shorter time), when the copper-based fungicide treatment wears off, blackened areas on roof. Clings to textured pvc/vinyl too.

Thats a poor masonry chimney cap. It should be replaced with one which extends over the sides of the brick to prevent water running down the brick.

I just see some soot from wood and charcoal burning, normal, and some places where rain washes the soot down over the edges of the chimney cap.
They should do what Raymond says, rebuild the masonry cap so that it has a lip or overhang.

A cap alone is not likely to solve the issues with this long-neglected multi-flue masonry (likely tile-lined) chimney.A full diagnostic inspection and analysis needs to be performed by a chimney professional.It appears to be typical poorly designed and poorly maintained 60s-70s construction.Obviously the crown wash has deteriorated/erroded a cap alone is not likely to "solve" the issues and is more likely to be an expensive endeavor where the obvious spalliing is occuring.There could be multiple sources of uncontrolled moisture entry and suppression of the brick chase from adequately drying freeze thaw cycles.I suspect the face photographed is south and/or east downwind of prevailing winter winds (drying) and downhill of the roof peak. A level II and quite possibly a level III needs to be performed for the entirety. Covering the exterior without evaulating, identifying, correcting the problems, especially those caused by poor design and lack of maintenance, is not a solution.

There seem to be a lot of these chimney caps without any overhang on houses ca. 1950s around here - mine being one of them. I also have spalling problems, and I believe the two are related. I suspect the cap is cracked - I see at least one, I believe, under the right-most flue. At this point putting on a better cap will not solve the problem, though it may slow it. The bricks are of inferior quality, and freeze-thaw cycles will cause continued deterioration.

Interesting article, HG, thanks for that. The fact that the black stuff is confined to a rough band around the edge of the cap and upper brick layer suggests to me that blue-green algae is not the answer here (nor is mildew). I can't see how soot would be washed down from the top of the metal caps, or why it would collect on the concrete cap. Where would it collect, and be subject to washing down by rain? It's the fact that the black stuff seems particularly dense around the juncture between the concrete and the bricks that made me think there might be smoke leaking into the chase from one of the flues. It's likely the mortar has deteriorated some, too, if it follows the pattern of my chimney in that respect.

I'm not flat-out disagreeing with any of the theories here, just skeptical by nature. Hard be certain of anything without getting closer to it.

"Need to cut back that tree canopy. Traps/cools exhaust, redirects rain fall, and source of moisture (respiration) too. Sap, pollen, debris, digested nitrogen, phos. more food for the farm."

I don't think the canopy even when leafed out is dense enough and close enough to trap exhaust or be an appreciable source of moisture through respiration, though the shade may play a role in slowing dry time and shed debris retain water. Usually tree canopies reduce the amount of rain penetration. Digested nitrogen from a tree?

FWIW, here's another shot of the chimney.

It could be indigestion or it could be acute appendicitis. You decide.
It is normal for a chimney to have a bit of black around the top. Not worth mentioning, even in a home inspection.
Here's some pics. I'm not saying these are all good chimneys, but they've all got some black gunk on them.

Jon, I have to laugh... those are all good old NW chimneys, Moss is a wonderful thing is it not

Actually, I don't know why I didn't see it before, but some of the "black" does look greenish brown. I'm going with Watson's suggestion.

It may be normal on chimneys in the Pacific NW, but it's far from normal around here. You do see "algae" on a lot of older roofs, though.

There is a concrete crown that does not overhang the chimney to form a drip edge directly but sits on brick corbelling. Consequently, the creosote and soot from the chimney caps is being washed on onto the face brick corbels. Meanwhile, the moisture laden chimney is also full of effloresence, which has resulted in spalling and damage to the chimney.

several possible problems with this chimney include:

crown allowing water penetration into the chimney but retarding drying
Portland cement based mortar is too hard
bricks soft fired or not SW grade
overhanging tree can impede performance and cause smoke spillage inside
concrete crown does not appear to have a bond break, which can result in the entire crown jacking up opening gaps and cracks.

Where there's exterior water damage, there's interior water damage, too. Sick chimney. Yes on the Level II, which should determine the need for a level III. At the very least this will probably require rebuilding the top of the chimney.

Ah, I was hoping you'd make an appearance, Bob! So you don't buy the algae hypothesis? I think you've nailed several of the problems, although I think the tree looks closer than it is, and it's hard for me to see how that could have much of an effect. It's a good 15' above the chimney.

Some have suggested I deal with my spalling chimney by attaching lathe and covering it with stucco (and putting on a proper cap with overhang). I'm not convinced this is an adequate solution. Seems like the crap bricks (probably from the same factory that made the ones in this chimney!) might not hold it well enough.

Oh, the algae can be real--I just can't see and feel it from here! ;-)

I have seen trees towering over 50 feet above a chimney cause smoke spillage problems---and documented on film.

Wrapping anything over a sick chimney won't cure it--just hide it from view until the damage becomes even more catastrophic. Stucco will simply improve the chimney's ability to absorb and retain water. Without a drainage plane behind it, it will take longer to dry and add more water to the chimney interior, which are both bad.

The mortar cannot be harder than the bricks. The bricks should be hard fired to resist moisture absorption but you cannot tell visually. Rilem tubes are used to conduct a Moisture Absorption Test (MAT tubes). When you have porous bricks, you are faced with razing the structure and replacing the bricks, razing the structure cleaning the bricks and resetting them with lime mortar or repointing with lime mortar. The best solution is to completely rebuild with hard fired bricks set in lime mortar but not too many people will go for that but it performs the best. Another tricks it to do a full lead sheet pan at the roofline with weep holes and a drainage plane so water absorbed from above has a place to go.

That tall tree will shade the chimney for part of the year and tend to cause higher moisture retention. It would be better placed as fuel inside the fireplace.

When you have an algae problem, consider switching to copper chimney caps. They will act as a natural algaecide but they may cause some surface staining depending upon what's in the water, mortar and bricks. If they cannot afford copper caps tell them to glue several pennies on the crown. Zinc works well, too.

Sigh, I had a feeling a rebuild was the only good solution. That thought has been plaguing me since I got my house. (The house in the post is an insurance case, and out of my hands; I put it in the forum for my own edification.)

After a bit of surfing I now think I understand the tree thing better. Couldn't picture how branches 50' above would disturb horizontal windflow around the chimney that much. It seems like it's the tree canopy to the side of a chimney that causes the bigger problems with eddying wind, as shown in the nice little graphics on this page. I can also see how branches directly over a chimney could deflect air downwards, just couldn't picture a few 15' up "trapping" enough smoke to make a big impact, unless maybe both fireplaces and the BBQ were going, and the wind quite still.

Carbon and water equals - carbonic acid.

Also condensation can escape into the brick due to poor alignment/gap in terra cotta liners allowing moisture to escape and push through face of clay brick popping them. Particularly in cold climates.

Originally Posted by Raymond Wand

Carbon and water equals - carbonic acid.

Well, um, not quite, or we'd be in trouble, since carbon is the 2nd most abundant element in the human body, and various forms of carbon are all over the place. Carbon dioxide dissolved in water creates a weak carbonic acid solution (only so much forms acid, the rest remains as CO2). This is one reason municipal water is usually made alkaline, otherwise pipes would corrode more quickly (but y'all probably know that ). The carbon in soot wouldn't form acid, though the carbon dioxide released from burning wood, etc. may mix with water in/on the chimney...but then lime from the mortar and cap would neutralize it, and some lime would be leached from the concrete in the process. Hard to say how big a role the carbonic acid would be in the breakdown of a given chimney.

Geez, with all the ways chimneys are susceptible to moisture intrusion, it's a wonder they stand as long as they do! And what were they thinking when so many caps were made without a drip edge?

Does that mean you didn't like the schematic?

Heck, no! It's a lovely little diagram, and I thank you muchly. But hey - that would also provide a way for smoke to escape the flue and squeeze out cracks under the cap...maybe my original idea of the black stuff being soot isn't so crazy after all!