Coal Burning and Acid Destroying Roofing

Anyone ever hear of this?

I don't know if what I was told has any truth to it or not, but I thought I'd get some input from all of you. Years ago I had an old timer from coal country tell me to be careful about burning a coal stove too low, too often. He said it allows acids to build up in the exhaust causing it to fall down around the stack and roofing and eat it away, asphalt or metal roofing. He explained that the stack doesn't get hot enough, therefore an acid builds up while in the stack and since the stack air is colder air than when hotter it allows the acid to fall on the roof and eat the roof. I can't explain it exactly the way he did and I certainly wouldn't be disrespectful to him and argue with him about it.

What I can tell you is that when I was younger and made the many trips with my family to south eastern Kentucky's coal country, I did in fact see an awful lot of old homes still standing from the 30's and 40's with metal roof with holes rusted in them, people still living in them...rust running all down the roof. Even modern homes of that time. During the depression I know it was hard everywhere, but in Appalachian mountain country is was especially tough. Most in coal country anywhere on the east coast have parents and grandparents that know about and/or experienced those times. Now I'm not saying the depression or being poor had anything to do with what I was seeing on those roofs, so don't misunderstand me. I just thought I'd throw that in there...as if some of you didn't already know. LOL!

Heck, I see similar roofs around here today from wood burners and the creosote problems people create by choking down their wood fires.

I have no idea if the old man was right. Perhaps it may have been caused by the type of coal being burned. Perhaps it was something other than what they thought. What I won't say is that he was wrong, because I don't know. All I know is that most old timers decades older than me usually know a thing or two and often they are correct. Could be hogwash too!

What say you?

What you are describing is the result of Acid Rainfall and Acid Snowfall.
So much also depends on the soft coal size, type of stove, stoker boiler, stoker furnace and whether a natural draft of forced draft heating appliance is being used for soft coal burning creating enough combustion air to burn efficiently with higher heat creating less ash.

Had asphalt roof replaced 2 years ago. It was not worse around steel chimney. The flashing was rust colored though, but is aluminum. I do think the old metal roofing would suffer near the chimney. The top sections of stainless chimneys certainly do.

He's right

lzaharis wrote:What you are describing is the result of Acid Rainfall and Acid Snowfall.

Let's assume that's correct. Then wouldn't the entire roofs look that way? I mean, after all it does rain and snow on the entire roof.

The roofs I used to see and what the old gentleman was describing were mainly around the stacks and directly under the stacks towards the gutters, or in the direction of the prevailing winds.

Does it only happen with soft coal? If not, then is it a result of fly ash landing on the roofs near the stack and mixing with rain and snow in those areas. I'm just guessing here. I can't say why so many of the roofs I've seen (in soft coal country) look the way they do. I have noticed the problem though.

Anyway, I just thought I'd mention it here since it was on my mind...because I just put a new roof on my house.

I must have the low sulfur variety....I've got 23 years out of the SS chimney and its still going. The original cap is on there minus the steel screen and the inner locking ring which did rust away. The shingles are also 23 years old and I plan on replacing them one of the next two summers while I am still physically able. They look no worse near the chimney than anywhere else. Maybe the effect is worse on metal roofs?

You betcha, it's a given T.

Ok, here is what is going on. Asphalt shingles are far less affected by chemicals than metal. The biggest problem with burning coal is having a cap on the chimney; as the flue gasses condensate on the cap & ultimately drip onto the roof. Now if you have a SS chimney than you are relegated to having a cap for obvious reasons. However if you have a masonry chimney than there is no need for a chimney cap, as it will do much more harm than good. My house is over 60 yrs old same masonry chimney, & never had a cap. As a retired mason, can't tell ya how many chimney repairs I've done because people burning wood had caps & the dripping creosote ate up the top blocks. So ifn ya have a metal roof & masonry chimney, ditch the cap!
JIm

A proper chimney has no metal.

coalder wrote:The biggest problem with burning coal is having a cap on the chimney; as the flue gasses condensate on the cap & ultimately drip onto the roof.

coalder wrote:However if you have a masonry chimney than there is no need for a chimney cap, as it will do much more harm than good.

That, is very interesting, I never thought of that. This further validates my "no chimney cap" policy. I removed mine originally because it drafts better without the cap. Now I know there are other reasons to take it off.

-Don

Don, I have a buddy who has a 30 by 50 metal building. He has two SS chimneys about 10 ft apart one is for his coal furnace the other is for his oil furnace. The building is built under a high embankment, so when on this embankment the roof/chimneys are clearly visible. Now mind ya both obviously have caps. Both chimneys showed advanced phases of oxidation on his metal roof under each chimney however the coal chimney was by far the worst. As far as the masonry chimneys go; while you are driving around look for wood burning chimneys with caps & I'll bet they are all coated with creosote. & some will visibly appear rotten at the top. The only exception would be burning 2 yr seasoned wood; but even then I am not so sure.
Jim

Modern shingles blow off or are shot long before coal will bother them.

Fly ash would probably have some impact on steel roofing over time, but I am not sure how long it would take to be noticeable.

My steel roof was 100 years old when I had it replaced, and that was only because the nails rusted out or pulled out. The house was heated with coal initially and then with high sulfur fuel for many years. We are on year 6 with the new steel roof and coal heat. So far so good.

I would opine that metal roofs generally fail due to "penetrations" and other things that scrape off the coating. Around a chimney this would be due to cutting the metal and leaving an unfinished edge. The reason that we painted any cuts in our metal roofs..... Over time nails and screws will work back and forth as the metal expands and contracts, plus it can rub on nails and such under it. I think that a roof penetration is where warm moist air will rise to and condensation will form in cooler or cold weather, this well accelerate rusting....particularly in older homes with stone foundations etc. I have gone into the attics of such homes that have been "tightened up" with new windows and blown in insulation and seen 2 or more inches of frost.......

Kevin

Guess I will be a test case for this as I put a standing seam metal roof on two years ago. I also have a cap on the masonry chimney so I just might take that off when the weather clears. Hope everyone is around in about 40 years when the warranty expires. Stand by.

Kevin , everything you say is absolutely true. However surface oxidation below each chimney & down to the gutters, & only in that affected area, gives credence to what I have posted.
Jim