How should I call this out?

It looks like corrosion caused by condensate should it be called out as staining and to monitor or as staining potential of corrosion of roofing material?

Is that poo stains?? What is that? A vent stack or a cleanout or ??

Its a vent stack from a furnace. NOT poo stains LOL

Point was… you need to give a lot more info if you expect any qualified replies!

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Report what you see:

Corrosion and staining observed below (high efficiency furnace exhaust?) on metal roofing. I recommend a metal roofing contractor correct the corrosive staining to prevent deterioration of the roofing material.


If that window was operable I would write it up as insufficient clearance.

As for the corrosion, see Larry’s reply above.

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I call out surface rust. Rust leads to corrosion. Corrosion = deterioration.

I write harder depending on the severity or possible consequence.


I know your point. I try to ignore you, but you have a way about you. You always have a way of not asking your question. So I make light of your condescending ways. I hope my clarifying what it was, helped. Thanks?

What is the vent for?


Really? Three questions on one line. Seems you’re the one with issues.

Hell, even Robert doesn’t get ya!

If the window was operable it had insufficient clearance. If it was a fixed window it’s fine.

I know the diagram is for a sidewall mounted vent but the same clearances apply.



Does the exhaust vent on a high efficiency furnace need an elbow? As far as I know the exhaust is supposed to go straight up and the vent is elbowed down. The condensation from a high efficiency furnace is corrosive so that may be the reason for that area of the roof starting to rust.

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High efficiency furnaces burn natural gas (methane) or propane and oxygen to produce heat. The byproduct of combustion is water and carbon dioxide. But the flame also needs air, which contains atmospheric nitrogen. The flame is hot enough to force the nitrogen to react with oxygen producing nitrogen dioxide and nitrous oxide, which dissolve into the water vapor in the exhaust stream to forming a dilute solution of nitric acid. The amount of condensate produced by a high-efficiency furnace is approximately 1 gallon per 100,000 Btu/h. For example, a 150,000 Btu furnace running eight hours per day would create 11 gallons of condensate. These high efficiency furnaces are designed to collect this acidic condensation that drains back down the PVC exhaust pipe. If any of this acidic condensation leaks into the furnace it will rust any steel it comes in contact with. The exhaust pipe should be straight to prevent the issue in your photo and the intake pipe turn down to prevent rain from entering.


Several issues.
1: Vent termination orientation. Open to weather and rodents.
2: Deformed vent boot.
3: Cable fastened to furnace exhaust vent. Suspect: Partial cause of metal roof staining.
4: Vent A: Clearance. B: Height.

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I used to call it out too based on that same diagram more or less. Then I started checking some furnace installation manuals and there’s a lot of them where their clearance recommendations are usually around 12 in. I also think that most aren’t going to be opening their windows when the furnace is running :wink: