What exactly is this on the cast iron vent pipe? Is it a repair or damage?
Need more detailed information than what a photo “shows”.
Got a roof view pic?
Did you ‘touch’ it?
Did you lick it?
Chris, cast iron, typically, rusts from the inside but it is hard to tell what that is from N. MI
Did you tap on the areas to see if they were thinner?
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Looks like Blackjack or asphaltic roofing sealant.
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Rust inhibitor.
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Thanks, I thought it might have been a repair. Crawlspace sewer stack was in bad condition but the vent looked ok. It recieves less moisture obviously
I think it’s just tar that dripped there from the original roofing job.
It’s a vent, so it’s not wet except during a rain, expected lifetime of that section of pipe is well over 100 years, maybe many times that.
Even if it was a repair – which seems unlikely – is the repair holding? All it has to do is keep sewer gasses going up to the roof.
Likely, there are higher priority issues on the property.
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It may be roof tar or it could be a repair. Give that cast-iron pipe a good wrap with a metal object and if it makes a sharp ring it’s solid if it makes a dull thump it’s cracked.
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Oh there were for sure. Thanks
Im with Bryce
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I’m in N. MI, myself. 
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clever, good one
That cast iron Hub and Spigot main plumbing stack vent drain pipe is clearly split to the Hub. Likely got banged by something during the attic repairs.
Relatively easy fix without having to replace the complete pipe.
Refer to a licensed plumbing contractor for repair/replacement.