3" plumbing stack vented into chimney with the water heater
Was it a working chimney?
yes it was shared with the water heater
Oh good methane in a chimney.
I find plumbing vents penetrating chimneys quite often. It causes excessive spalling of the liner (if equipped) and bricks.
I simply recommend removal.
Most of these were done this way due to the slate roof. The Plumber didn’t want to penetrate the roof.
You are correct the garage did have a slate roof. This one had been replaced a while ago.
That’s usually the case when I find these plumbing vents penetrating chimneys in the attic.