On a recent repair, I saw a severe rust problem coming from UNDER the flashings. I removed the top pipe and flashing from the tile and got no resistance from below (from being attached).
You can see all the rust that had occurred to the bottom of the flashing and portion of the pipe in the attic. The outside exposed part of the pipe had no rust.
This is looking in at the failed connection. There was only a little bit of rust at the joint.
This picture shows the results. You can see the rust stains on the tile that was under the pipe flashing and on the step flashings in the background. More importantly though, you can see the charring of the edge of the plywood and the tile batten. It was only a matter of time until this started a fire. I think the original roofer found the base flashing nailed down at the plywood level and when he raised it to the tile level, he broke the pipe joint and almost caused a fire almost two decades later. I coordinated with a heating guy and got this thing fixed.
Good catch Woody... Most roofers don't pay much attention to heater vents, they just flash them and roof on. I remember TRG had one a coupla yrs ago where there was a risk of the occupants suffering carbon monoxide poisoning 'cos the wrong type of vent was used.