I had my roof replaced 6 years ago. The roofer installed the flashing device pictured here. I now believe there is a leak at the stack. (During some bad storms a stain develops on my bathroom wall above the toilet right where the stack pipe runs.) My sense is this IPS flashing device is not for cast iron stacks and the rubber collar may well have been compromised by being forced over that existing old ring of cement. My inclination is to attempt to seal around the rubber collar with roofing cement. Would that be just making more of a mess? Or might there be a problem elsewhere around the flashing? The roofer won't respond to my calls. Thanks for your advice.