How can I waterproof this weak spot in my roof tile where pipe goes through it?
I would take a piece of flexible rubber sheeting approximately 12"X12", cut a hole in the rubber about the size of the tubing (make it a tight fit), near the center, and slit the rubber from the edge to the cut hole. Fit the rubber sheet around the tubing and use silicone caulking to seal the underside of the rubber, the slit area and between the tubing and rubber sheet. Use a lot of a good silicone caulk (not the cheap stuff or spray foam), 2-3 Tubes of caulking should suffice. Lastly, coat the whole top of the rubber sheet with a thick film of caulking and spread with a cheap paint brush to coat the whole rubber sheet out past the edges.