Most of the current roof jacks available on the market today can be installed after the roof has been laid. I prefer the metal plates with the rubber bushing for the pipe. If the nail pattern of the roof doesnít permit you to slide the whole plate under, just cut back the top as little as needed to fit. If you take the time, it possible to cur the shingles to wrap around the pipe to hide as much of the roof jack as possible. Iíve done this with every type of roof except tile roofing. Iíve been plumbing for over 10 years and have put more then my share of vents through existing roofs.