Most roofers, if asked, will install screens on your plumbing vents after they re-roof or re-flash. However, these are often not extended down to the bottom of the vent due to the roof pitch. in fact, sometimes they just cover the top of the vent. This is not effective as this picture shows. In this case the squirrels have just chewed the base off, producing a major leak. A CQ covers the entire vent and looks a lot better. So protect your vents and your home, buy some Critter Quitters.