On most homes today when you look up at the roof, you see a white PVC pipe sticking up from the home. This pipe is usually a plumbing vent that goes out of your home through the roof. Most homes today use a neoprene ‘boot’ around the PVC pipe that over time dries out and the gasket or flashing cracks. When this occurs, usually the first sign is a small stain on your ceiling. If you see a small stain on your ceiling after a good rain, your first look should be this boot and pipe area. I recommend calling a professional roofer for the repair, they are trained to walk on roofs and can make this repair fairly easy.