The most common cause of a leaking vent pipe occurs at the collar surrounding the base of the vent pipe. The base of the vent is usually aluminum or galvanized steel, but the collar is made of rubber. As described in the video, these leaks around pipe collars often go undetected. The roof leak is never a major one but it does allow rain water to enter the attic space. If the attic is properly insulated, the insulation will absorbed the water, completely drying before the next rain. This cycle will go on-and-on until the plywood substrate surrounding the pipe collar completely rots. These small leaks work like compound interest. If you leave it alone long enough, the monetary value will be huge.