Al wants to know how to clean out a vent stack.
However some people do get birds nests in there, or clogs that for some reason are difficult to reach through the regular plumbing branches. You can run a plumbing snake into the stack from the roof, but be careful not to fall off the roof when you're up there. In part of the country the basin style roofs, which are almost flat roofs that drain to the middle, use the vent stack as a roof drain. These can collect gravel and even asphalt in the stack. In this case you should have a plumber clean it out with steam equipment capable of melting the asphalt and moving it on down the line.