in buffalo ny: none. sewer vents are open to the sky.
see:
http://www.free-ed.net/sweethaven/Bl...p?iNum=fra0308

Nexus7 wrote:
The plumbing stack sticks out 2' from my roof. It is a 4" iron pipe (OD
is 4.66"). That's all there is, just the pipe sticking out. What kind
of cap, if any, is appropriate for it? I can get a cap that is made for
appliance vents and is adjustable for 3-5" diameter. However this is
designed with an upward flow of combustion gases in mind. Thanks.