Please explain why a sink pipe needs to be vented , and the location of the sink pipes "in" the attic ?
BL, I know you know the answer to this.
all drains in a house require an air vent so the draining of the receptacle does not create a vacuum as it tries to drain. That vacuum can prevent a receptacle from draining.
it is typically called a "stack" or "vent stack".
typically, the stack is run through the roof. Since the inception of those little vent caps, air admittance valves, that only allow airflow one way, some localities allow the vent to not have to go through the roof but use one of those to prevent outflow of sewer gasses.
the problem with an open stack is, it is open from that point clear to either the municipal sewege line or to the septic tank with no p trap to prevent backflow of sewer gasses. If this is as such in an attic, sewer gasses can accumulate in the attic if there is improper attic ventilation.
generally, it is not allowed by code to do this.
it often causes no problems but it can cause some very extreme problems if things all go wrong. It is best to fix this as soon as possible. If the locality would allow a vent cap, such as this:
http://www.oatey.com/aav_public/resources/photos/6_sink_app.jpg, it would be a simple fix. If the locality would not allow it, then the pipe would need to be extended through the roof. That would obviously be more expensive due to the roof repair involved.