why is it necessary to vent a pipe to drain it?
Separate Vents. Sometimes, itâs just not feasible to vent only the sink, maybe because a shower or tub is sitting too far away, or maybe because one fixtureâs drainpipe does not connect to the main bathroom drainpipe, which is usually the line running from the commode. When this occurs, an additional vent is necessary. In new construction, the plumber will install a separate vent for any plumbing fixture that does not use the vanity vent. If a fixture has its own vent line, the line will run vertically to the attic and tie into the vent-and-soil stack if possible.