Does a basement bathroom that's already roughed in need to be vented?

Hey Geno... The answer to your question depends on how the basement bathroom was roughed in... That is, was it roughed in by code.. or not..

A basic full basement bathroom has at least 2 vents connecting into 1 vent that runs somewhere into the main vent stack or 1st floor bathroom vent system.

That may sound complicated or not... point is that if you are lucky all the work is done for you (*crossing fingers*)... you need to check behind walls or at least into ceiling/joists and see if there are vents present.

Start at the sink. Do you see a tee type fitting (in the wall) stubbed out and into p-trap of sink? If you see a tee type fitting then that sink is vented. It may even be the wet vent for the toilet (discuss later). If you look by the tub waste or the shower valve end of shower you should see a pipe coming out of ground somewhere near... that is tub/shower vent. If you see any of these thing... IT IS A GOOD SIGN!

Geno... after you check these things... check back and let us know what you find.