new bathroom

am putting a bathroom in my garage. It is above the garage, on the second floor. How far from the toilet can the vent be? Do all the fixtures need to be vented? There is a sink a tub and a toilet.Is one vent downhill from the toilet eenough?

All of the fixtures need a vent, but that doesn't mean three pipes through the roof. You can combine them anywhere above the flood plane of the highest fixture (the sink) into one pipe up through the roof.

layout

Thanks for the rep;y. The sink is next to the toilet on one wall. The tub ,a clawfoot is opposite them on the opposite wall. Can you give me a basic layout that I can somewhat follow?

Best left for one of the pros. I know the concepts, but not all of the details...don't want to lead you astray! Each situation is a little different. Where is the main drain line yo will need to connect to? It must be at least a 3" line for the toilet. The sink and tub can be 1.5", although you might want to run 2" for the tub and reduce to 1.5" for the drain (that would make it easier if you later decided to make this a dedicated shower since that requires a 2" line - tubs drains are 1.5"). All drain lines need to slope 1/4" per foot down. Depending on the depth of the floor joists, this can be a problem to obtain the slope needed, so keep thatin mind. Also, there are limitations on how big and where you can make holes in the joists, so depending on which way they run, and the locations of the fixtures can be problematic as well. Sometimes, you need to make a pipe chase (box) below the floor in the ceiling below to accommodate the drains.

venting new plumbing

So can I run a 3 inch line to my toilet{ the farthest from the waste pipe} with the proper pitch, drop a 1 and a half line in from the sink , and then come over from the tub with a 2 ihch line ? Where in this order would the vent come in?Is the vent to prevent gurgling or for odors or what?

Each drain needs its own vent line. The vent line prevents the drain from being siphoned dry. Each time you run waste down the drain, it displaces air. If you can't make that up you run the risk of sucking an adjacent drain dry, then you have a path for sewer gas to come back out of the pipe into the room. So, for three fixtures, you need three vent lines. As noted, you can tie them together so you only need one going through the roof, but it has to be done above the highest water - the flood rim of the sink. Usually easier to do it in the attic.