I want to put in a downstairs toilet and also a toilt in my garage which is away from the vent. I know I have did up the ground and connect new pipes but does each one need a vent or can they be vented from the existing vent stack!

Hi Mike...

As you suspected, all fixtures need to be vented.

In the case of adding toilets I'm figuring that you will also be installing new sinks... right? If that is the case then you may be able to use the sink vents as WET VENTS for the toilets. Here, the sink drain and vent will need to be 2" and those vents will need to connect into a 2" or larger vent.

Double check with the local plumbing department in you area to be sure that wet venting is allowed... OK?

For piping details let us know more about your plans.

Mark

Can a air addmitance valve be used so I don't have to take vent stack through flat roof[for the toilet]

If they are allowed in your area then AAVS can be used.

AAVS must be/remain accessible for future change outs in case they fail.

Here, you would install a wye fitting just behind the toilet elbow and then run 2" up to pick up the sink. Install the AAV with a readily accessible access panel or install it under the sink base as high as you can get it and you should be all set.

I'm off to the next job... any more questions just leave them and I'll respond when I get back in... OK?

Mark