Thanks! I'm assuming that 2" line in the slab running from the toilet to the shower is the vent for the toilet, or meant as venting for the whole system?Unless there's a bathroom upstairs you won't have a stack. Size is based on the fixtures draining into the pipe. Toilets require 3", most residential main building drains are 3". Toilets require 2" minimum vent, most everything else requires 1 1/2" minimum. You can't reduce the size of a drain. What is the 2" line going from the toilet to the shower.
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