drain-waste-vent
I'm adding a secondary bar sink in a kitchen area, which will extend the waste pipe to it's farthest length, so I need to put in a waste drain vent pipe up the wall and through the roof.
The drain pipe is 1-1/2" diameter and I'm wondering if the vent pipe going up the wall can be a smaller diameter, like 1" or 3/4", so I don't need to cut such a wide channel in the drywall and fight with fiberglass insulation.
I am not in the UK but my city uses the International Plumbing Code.
Everything needs to have the required slope. The required slope depends on the size of the pipe.
IPC table 704.1 shows that a pipe with an ID (interior diameter) of 2.5" or less needs 1/4" per foot. While a pipe that is 3" to 6" only needs 1/8" per foot.
You must have a vent above the vertical run for these drains. As drawn, water from your new work cannot draw air behind it from your existing work.
This is what vents do: draw air in behind draining water.
You could functionally combine all the new work drains as drawn into a single vertical vent, but remember that it needs to be able to draw air from above their combined drain line.
Depending on your code you may or may not be able to combine these into a single vent, but they will need their own vent.
Best Answer
I've found this:
source: plumbing-basics.com