I am building a new full bathroom on what used to be my back porch. I am planning to extend the existing drain at a cleanout and to run it outside the house underneath the floor.
I need to know if I need to add a new vent to the extension, or if I can rely on the existing vent. The total length of the new (4") drain will be about 15' from the existing vent to the farthest fixture.
I am in Virginia. I've looked at the code but I can't figure out which rules apply to my situation.
Best Answer
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.