Consolidating Plumbing Vents in Attic

[Devinc]

We currently have five plumbing vents through our roof. There are two 2" vents (toilets) and three 1.5" vents (sinks/tubs, washing machine). Before putting on a new roof I want to consolidate them all into one new 4" vent. The two existing 2" vents are furthest away from the new 4" vent, and there is am existing 1.5" vent roughly inline between each existing 2" vent and the new 4" vent. Diagram attached. I will use 2" pipe from each existing 2" vent to the existing 1.5" vent. Can I use 2" pipe the rest of the way to the new 4" vent even though it's taking on the traffic of a 1.5" vent along the way? Or does it need to be larger to account for the combination of a 2" vent and a 1.5" vent? I'm looking for "X" and "Y" in the diagram attached. Thanks in advance!

 

[John Gayewski]

It's all about how many units are attached to each one. Generally 2 inch will cover you. But when you start to get a few toilets and washing machines it'll need bumped to 3". Your plan should be fine unless you know one or more of those pipes is loaded down with fixtures.