3″ or 4″ Drain Pipe?

3″ or 4″ drain pipe – which is better? That is actually a very good question. Here is the very simple answer – 4″ is usually better. But Why?

A 4″ drain pipe can “wet vent”. Basically that means that the pipe is big enough that the water and air can pass through it with very few venting issues. That does not mean that venting is not needed though. It just means that it is not as critical with a 4″ drain line.

A 3″ drain line on the other hand is acceptable in a lot of homes. It just needs more help venting it properly. Either way, if you can do it, try to opt for a 4″ main drain line if you can.