I'm using 2" DWV pipe for bathroom/kitchen sinks and vent. I'm in the
process of constructing the walls. Can I get by with a 2x6" wall or do I
need 2x8"? I will be boring the studs to accommodate the pipe.
Is this legal in your area? Here you must have at least one 3" vent line,
secondary vents for additional sinks, toilets, etc. can be 1 1/2" pipe and
if they don't tie back into the 3" main vent they must expand to 3" as they
exit the roof.
There is also typically a requirement that the aggregate vent size be
at least as large as the building sewer (main drain). Or sometimes
there is the stronger requirement that at least one vent is as large
as the building sewer. Just FYI.
If the walls are not load bearing, then you can bore a hole up to 60%
of the stud depth, which is 3.3" for a 2x6, plenty for 2" DWV.
If a wall is load bearing, then you are restricted to 40%, which is
2.2" for a 2x6, not quite enough for 2" plastic DWV (but OK for 2"
copper DWV). However, you can bore a load bearing stud up to 60% if
you double the stud, although not for more than two consecutive studs.
As Wayne mentioned, the largest hole you can put in a load-bearing stud is
2.2" (40%), or 3.3" (60%) in a non-load-bearing stud. If you double-up the
studs, you shouldn't have a problem with 2-1/4" holes (41%) for a short
horizontal run. I had to run a 2" drain for our kitchen sink through an
exterior 2x6 load-bearing wall under a window to reach the vent/drain off
to the side (about three feet run total). My inspector said it was fine.
As for using 2" pipe when 1-1/2" would do, I did it for simplicity. It's
easier to pick up a supply of 2" pipe and fittings than to keep an
inventory of various sizes on hand. Especially when you're working a long
distance from the nearest store. I plumbed my entire house with 2" and 3"
You generally need the equivalent vent as the main drain leaving the
building. For instance, if you have a 3" main drain, you need a 3" vent. Of
course, you could use three 2" vents to get the same venting as a single
3" vent. On the other hand, if you live in a cold climate, smaller vents
can ice over and potentially block the small vent. So 3" vents are often
the minimum size in cold climates.
In my case, I wanted to minimize the number of pipes penetrating the roof,
both for appearance and the increased potential for leaks. So I installed a
single 3" main vent through the roof, and tied all my fixture vents into
the one main vent. I have a large 3" trunk line running through the attic,
and all my smaller 2" vents tie into that. It meant a little extra pipe,
but PVC pipe is fairly inexpensive and the tradeoff was well worth it in my
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