I have a bathtub that has horrible drainage and its because of an old galvanized horizontal pipe that connects to the stack.

I cannot replace this pipe with PVC with a better slope without compromising the floor joist structure in the ceiling. Home is from early 1900s.

However, I do have the ability to run a new drain all the way down to the basement and add a y connector that would keep the old galvanized pipe as mostly a vent.

I am a little concerned with the space I have in the ceiling to pull this off but I think I can do it. I am more concerned about the small 2 in drop where the pipe exits to the stack if I use it as a vent. (Please see images below)

Can someone give me an opinion of what I have proposed? Would the existing horizontal pipe work as a vent in this situation or would I be making any regressions here?

Today: Today

Proposed: Proposed

Photo: Photo

  • are there any other things feeding the stack above where the existing drain connects? (and what are they?)
    – Jasen
    Commented Jan 29, 2020 at 2:54
  • The toilet and bathroom sink drain into the stack at around the same height as the tub drain.
    – Sealer_05
    Commented Jan 29, 2020 at 3:27
  • your new setup may need to meet the building code and I don't have the skill to answer that,
    – Jasen
    Commented Jan 30, 2020 at 5:13
  • I'd think you could just cut out the galvanized pipe and replace it with PVC. Why don't you want to do that?
    – gnicko
    Commented Jan 31, 2020 at 20:02
  • Because there is no room to piece one in between the joists because it would add knuckles and it's already extremely tight.
    – Sealer_05
    Commented Feb 1, 2020 at 21:02

4 Answers 4


Depending on your location, this may not be up to code. Under UPC, a dry vent cannot be horizontal until it is a few inches above the fixture it vents. If your drain pipe backs up over time, you'll have water enter your horizontal vent section, where it can deposit solids and cause the vent to clog. A wet vent can be horizontal because it's expected to be flushed out by other fixtures that drain through it, but there's nothing to stop a horizontal dry vent from becoming clogged over time. Your proposed setup is more likely to suffer from a clogged vent than your current setup.


It appears your floor joists are already compromised by being notched out for the old drain pipe. But, to answer your question, since you have no vent now I think your plan is at least an upgrade.


When I have had a room gutted like yours I try to do the best job I can for the future. You plan isn’t bad but I think you can do better. If that galvanized pipe is bad enough that it is a poor drain the it might not be much better as a vent. Can’t tell by the photo but it looks like 2 “ pipe. One method to change the pipe with PVC in a confined area is to cut shorter lengths and connect them with no hub clamps. This install method allows you to make adjustments without having to worry about glue drying It works well and is quick on a retrofit like yours. You would then have a new drain that would function as both vent and drain if there are fixtures above that point on that stack.


As you drew it if the new "vent" becomes even partly clogged that big drop stands to siphon the bath trap. Is there room between the tub wall and the room wall to shove in a "kitchen island" vent arrangement? This is where the vent (your old horizontal pipe) is protected by a loop of air that extends above the water line. Whether this is to code I don't know, but I think it would work well.

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