My bathroom sink and tub are directly adjacent. The tub drain appears to connect to the sink drain line. Photos follow.

Under normal use, there is no problem and both the sink and tub seem to work normally. However, if the sink is allowed to drain from full (for example after shaving), water backs up into the tub.

This has occurred for as long as I have lived in this house, which is only for about the past 3 months.

I have read a number of posts indicating that backflow of this type could indicate a blockage in the line. But I thought a blockage would cause backflow all the time. So could it be a partial blockage? Or is it a problem with slope on the drain line? Inadequate venting?

The tub drains from a horizontal line, under the vanity. It then connects in with the sink drain.

Fixture Layout fixture layout

Sink Drain sink drain

Tub Drain (under vanity, at tub end) tub drain

1 Answer 1


A partial blockage allows some draining, but will backup when the water flows fast. It's not unusual.

You can clean the line with a snake - let the water run the whole time (to wash the dirt away, and keep down the smell), and go back and forth with the snake letting it spin and clean the walls of the pipe.

I also don't see a vent, but it may be hidden.

  • Thanks for your feedback. I don't know if it has a vent either - but would a missing vent cause this drainage problem, or is that an aside?
    – Roberto
    Jan 14, 2016 at 2:27
  • The Vent was an aside. It's sometimes inside the wall, but that's more common when the pipe is horizontal, and yours is vertical. You probably need to add one, you add it right where the black and silver pipes meet.
    – Ariel
    Jan 14, 2016 at 6:46
  • OK, got it now. If it's not vented, can I assume there is no vent nearby (otherwise they would have put one in the first place)? And that means adding a mechanical vent now?
    – Roberto
    Jan 14, 2016 at 11:47
  • @Roberto I don't know, I can't assume anything. But it would not hurt to add a mechanical vent. But again this is unrelated to slow drains. For that you need a snake.
    – Ariel
    Feb 4, 2016 at 3:02

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