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.