How drainage and venting work

Household drains work by gravity: water falls through progressively larger pipes toward the connection. For that to happen smoothly, air has to move through the system too.

This is why sewer symptoms travel. The fixture where you notice something is not necessarily near whatever is causing it.

What this page cannot establish

this page cannot establish what is happening in your system
it cannot tell you whether venting or the buried line is involved
the fixture where a symptom appears does not establish where the problem is

Why one fixture responds to another

Fixtures on the same branch share the run downstream of where they join. When water moves through that shared run, it displaces air, and that air has to go somewhere. Where the system is behaving normally, it moves through the venting arrangement. Where something restricts flow, the air can find its way out through a fixture instead.

That is the general mechanism behind the common observation that flushing upstairs produces a noise or a level change downstairs. It is a description of how systems behave, not a finding about yours.

What this means for what is worth writing down

the pairing matters more than the symptom — which fixture responds to which use
the fixtures that behave normally are as informative as the ones that do not
the level a fixture sits at within the building is relevant
whether the pattern is consistent or changes is relevant

Observations this explanation relates to

Gurgling

Sounds from a drain or fixture when water is used somewhere else.

A professional hearing this is generally trying to distinguish: which fixture makes the sound and which use triggers it; whether it is consistent or occasional; whether it coincides with slow drainage anywhere.

Does not establish: the cause of the sound; the location of any restriction; whether venting or the buried line is involved.

More than one drain affected

Several fixtures are draining slowly or behaving unusually at the same time.

A professional hearing this is generally trying to distinguish: whether the affected fixtures share a run downstream; whether the lowest fixtures in the building are involved; whether one fixture backs up when another is used; whether the pattern is consistent or changes day to day.

Does not establish: the location of an obstruction; that a shared run is involved rather than several separate issues; the condition of the buried line.

Slow drainage

Water leaves fixtures more slowly than it used to.

A professional hearing this is generally trying to distinguish: whether the change was gradual or sudden; how many fixtures show it; whether it varies with the amount of water used; whether it has ever fully blocked.

Does not establish: that the line is damaged; that the line is not damaged; how much of the line is involved.

What would change the answer

nothing to read this
to apply it: a note of which fixture responds when which other is used

What to do next

The pairing — which fixture responds to which use — is the observation a professional will ask for, and it is easy to lose.

Record the pattern you are seeing

If your situation is different

Several drains are affected at once: More than one drain affected

Gurgling
The observation this explanation makes sense of.
Multiple drains affected
Multiple fixtures behaving together is the same phenomenon.
Symptoms
Every observation this explanation makes sense of has its own page.