Suds in the Downstairs Tub

Location: downstream of the point where 1A's fixtures tie into the stack - the shared stack or building drain below 1A, not 1A's fixture branch and not 2A's washer line.

Reasoning:

• For 2A's washer water to appear in 1A's tub, the water has to fill the shared line until it reaches the level of 1A's tub trap and overflow into the fixture. That means the restriction sits below 1A's connection, which makes it a shared building line.
• 2A drains normally because the water is leaving 2A's branch fine - it just cannot get past the restriction further down, so it backs up into the lowest open fixture.
• Gurgling means the fixture trap is being siphoned or the line is partially air-bound because the blockage is reducing the air path. Gurgling alone is not proof of a vent defect; here it is a symptom of the partial blockage.

Conclusion for the manager: this is a common-line problem, not 1A's misuse, so it is an owner cost. Put that in writing with the reasoning, because 1A is already being treated as the culprit.

1. Stop the addition of chemicals. Do not bring more, and do not run water into a line full of caustic. Adding water to an already-treated standing column can splash back. Wear chemical-resistant gloves and eye protection, and ventilate the unit.
2. Tell the resident to stop using 1A's tub, toilet, and any drains that back up, and ask 2A to hold off laundry until you clear the line. Put a sign on the washer.
3. Protect the unit: move or elevate belongings near the tub and toilet, place towels or a wet vac, and check the ceiling below 1A if there is a lower level.
4. Snake from the two-way cleanout in the utility closet, working downstream toward the building drain - that is the correct access point for a shared-line blockage and avoids pushing debris through the fixtures. Use a properly sized auger with a blade appropriate to cast iron, and do not force it against a fitting.
5. Flush with plenty of water after clearing to carry the debris out and to dilute any residual chemical. Watch for the line to take water freely at the cleanout before you reopen the fixtures.
6. Inspect for cast-iron corrosion while the cleanout is open. On 1970s cast iron, a scale-narrowed or channeled line will re-block in weeks.
7. Document: time, access point, depth of the obstruction, what came back on the cable, and photos. Notify both residents in writing when service is restored.

Next steps:

1. Re-check flow at the cleanout - is the main line now fully open, or is it only partly clear? If the cleanout runs free but the tub is slow, the remaining restriction is in the fixture branch or its trap.
2. Camera the line from the cleanout downstream to look for scale, a broken pipe, a belly, root intrusion at the exterior, or a shifted joint. On 1970s cast iron this is the decisive test, not repeated augering.
3. Test the vent - the roof vent or an AAV may be blocked by a bird's nest or debris, or the stack may be partially blocked by scale. A slow drain plus gurgling that persists after the main is clear points at venting. Confirm by observing whether the trap seal is pulled down after a flush, and check that the fixtures share a wet vent correctly.
4. Pull and inspect the tub trap - soap and hair buildup in the trap or at the overflow plate can mimic a main-line problem.

Possible causes, ranked: partially cleared main still restricted by scale; separate blockage in the 1A tub branch; blocked or inadequate vent; a break or offset in the underground cast iron at the building exit.

What to tell the manager: do not declare the job closed because the water went down. If the camera shows a deteriorated main, that is a capital repair and the work order needs to say so in writing, with the camera footage attached.