Plumbing backup

Master bathroom had two sinks, shower and a tub. When I filled the two sinks up and drained them at the same time, the water backed up and rain out the shower drain. What would be some causes of this? The shower had no trouble draining water.

Heres a link to the manufacturer with more info…

Sorry David…this was for your other post…lol

could be a vent problem, or drain configuration.

Do we need to diagnose the cause, or just note the observation?

What Wayne said.
Seems when that occurred on me it was always a drum trap configuration involved.

What Wayne said…too small, none, or incorrect venting. they are venting at the shower drain.

Would be nice to have an answer when client asks what causes that other than I don’t know or I don’t need to know or give you a cause why it is happening.

Aren’t you then assuming some responsibility, liability, that should be the job of a professional plumber? What if you misdiagnose the cause and it turns out to be something much serious?

Mr/Ms Client there are numerous causes for this. It could be as simple as a clogged drain or improper venting.

Yes something like this. Give them several possible causes but always refer them to a specialist.