My bathroom drain in older apartment building when both taps are running full force, backs up the drain in about 30 seconds.

P trap is clean. Half force nothing happens. Neighbor above has similar problem.

What is the cause and how is it fixes? Adjoining bathtub doesn't seem to be an issue.

You have a clog or partial one in a drain line common to both, could be two clogs at different levels. Snake as far a necessary into the P trap. Clearing yours might clear his, if not snake his too.

What is meant by half force?

If it is an apartment, have their staff do the work. If YOU do something wrong, it is going to be YOUR problem.

Agree with ma0641... if apartment building leave it to the landlord... you break it, my guess is that you'll have to fix it, too!

Otherwise, if the water backs up after 30 seconds then you know the "partial" blockage (as mentioned by talaniman) is probably a number of feet down the drain. Here, remove the Ptrap if possible and snake the distance from the sink over to the toilet drain... the branch drain off the toilet drain is where the blockage is in most cases like this (connection can be close to toilet or further away, but lavatory drains usually connect into the toilet drain).

Be aware that the snake might go up a vent just as quickly as it can go down the drain so plan to snake the drain a number of times to be sure you are snaking the drain and not the vent...see the picture:

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Your neighbor would need to do this as well if he has the same problem.


Mark