My bathroom sink drain routinely (~5 months) becomes so ridiculously clogged with build-up that I have to remove the trap and outlet arm in order to clear it out. The build up even extends beyond the removable arm and into the drain outlet in the wall.

With the parts removed, it's easy enough to clear out the pipes. I try to clear out the wall drain outlet as much as I can reach with my tools.

However, when I spray water into that pipe, rather than flowing away, it flows back toward me (or the trap, if it was hooked up). Is this a normal design?

I'm guessing that it is exacerbating the build-up issue, since it makes it harder for the stuff to flow away. The stuff that never makes it all the way to the final drop ends up flowing backwards toward the trap. It settles and hardens.

My thought is that if the pitch was adjusted, it would reduce the amount of build-up. But the current pitch could be the intended design, and there might be consequences with changing it that I'm not aware of.

I appreciate any thoughts and help.

1 Answer 1


It definitely needs to slope away down to the drain. Specifically a quarter inch per foot. No more, no less. The reason for this is that at a quarter inch per foot waste water tends to sort of oscillate up one side and then the other of the pipe. This effectively washes the pipe as it goes. If you have a negative slope any debris will have plenty of time to firmly attached itself to the bottom and side walls. Too much slope in the water shoots down center of the pipe and debris build-up into lumps on either side of the pipe.

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