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Hey guys, newbie member here who just had some house renovations done and wanted to seek some advice.
So amongst everything that got replaced, we have a completely brand new kitchen - including a new dishwasher and a two-piece sink. Today whilst I was hand-washing dishes, I noticed that that if one of sinks was drained after being semi-filled with water... it'd come back up the pipes and water would accumulate in the second sink too. That and both sinks were draining far slower than I'd like, and for a brand new system too.
I had a look underneath at the pipes and just wanted some other opinions or advice. Now, being no plumber or plumbing expert myself, I was wondering if there were perhaps too many bends in the pipe configuration that may be contributing to the sluggish drainage? Or the fact that the trap is lower than the drainage outlet leading into the back?
All contributions and advice would be greatly appreciated, as I would like to know if this is something I should confront my builder about before the final stages of the renovation are completed so it can be rectified. As this is a brand new system, I am confident that there are no clogs as of yet... it was literally my first day doing dishes! Thank you!
Welcome to the Bunnings Workshop community @zackwho. It's to have you join us, and many thanks for your question about plumbing configurations.
The bends in the pipe and levels are all spot on. The bends are an S trap and a requirement to have. They don't significantly slow the water drainage as although there are many bends, the exit pipe is below your sink. If you had both sinks full and tried to drain them simultaneously, or if the dishwasher is in its drain cycle and you release a sink full of water as well, there might be a bit of delay.
After that 50mm S trap, it appears they've dropped it to a 40mm pipe when it exits through the back of the cabinet. See how the pipe through the cabinet is smaller than the S trap and other pipework? This could restrict the flow enough to see water backfill up through the S trap and into the second sink. Try to measure that tiny section of pipe exiting the cabinetry; it should be a similar size to the horizontal pipe connecting the two sinks.
Water should not accumulate in the second sink when draining the other. This is an issue I'd suggest raising with the builder.
Please let me know if you have any questions.
Mitchell
Thanks for taking the time to reply Mitchell. 
Very handy advice.
Hello @zackwho
It's great that you've received excellent advice from @MitchellMc. Just to add to the suggestions made, I recommend filling the kitchen sink with water and using a Kinetic Small Sink Plunger to agitate the drain. There is the possibility that part of the construction debris might have lodged itself in the pipework. Perhaps by agitating it, it will open up the drain and you'll get better flow.
However, if after several attempts of agitation and you still get no results, I suggest speaking to your plumber as recommended.
Please keep us updated with your progress, we look forward to seeing your sink drain repaired.
If you need further assistance, please let us know.
Eric
