I am relocating my kitchen sink to a new location. The new sink will be 12-15 feet from the vented drain in my basement. Do I need to install the new drain pipe on a downward angle to the vented stack to help drainage.

Hey Fred:

You need to do more than install the new drain pipe on a downward angle... you also need to add a vent to the kitchen sink and the vent needs to come off above the trap.

The drain line needs to connect directly into the vent TY fitting or can be up to 5-6 feet away MAXIMUM.

Drain pitches back to basement drains at 1/4" pitch per foot of drain line.

Install a full size cleanout under the kitchen sink and also where drain connects into basement drain.

I recommend 2" pipe to a kitchen sink... 1.5" vent is correct here!!

Let me know if you need more... MARK

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You might get away with an air admittance valve (AAV) but you should check the code in your area, some towns do not allow AAV.

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Originally Posted by fredruss

I am relocating my kitchen sink to a new location. The new sink will be 12-15 feet from the vented drain in my basement. Do I need to install the new drain pipe on a downward angle to the vented stack to help drainage.

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OK! You've got the wall open to run the drain line already. But you're too far away from the vent, (see Marks post).
You're going to hafta vent your sink so simply cut in a 2 X 2X 1 1/2"tee on its back about 4 feet down from the trap, on the drain line, and put a raiser on that tee that will reach 6" over the top of the sink rim and elbo on down to a 2 X 1 1/2"inverted tee that you will cut into the kitchen vent. This line will be sloped back from the vent to the drain line.
**or** you could get lucky and be allowed a AAV, (Air Admittance Valve) as Bob suggested. Good luck, Tom