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I've read that the rule is 1/4" per foot of drain line. From what I know, the rule is based on the height of the P-trap to where you tie in. What part of the P-trap do you measure from? How low can you tie into an existing pipe?

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You'll almost always measure from the trap weir (crown weir), which is the point within the trap where water would overflow the trap and spill down the drain line. –  Tester101 Aug 8 '13 at 12:07
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up vote 2 down vote accepted

You're limited by the size of the materials you're using.

2012 International Residential Code (IRC)

SECTION P3105 FIXTURE VENTS

P3105.1 Distance of trap from vent. Each fixture trap shall have a protecting vent located so that the slope and the developed length in the fixture drain from the trap weir to the vent fitting are within the requirements set forth in Table P3105.1.

Exception: The developed length of the fixture drain from the trap weir to the vent fitting for self-siphoning fixtures, such as water closets, shall not be limited.

Table P3105.1

If you're using a 1 1/2" trap, there must be a vent within 6' of the trap weir. The overall length of the drain is also limited based on the pipe size (P3105.2).

P3105.2 Fixture drains. The total fall in a fixture drain resulting from pipe slope shall not exceed one pipe diameter, nor shall the vent pipe connection to a fixture drain, except for water closets, be below the weir of the trap.

So if you're using 2" PVC, the drain must be less than 8' long horizontally.

1/4" * 8' = 2"

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That's odd. I have a drain line that is running through my ceiling that is AT LEAST 15.5' long and probably closer to 18-20'. Is P3105.2 only the distance of the vent to the trap or is it really the max length of a drain run? –  2 Left Thumbs Aug 8 '13 at 21:15
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@2LeftThumbs The drain AFTER the vent is not limited in length, just by the number of devices that may be connected to it (for a given diameter) –  HerrBag Aug 9 '13 at 3:41
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@2LeftThumbs Notice the wording "fixture drain" in the code. As HerrBag says, this is the length of pipe from the fixtures trap to the vent pipe. The pipe after the vent is governed by a different code section, and can be much longer. The reason I used this code reference, is because you mentioned tying in to an existing line. So unless you intend to install a vent, the fixture drain code is important. –  Tester101 Aug 9 '13 at 11:45
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