To have the drain centered in a new shower location would position the drain outlet hole 10’ from vertical drain stack which drains to the basement where the main sewer line leaves the house.

Proposed Assembly to not exceed .25"/foot slope for 2" pipe 8': Following a 2" P-trap directly under proposed Shower drain, can a 3"x2 increase/reduce coupling [Charlotte PART NO. PVC 1102] be introduced, then 3" PVC Sched. 40 pipe sloped in for a 36" including a 3"x2" PVC Sched.40 Eccentric Reducer [Spears part No. 429-338E] followed by 2" PVC Sanitary Tee for dry venting [Charlotte PART NO. 400] followed by 7' of 2" PVD sch. 40 pipe sloping .25"/foot to the vertical drain location.

Or can a 2" pipe travel 10' without siphoning out the P-trap and sufficiently drain when vented properly?

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  • the bottom of the eccentric reducer would eliminate the upslope between reduction from 3" to 2" pipe it would seem by design. supplyhouse.com/… That said, pdd recommendation is how I will proceed, vent within 8' of p-trap. THANK! Nov 22, 2023 at 21:52

1 Answer 1


Yes, a 2" pipe can travel 10' without siphoning out the p-trap and sufficiently drain when vented properly.

The key is when vented properly. The distance between the vent and the p-trap is limited to a maximum of 8' for a 2" pipe, but the 2" pipe can continue to run horizontally downstream of the vent connection without limit.

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In your picture, the distance between the dry vent and the p-trap looks to be less than 8'. If that is the case then it is fine. The distance after the vent to the stack is not limited.

As noted by isherwood, you cannot reduce a drain size.

  • Does it make a difference if the vent stack pipe is 2" vs 1.5" considering the need to route the pipe around pluming valve and wall niche possibly? Nov 22, 2023 at 22:13
  • 1
    The vent size does not limit the length of the fixture drain. The size of the vent would determine the allowable length of the vent to open air, a vent stack, a stack vent, or a vent header. However, this would be considered an individual vent and as such, its length is not limited, and its size is based on the size of the trap it serves. The minimum vent size for a 2" p-trap is 1-1/2.
    – pdd
    Nov 22, 2023 at 22:40

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