I'm planning a reconfiguration of my basement and want to move both the powder room and w/d to new locations.

The powder room would be close enough to the stack not to worry me, but I'm looking at a potential run of about 35 feet from what would be the washing machine's drain back to the main stack.

First question - from a drainage perspective, is that just too long for a laundry drain?

Secondly - for venting, can I tap into the main stack? Say, something like this? Disregard venting question - found my answer which is "no"

  • How many other things use that stack? Commented Aug 8, 2011 at 17:45
  • @The Evil Greebo, don't forget to put in adequate access (roding) points, especially important if the new run isn't straight ie Has bends in it.
    – Mike Perry
    Commented Aug 8, 2011 at 19:19
  • 1
    Additionally to help drainage and snaking try and using more gradual bends if you have the space. Prefer "long sweep" 90 degree elbows instead of shorter elbows. If you can get away with the space you can even use two 45s (with some straight pipe in the middle) to make nice slow bends. For a washing machine it may not be do much of an issue (it is going to be dumping lots of water in a hurry). Also I would not worry about it too much if you are not hiding this pipe inside a wall or the floor (PVC is easy to cut into if you really had to due to a serious clog).
    – auujay
    Commented Aug 8, 2011 at 20:08
  • This is a great point - there is only one corner involved and that corner is under the basement stairs, so I can definitely get away with a long 90 and a cleanout point in that area. Commented Aug 9, 2011 at 12:49

2 Answers 2


I don't think there is ever a "maximum" you can have on a horizontal run. Of course everything needs to have the required slope and you may not have enough vertical space to get it. The required slope depends on the size of the pipe.

IPC table 704.1 shows that a pipe with an ID of 2.5" or less needs 1/4" per foot. While a pipe that is 3" to 6" only needs 1/8" per foot.

In your case, with 2.5" or smaller pipe and a developed length of 35 feet you need a total drop of 8 3/4 inches (or put another way, the pipe must start 8 3/4 inches above where you will connect to the stack). For 3 to 6 inch pipe you just need 4.375" of drop.

  • Thanks - 8" won't be a problem since it'll be coming out of a washing machine drain that'll be more than 3 feet up off the ground. Commented Aug 8, 2011 at 18:40

It's not so much the run of the drain pipe to the main stack as it is the proximity to a vent. Every drain needs a nearby vent to allow air to get in behind the water. Think of it like a bottle of water; upend it, and you get the familiar "glug-glug-glug" of some water going out, then air coming in to break the vacuum. Stick a straw up through the mouth of the bottle into the air gap, and not only does the glugging stop, but the bottle drains very quickly. Your household plumbing works similarly.

The ICC organization's plumbing codes state that there should be a vent pipe within five feet of any drain. With your new location 35 feet away from the main stack (which is of course vented), you're too far away from that one, so you'll need to tie into another vent stack. Some codes allow for a wash basin and clothes washer to be each other's vent if they're within five feet, but this introduces new challenges. For instance, you can't have a P-trap on the sink itself as that closes off the pipe and prevents it venting; the trap must instead be downstream of the washer, and as this is a basement installation, if the main stack backs up from upper floors it will do so into the wash basin, so a backflow preventer at the stack end of your new drain run is highly recommended if not required.

  • Good point. IPC table 906.1 shows the maximum distances from the vent to the trap. It changes based on pipe size but is always way less than 35'.
    – auujay
    Commented Aug 9, 2011 at 16:22
  • Can you run a parallel pipe from the drain location to an existing vent stack?
    – TomG
    Commented Aug 11, 2011 at 1:05
  • If you put in the required slope (to make the water run downhill) and the vent is within the maximum distance, then yes. However, a vent stack is usually also a drain stack, so if you have a vent within 5 feet, you should probably be tying in to the whole stack.
    – KeithS
    Commented Aug 11, 2011 at 14:36

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