Can I Plumb (Per Code) My Washing Machine Drain into the Same Group of Drains for the Bathroom?

Updated December 11, 2020
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Question by G3: Can I plumb (per code) my washing machine drain into the same group of drains for the bathroom?

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The washing machine drain (1.5") would run horizontally along wall, pick up the lavatory drain (still 1.5") and wye into a 2" drain with the shower, which then wyes into a 3" drain with the toilet and on to a sump. This install is in a basement, so it would be very difficult to cut more concrete or run a separate line to the sump. This is in Virginia.

Answered LCD: Pipe sizing is determined by DFU's - Drainage Fixture Units - but I think you need to check your plumbing code because it sounds like you are using one from the 60's. Washing machine drains today, in at least most jurisdictions, are 2" because of the higher pump-out capacity on new washers, bathroom sinks 1-1/2 or sometimes 2", toilets always minimum 3", tubs 1-1/2 or sometimes 2", showers 2", main stack or collector branch 4".

So - sounds like your washing machine drain is small, as is the main stack or branch collector, at least for most locales- and especially if this is underslab because the low slope there will promote clogging with smaller lines. Also - in many areas any line under a slab has to be 3" minimum for routing and snaking ease, so that might come into play too.

Also, almost always when you run two lines together, the line from there on has to be upsized to the next size for the combined flow capacity from the two sources - so even if the washing machine used the old 1-1/2 drain line rather than 2", you cannot connect it into a 1-1/2" drain from the lav - it would have to be 2" from there to the 2" shower line connection, then 3" from there to the main stack 3" line. In some areas even that 3" to 3" connection would be illegal and would have to be 4", which would be the standard in modern construction. So - I would do a bit of code checking - and also look into the newer requirements for the height of the washer standpipe, and how far it has to be above any junction - many people get into trouble with that too.

Answered by G3: LCD: I couldn't figure out how to respond directly to your answer, so I'll add another response to the original question and hope you see it.

Thanks so much for your detailed response. That was very helpful. After doing a little checking, I believe you're right in that I'm using an old DFU table, so that observation is greatly appreciated! I can handle pipe sizing however I need to (I want to make sure everything is done safely and legally).

That you're aware, are there any fundamental issues (assuming the pipe sizing is correct) with adding the washing machine onto the bathroom group itself (not sure if it's significant that the washer would be upstream of all the other fixtures)? The reason I ask is I was told by someone who was unable to substantiate their claim that they didn't think adding the washer to the bathroom group was code compliant. I can't find any references that either support or refute their claim (I don't have a copy of the code, so I'm relying on what's on the web as my resource). Unfortunately, the code compliance people here are of less than zero assistance, so I'm left feeling around in the dark for answers.

Any insight you (or anybody else) can provide would be greatly appreciated!


Answered by LCD: There is a general rule - though I have never seen it spelled out in the code - that you do not put a "major fixture ahead of a minor fixture" - meaning as you go downflow in the drain lines (except in the main 3 or 4" stack) you should not have a lower DFU fixture downflow on the line. I think this rule of thumb may be from the days of wet venting where a major flow (like tub or washer emptying) could pull the water out of the trap from lesses downflow fixtures, so may not apply fully with individually vented fixtures - but still a good rule of thumb because a washer discharging into a line with long run or bends could well back up partway in the drain line, potentially overflowing into the shower or any floor drain. Obviously, you do not want to get it all installed and then find out there is an issue. - much less with the toilet doing the same thing, like if flushed while the washer is discharging.

Having the washer "upflow" of a lav would not overly worry me because soap suds and water would have to rise about 3-4 feet to overflow that trap. However, having the shower downflow on the same line with only a foot or less rise to the drain opening would worry me - you could well get backup into the shower unless the downflow 3" line from there is very easy flowing without a lot of length or bends before hitting the main stack.

Note - you said the washer drain (presumably now 3" after merging with the shower drain) would be going into a 3" main drain to the sewage grinder pump - that would I believe violate the rule against merging two same-size lines without an upsize, so I think the code would require 4" line for the run to the pump. Depends on the DFU rating for the type of connection you use and the DFU ratings for each leg of the connection - you might be OK but I don't remember 3" rules - with a 4" stack this would certainly be OK but maybe not with 3" - which might mean having to either separate washer piping all the way to the pump or upsizing the main drain line to the stack.

Of course, don't forget venting for each fixture, properly spaced per code downstream of the respective trap. And Studor or AAV (Air Admittane Vents) are not kosher for a washer because the vent has to be able to discharge air as well as let it in, to vent air from the drain line as the slug of washer water enters it and starts backing up at bends - because otherwise that air can push the water out of other traps like at the shower.

And related to venting - many or most code areas require the trap for the washer be a certain elevation ABOVE floor level or above the horizontal run on its drainline to prevent backup through the trap or potentially noxious matter from further down the line. Usually not an issue with deeply buried sewer lines under slabs, but can be where the sewer line has been installed shallow.

And you have to consider grinder pump (if this is new or has to be upsized because of the larger capacity needed to handle the added washer if it is new in the basement) selection and sizing, backflow prevention, discharge line size selection and routing, etc.

One other possibility - sized based on your floor heights and washer capability and such, would be installing a booster pump with backflow preventer on the washer and run its discharge to a standpipe located on the floor above, bypassing the lift/grinder pump entirely. Plumbing would (other than the booster pump, which is wired by solenoid into the washing machine discharge pump wiring so they run together) be same as for a normal washer, but the standpipe and airgap and such would be on the floor above as if the washer were there. There are also higher-capacity replacement pumps for washers out there (at least Whirlpool at a minimum used to sell them) - totally replaces the existing pump or the existing pump has its drive and impeller removed to allow free flow to the higher head capacity pump.

This is quite rarely done for individual washers (has historically been done for basement laundry rooms in apartment buildings with multiple washers but no reasonable drain access because the basement or subbasement where the laundry room is lies below the sewer exit from the building to the street), so you might have trouble finding an Appliance Repair guy or Plumber who can locate and wire in the booster pump - normally only used in basements with no plumbing installed but they want the washer down there. There are two installation details I have seen - one is to use am in-line high-capacity pump that can handle what the washer discharges, the other is to go to a washtub or holding tank properly sized to be able to hold all the washer capacity, then a smaller pump able to evacuate the tub in substantially less than the time between discharge cycles from the washer, but not necessarily be able to handle the full discharge rate of the washer. Needs to be wired with a high-water level alarm to automatically shut off the washer if the water gets too high in the tub. If separate in a tub or tank should also incorporate an airgap lint trap where the water dicharges to the tub to protect against pump clogging. Lot of downsides to that type of solution, but sometimes is the only viable solution with basement laundries.

Of course, discharging directly to a lift pump would accomplish the same thing. But I am almost positive a lift/grinder pump is not allowed to have more than one inlet pipe because it could pull from one and end up backflowing to the other, or pull air from one and not drain out the liquid in the other.

One other possibility, depending on your situation and what the washer elevation is relative to the sewer line elevation, would be a totally separate connection for the washer to the sewer line under the slab.

Oh - one other thing on backup of suds issue - if there is any floor drain downflow of the washer discharge, then if the sewer line is shallow you can pretty much count on at least suds backup from the floor drain, if not water backup too - so in that case the minimum 18" (or whatever your area requires - I have seen up to 3 feet in some areas) sewer line burial below the lowest trap elevation rule comes into effect again. [Though having a floor drain in a basement system with a lift/grinder pump is a tough decision - because it can protect against flooding of the basement from an overflowing washer or pipe break or outdoor water overflowing into the basement or such - but raises the risk of sewage backup if the lift pump fails or there is a backup in the upstairs main stack which it discharges into and its backflow preventer fails (sewer backflow preventers commonly at least partly leak).

To be honest - I really think, since your local code enforcement people are not very helpful, that you should pay a plumber if not to actually do the work, at least to come advise on the job (probably $300 or so for that) because you have venting, different allowable slopes depending on DFU's and pipe size, selection of types of Tee's and/or Wye's depending on what is meeting up with what type of line, cleanout requirements, etc - leaves a lot of opportunity for getting something very wrong, especially since you are below the slab which can complicate venting immensely. Oh - and depending on elevations of shower and any floor drains, there is a minimum required burial depth below the slab too - commonly 18", which would then affect the setting depth for the grinder pump if required in your case.

It may also be that uyour code jurisdiction (state and/or city or county) REQUIRES that a licensed plumber sign off on plumbing installation even if a DIY job - so you might have to find a plumber who will advise on the layout, inspect ones everything is laid out but before connection, then reinspect after connections and for the required pressure test (forgot about that one, I bet) and sign off on the plumber's certification. And of course he can tell you where code enforcement inspections are required, if applicable for DIY work in your area.

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