Does a Laundry Tub Addition Require a Vent Line?

Tom PultzMay 20, 2009

I'm adding a laundry tub next to the W&D but don't see how I can tie into the existing vent system for the washer.

It appears the existing washer drains through the laundry room/garage wall into a 3 inch DWV line coming from the upstairs bath (the 3 inch line is exposed on the back of the garage/laundry room wall). That 3 inch line serves the upstairs toilet and shower. Near the floor of the garage it turns and runs down into the crawl space below the family room where it eventually ties into the main drain.

I thought I read here once that if the fixture arm for an appliance drains into a larger diameter line... then no additional vent is required.

So... could I just use a 1-1/2 inch fixture arm and drain line and run that down into the crawl space and merge it with the previously mentioned 3 inch line that's running sloped horizontal at that point?


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I think I have your question figured out.
Yes tapping into an existing drain line with no individual vent pipe is fine. All drains do not have there own vent pipes, many share vents. Just make sure you have an air stop/ pee trap drop that is below the laundry tub but higher than the drain pipe you tap into.
best of luck chris

    Bookmark   May 20, 2009 at 11:39PM
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one can add a laundry tub to drain into the same standpipe as your Washer.

check with others before proceeding; do not rely on anonymous internet strangers.

i don't like the description you used in your last sentence. it's not reassuring; you didn't mention where the P trap would be in that description.


    Bookmark   May 21, 2009 at 1:06PM
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Tom Pultz

Hmmm, anonymous strangers... such at hth :-)

Yes, there will be a p-trap. That's pretty much understood isn't it.

Not sure I can tie into the standpipe for the washer as the new laundry tub (Kohler cast iron unit) is quite deep, which means the fixture arm height will be about 12 inches above the floor, and that is probably lower than the existing p-trap for the standpipe based on the piping I can see in the garage. The only way to know for sure is to remove some drywall.

    Bookmark   May 21, 2009 at 4:14PM
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tom, you have to say Where the P trap would be in that description, what height, how many bends etc. Not the fact that there will be one.

HTH = hope this helps.

Use the washer's P trap. Check this with others. A laundry standpipe P trap can receive a washer and a laundry tub. Hope this helps.


    Bookmark   May 21, 2009 at 6:51PM
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Tom Pultz

The nominal height of the counter top in the laundry room will be 36 inches. The dept of the sink is ~ 15 inches... allowing 3-4 inches for the strainer and tail puts the height of the p-trap discharge (Kohler K-8998) at 17-18 inches (at most) above the floor.

Based on the pipe connections I can see in the garage this height is going to be very, very close to the height of the existing p-trap for the washer standpipe.

Therefore, I still think the easiest way to hook this up is to run the p-trap as described above and then run the drain down through the wall into the crawl space and tie into the 3 inch DWV line directly below. The total length of the drain pipe for the sink would be about 5 feet to where it ties into the 3 inch DWV pipe.

As for bends: there would be 1-90 deg in the wall, probably 1-45 deg below the floor, and 1-90 deg long sweep before it ties into the 3 inch with a 1-1/2 to 3 inch merge.

The only thing flowing through this line will be water. I can't see it being a problem.

    Bookmark   May 22, 2009 at 12:15PM
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"The only thing flowing through this line will be water. I can't see it being a problem."

Depending on the actual vent location the stand pipe may siphon the the sink trap.

Clothes dryers pump a lot of water very quickly.

    Bookmark   May 22, 2009 at 2:58PM
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Tom Pultz

If attaching the sink discharge to the stand pipe may siphon the sink trap when the washer empties, then avoiding hooking the sink to the stand pipe sounds like the way to go.

    Bookmark   May 22, 2009 at 5:41PM
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Now that everyone has taken their best guess, would anyone be interested in what the code says?

In the original post Tom did not tell us what code he is under or where he lives, but given that his screen name is Tom-In-Seattle, I am going to go out on a limb here and assume he is in Seattle, Washington. If that is correct, then he is under the Washington State Plumbing Code, which is modeled after the UPC (Uniform Plumbing Code). Unfortunately the UPC is extremely restrictive on venting.

Not only can he NOT connect the sink to the existing washer standpipe, based upon his description, the washer standpipe is in violation of the code as well.

The post clearly states that the 3" vertical stack is serving a shower and W.C. (W.C.=Water closet, commonly although improperly called a toilet).

First of all, under the UPC all fixtures sharing a common vent MUST BE on the same floor.

Second, both the UPC & IRC prohibit connecting a fixture arm on a vertical stack below a W.C.

Now to answer Tom's original question, "could I just use a 1-1/2 inch fixture arm and drain line and run that down into the crawl space and merge it with the previously mentioned 3 inch line that's running sloped horizontal at that point?" The answer here is positively MAYBE, depending upon the angle of the slope on that line.

In plumbing any line which rises at 0 to 44deg above horizontal is said to be a "horizontal line" and any line which rises at 45 to 90deg above horizontal, where 90deg is vertical, the line is said to be a "Vertical Line" so we would then have to measure the actual angle of the slope. If that sloped section of line is rising at 45deg or more it would then be a vertical offset in the vertical stack, and here again, we may not connect a fixture arm to a vertical line below a W.C.

If the measured angle is 44deg or less we could tap into that line however, when doing so, we must use a Wye & 1/8bend or a Combo to make the tap. A tee would be prohibited.

Can he use 1-1/2" line to make the drain line from the sink to the 3" in the crawlspace? NO!

Under the UPC a laundry sink is rated for an 1-1/2" P-trap and fixture arm from the trap to the wall but an 1-1/2" drain line is only rated for 1DFU (Drainage Fixture Units), whereas a laundry sink is rated at 3DFU's.\ therefore the load would exceed the limits of the 1-1/2" riser in the wall and the horizontal drain line in the crawl space.

Where the fixture arm enters into the wall you must have a 2" quarter bend at the top of the riser, then a 2" riser dropping down to the crawl space. At the base of that 2" riser you must install a 2" Wye & 1/8bend with the riser connected to the side opening and a thread adapter and clean out cap on the top end of the Wye.

From the base of the riser the 2" horizontal line may run a maximum of 5' to 3" drain line you are connecting to. Keep in mind, that is 5' maximum developed length of pipe, which means a maximum of 5' when measured on a line on the top of the pipe, including the offset for the wye at the 3", and believe me, UPC inspectors generally have a very critical eye. At 4'11" I generally get nervous and hope the inspector is having a good day. You can forget trying to get by with 5'1".

    Bookmark   May 22, 2009 at 9:02PM
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Tom Pultz

Wow, lazypup comes through again with his usual detailed sage advice. Many thanks.

Now... although my screen name is tom_in_seattle I actually live in Sammamish (nobody would know where Sammamish is!), which was unincorporated King County up until 1999. The house was built in 1984. Does that mean existing plumbing work is grandfathered in?

Judging by what "pup" has written here and his other postings I think there are various parts of the DWV plumbing in our house that would not pass current code, particularly with venting, and for example, the plumbing in the enclosed picture. Is that vertical WYE in the center allowed?

The near horizontal line in the center of the picture is the line I'd like to use for the laundry tub drain connection. This carries waste from the master bath WC and shower, and also the washer in the laundry room. The attachment point would be further to the left, and thus closer to the floor above. Looks like the 5 foot rule is attainable.

Since "pup" says I need to merge with this line from the side (like the lower horizontal connection), it might be possible and easier to run the 2 inch drain from the tub down through the bottom of the cabinet in line with the connection point, instead of through the laundry room wall, which is to the left in the picture... on top of the concrete wall. Going through the wall would require more bends and linear distance, although it might be a cleaner installation inside of the laundry tub cabinet. Need to do more checking and measuring.

Other DWV piping in the picture: 1) the 3 inch vertical WYE is the drain for the upstairs WC, 2) the other vertical 3 inch line is the drain for the downstairs WC,which also carries waste from the upstairs LAV, and 3) the bottom horizontal line is the main disharge, which also picks up waste from the kitchen, located to the right, and also the master bath and spare bath tub.

BTW, I had the crawl space totally cleaned out due to rodent damage, so all the insulation shown here is gone, and will probably be replaced with spray foam.

Thanks again for the help.

    Bookmark   May 23, 2009 at 1:52PM
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I have examined the photo with great interest and from what I can see you need not worry about being grandfathered in.

The only code violation that I can see in the picture is the galvanized steel perf strap hanger on the horizontal 3" line.
Code now specifically prohibits metal strapping or wire hangers for plastic pipes. The reasoning here is two fold:

1.When liquids flow through pipes there is a slight high resonance vibration in the pipe. As a result, in the course of time a metal hanger will cut through the plastic pipe.
2. When cold water flows through the pipe, the pipe wall cools below the ambient air temperature surrounding the exterior of the pipe, and in some circumstances condensate moisture will collect on the metal hangers. The moisture then leads to rust forming on the hanger and in the course of time the hangers rust through and break.

The codes are very explicit on pipe hangers.
Plastic pipe should be suspended by means of holes bored through wood, Plastic pipe hangers or plastic perf strapping. When using plastic perf strapping you should install wood blocking down from the joist above the pipe to the top of the pipe. This will prevent the pipe from rising up and ultimately changing the pitch of the run. The perf strapping should then run under the pipe and be attached to the wood blocking on either side or it may be attached directly to the joists from above in the same manner as that metal strapping is now.

Steel strapping or steel pipe hangers should be used for galvanized iron, black iron or cast iron pipes and copper or copper clad pipe hangers or strapping should be used for copper pipes.

The 3" line running from left to right, which is supported by the galvanized strapping is a Horizontal line. We may attach a branch line to a horizontal line from either side, from the top as shown in the photo or at any angle in between horizontal or vertical however, when attaching a branch line to a horizontal line we are required by code to use a "Wye & 1/8". A sanitary tee is expressly prohibited.

here again, the reasoning is simple. If we had a tee where that vertical line ties into the horizontal, as the waste coming down the vertical enters the horizontal it would immediately fall to the bottom of the horizontal, the expand outwards, some flowing downstream, while some of the liquid would immediately back flow upstream. This causes two distinct problems;
1. When the liquid hits the bottom of the line, then changes direction it reduces the velocity of flow.
2. The liquid which immediately back flows upstream could be carrying solid particulates. When that liquid stops its upstream movement then reverses and runs back downstream, there is no longer sufficient velocity to carry the solids and they are left behind. Once the liquid has receded the solids are then left on the bottom of the pipe and they will dry in place forming a clog.

To prevent that from happening the codes require that all connections to a horizontal line must be made by means of a Wye. The side inlet of a Wye is configured at a 45deg angle in the direction of flow therefore as the liquid entering through the Wye goes into the horizontal line it is directed downstream thus maintaining the velocity of flow.
When making a 90deg tap into a horizontal line we then use a Wye & 1/8 bend. An 1/8 bend is a 45deg sanitary elbow which when attached to the 45deg input of a Wye it completes the required 90deg turn.

You could use a common 1/8 bend which has a hub on each end, but when doing so, you would need to cut a short nipple of pipe to connect the Wye & 1/8 bend hub to hub. A simpler method would be to use a "Street 1/8 bend.

A Street fitting has a hub on the input end but the output end has a raw pipe size, called a "spigot". The spigot can be inserted directly into the hub of the Wye, thus you would not need a pipe nipple between the two fittings.

The Wye shown in the center of your photo is technically not a Wye, it is a "Combo". Understanding that when making a right angle connection to a horizontal line we are required to use a "Wye & 1/8 bend" the fitting manufacturers produce a "Combo" Which is simply a Wye & 1/8 bend made into one fitting. Functionally there is no difference between a Wye & 1/8 bend" or a "Combo" but it is more convenient for the installer because it is one less glue joint to make up and in most instances a Combo is a few pennies cheaper than the cost of a Wye & 1/8 bend. Generally this is not a big deal in single family residential plumbing but believe me, if you were plumbing an apartment complex a couple pennies on the fittings and not having to pay a man to make the extra glue joints can add up very quickly.

Now before we get into the final solution for your new drain line, let us take a moment and discuss how Tee's and Wye's are identified.

If all three openings on a Tee or Wye are the same size the Tee or Wye is identified by the pipe size, I.E. a 3" Tee however for your application you need to connect a 2" line to a 3" horizontal so the easiest method is to use a Reducing Wye. For a reducing Wye or Tee each opening size is listed in a specific order;
1. The input on the straight through, 2. The output on the straight through and 3. The side opening by example, you will need a 3" wye with a 2" side opening so your Wye would properly be called a 3x3x2 Wye. (you could use a 3" Wye but by using a reducing Wye you save the cost of a reducer for the side input plus you will have one less glue joint to make.) I would recommend a "3x3x2 Combo".

I can see that the horizontal 3" is running parallel to the wall, but it is difficult to tell how far away from the wall it is. Here is a tip. When you install the Wye & 1/8bend or combo on the 3" horizontal configure the side opening to rise at a 45deg angle towards the wall, then make your offset with a line running up at a 45deg angle. In this manner your offset will be a vertical offset so the horizontal distance of that offset doesn't count off your 5' maximum fixture arm length.

Send me an email so i have your email address and I will return you some illustrations I made for my apprenticeship class that will help you.

    Bookmark   May 24, 2009 at 5:22AM
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Tom Pultz

So, I finally got around to working on the laundry room again... and here is what I have.

The 1-1/2 inch piping in the wall is 24 inches from the sanitary tee and slopes at 1/4 inch per foot. I added a Studor Mini-Vent AAV as shown.

The vertical piping is 2 inch and it drops down into the crawl space about 10 inches, then over about 6 inches to another vertical drop of about a foot, where it mates up with a 3 inch horizontal drain line via a combo fitting.

Does this look OK for UPC or should I have added a cleanout fitting on the fixture arm in the wall? It doesn't seem necessary considering how easy it would be to remove the p-trap and metal piping.

Re: the connection of the metal drain pipe to the ABS... is there a requirement for about how much (or how little) piping should extend past the seal? Although the sink will obviously be flipped over when installed the proximity of the p-trap and metal piping will be approximately as shown... so I just need to decide how much to extend the ABS out of the wall.

Thanks for any thoughts, and Happy New Year.


Ps - there will be a base cabinets along the wall, with the recess box with vented cover and AAV hidden behind a wine chiller.

    Bookmark   December 29, 2010 at 9:43PM
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Your configuration is great but the horizontal waste arm from the vertical drain to the trap location is too small.

Your load (sink) is rated at 3dfu's and an 1-1/2" trap is correct but under the UPC table T-7-5, an 1-1/2" horizontal drain line is rated for a maximum of 1 dfu. You must make the horizontal waste arm from the trap adapter to the vertical drain line 2", which is rated for up to 8dfu's.

The 1-1/2" riser up to the AAV is fine.

If the trap can be removed that serves as the cleanout on the upstream end of the horizontal waste arm.

Your AAV will be immediately accessible and it is easily removable so it qualifies as the cleanout on the vertical riser.

Keep in mind that under the UPC you are only permitted a maximum of one AAV per structure, so if you have no other AAV's you will be okay on that.

    Bookmark   December 31, 2010 at 12:42AM
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Tom Pultz

Thanks for the reply lazypup... I was afraid you were going to say the horizontal waste arm was too small based on your previous reply last year.

A rating of 1 DFU for a horizontal 1-1/2 inch pipe is based on an UNLIMITED length of piping is it not? (see table in link)

I'm probably wrong, but I thought the horizontal section of this "fixture arm" installtion was considered a continuation of the p-trap and discharge, which is why I made it 1-1/2 inches.

Fixture arms have defined allowable lengths, i.e., they are not allowed to be an unlimited horizontal length. For 1-1/2 inch pipe under UPC the maximum fixture arm length is 3 ft, 6 inches (6 ft under IPC); for 2 inch pipe it is 5 ft (?) (8 ft under IPC).

Using these lengths the IPC basically says the pipe may fill to the top before it meets the vent line, and for UPC it says the pipe may fill about 1/2 way before meeting up with the vent line.

If a 2 inch horizontal pipe is required for basically ANY kitchen or laundry sink rated at 2 DFUs, how did the original installation of our kitchen sink in 1984 meet UPC code... it has a 1-1/2 inch "fixture arm" in the wall, running about 32 inches over to the vertical riser, which is 2 inches, and the vertical vent, which is 1-1/2 inches.

And... how can ANY installation of 2 inch piping be legal through vertical 2x4 studs? Boring a hole large enough for 2 inch piping through a 2x4 stud in a load bearing wall removes too much material. Running a 1-1/2 inch line barely meets code, which requires double studs (as I have done) in load bearing walls.

Re: the 3 DFU rating for the laundry sink I'm not sure where you found this as every table I've seen rates a laundry sink the same as a kitchen sink: 2 DFU's (see table in link for King Co Washington where I live).

As it stand snow I'm not going to change anything until I can confirm with the local inspector what is required locally. In retrospect I should have done this in advance.

This AAV is the only one I have in the house, and they are allowed in King County, WA.

Thanks for the response, but in the case I'm now more confused.

Happy New Year, Tom

Here is a link that might be useful: Drainage and Fixture Unit Values

    Bookmark   December 31, 2010 at 2:34PM
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Tom Pultz

Just as an update... I had my plumbing inspection today and all 10 items I had changed or added passed, including this laundry sink addition.

Since this was my first ever inspection I was a bit surprised with the process and what wasn't checked or verified.

For instance, since I added two water spigots to the rear of the house, and replumbed the one for the front, I thought the inspector might have wanted to see how I ran the new lines in the crawl space. Nope. All he cared about was whether they had vacuum breakers.

Since I moved the kitchen sink I thought he might want to measure the length of the fixture arm in the wall to make sure it was within the UPC limit. Nope. Maybe he just eye-balled it and figured it was OK since it's just a bit less than 3 feet.

He also did not care to see where in the crawl space I ran the PRV discharge line for the point of use electric water heater I installed in the laundry room. He said as long as it ran into the crawl space that was fine. I actually ran it so it discharges above a grate I added to the existing storm drain... the discharge point being a minimum of 4 times the pipe size above the grate.

Does this sound typical, or being the anal engineer I am, was I too paranoid about the details that might be checked?

Thanks for all the input from the professionals on this site... it was very valuable.


    Bookmark   January 31, 2011 at 4:43PM
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