Improving laundry plumbing - p trap located 40 feet away in basement

[John King]

I’m redoing our laundry room and looking to add a laundry sink. After opening up the walls, I discovered several issues with the existing drain/vent layout, and I’m trying to figure out the correct way to bring everything up to code—or at least avoid making the situation worse.

Current Plumbing Setup
Washing Machine (Second Floor):

• There is no visible P-trap on the standpipe which is 1.5”
• The standpipe drops into the floor and appears to run about 6 feet horizontally through the joists to an interior wall.
• I have not been able to locate any vent coming in that wall. I found the stud bay with the drain line, but there is no visible vent riser continuing upward.
• From that wall the drain line drops into the basement and then runs another ~8 feet horizontally.
• After the horizontal run, the line enters a long-sweep TY that is open on top with a condensate line drains into it.
• The TY then feeds into a P-trap.
• Immediately after the P-trap is a sanitary tee that sends one pipe upward (seemingly a true vent) and the other downward appears to tie into the main stack.

Floor Drain in laundry room

• The nearby floor drain also has no visible trap. I put a snake in and it went a few feet without hitting any resistance, which leads me to believe it ties into the same long run before the basement P-trap.

What I’m Trying to Solve
How do I bring this closer to compliance—or at minimum not make the situation worse—while adding a new laundry sink and maintaining proper function for everything? We have lived here for over a year with no sewer smell.

I understand the current layout is not ideal: long horizontal runs before the P-trap, no local traps for either the washer or floor drain, and no clear vent for the standpipe. I’m trying to determine the safest and most practical way to improve things without tearing apart half the house.

Options I’m Considering

1. Add AAVs (Air Admittance Valves)

• an AAV for the new laundry sink, and
• an AAV for the washing machine standpipe (after installing a proper P-trap upstairs)

This would likely help the washer and sink vent properly. However, this does not address the floor drain, which is still untrapped locally.

2. Run New Vent Lines to the Attic
Another option would be to run a proper vent line from the laundry room up into the attic and horizontally over to tie into the bathroom vent.

• This run would be ~30 feet across the attic (not easy to access, but possible).
• This would potentially bring the washer and new sink into compliance with a true vent rather than relying on AAVs.

However, the floor drain would still have no local P-trap. Even with new venting upstairs, the only trap would continue to be the basement P-trap downstream.

Floor Drain Dilemma
No matter which option I choose above, the floor drain is still a sticking point:

• To make it compliant, I would need to install a proper P-trap locally (I’d like to avoid replacing the floor (or the 1st floor ceiling)
• If I leave the basement trap as-is, the floor drain technically remains untrapped locally, but it has been working and hasn’t caused issues so far.
• Eliminating the floor drain entirely is an option, but not a desirable one.

Looking for Guidance
At this point, I’m unsure whether the best approach is to leave the basement P-trap alone (“if it’s not broken, don’t fix it”) or whether I should be removing or reworking the floor drain entirely.

I’d appreciate advice from anyone familiar with this kind of legacy plumbing situation. What would be the most practical and code-appropriate way to improve or re-vent this system while adding a sink—particularly given the complications of the long trap arm and the existing floor drain?

Huge thank you in advance for any help here! Hopefully I’ve described this all clearly enough, but please let me know if clarity is needed.

 

[Reach4]

. Add AAVs (Air Admittance Valves)

• an AAV for the new laundry sink, and
• an AAV for the washing machine standpipe (after installing a proper P-trap upstairs)

To be code compliant, you would be rid of the p-trap at the santee. If you want to use the 1.5 inch drain run, do this, but use an AAV for the vent.

This lets you use 1.5 for the drainage from the standpipe even tho the picture says 2-inch. Otherwise you would want 2 inch for the standpipe drainage.

 

[Sponsor]

 

[John Gayewski]

Your saying the "standpipe" is 1.5". Is the rest of it 2"?

 

[Sylvan]

The sink and standpipe have to be individually trapped.

Also, the minimum size of a WM waste is 2."

A lot of installations use the sink as a receptacle for the WM discharge

The reasons being

1- You use only one trap

2- The sink acts as a reservoir in case there is a stoppage downstream, the sink can help prevent flooding or overflow

AAV is not legal in some areas

 

[John King]

Thanks everyone for the replies.

The entire waste run down to the basement is 1.5” and is unfortunately not something I’ll be able to change. However I did decide to do this right, and I’ll be added a vent for the washing machine and another for the sink.

I’ll also be tearing out the flooring and replacing the floor drain with a p-trap and vent, however I’m not entirely sure how I need to run that. I’m hoping I can tie into the washing machine vent somehow, so any examples that can be shared would be helpful.

 

[Reach4]

I’ll also be tearing out the flooring and replacing the floor drain with a p-trap and vent, however I’m not entirely sure how I need to run that. I’m hoping I can tie into the washing machine vent somehow, so any examples that can be shared would be helpful.

Take a look at https://www.iccsafe.org/wp-content/uploads/20-18927_GR_2021_Plumbing_Venting_Brochure.pdf page 14.

 

[John King]

Following up on this now that I have the wall and floor opened up.

Here’s where I am at:

1. Washing Machine

• Installing a new washer box on the left side.
• Running a 24” standpipe down into a P-trap, then over to the main drain line.
• Connecting with a sanitary tee on the vertical drain.
• From that same tee, running a true vent straight up and out through the roof line.

2. Sink

• Adding a sink on the right side of the photo.
• Sink will drain through a P-trap into a sanitary tee, then drop down into the floor joist cavity.
• Using a long-sweep 90° to turn and tie into the existing drain line.
• Running the sink vent up, then horizontally over to the left to tie into the washing machine vent.

3. Floor Drain

• Adding a P-trap for the floor drain.
• I am uncertain how to run this and could use some visual help.

 

[Reach4]

1. Washing Machine

• Installing a new washer box on the left side.
• Running a 24” standpipe down into a P-trap, then over to the main drain line.
• Connecting with a sanitary tee on the vertical drain.
• From that same tee, running a true vent straight up and out through the roof line.

Some comments on that WM aspect:
1. You did not say if the trap and drain would be 1.5 or 2 inch.
2. If 1.5 inch, you might consider increasing the vertical part to 2 inch to have a little buffer capacity. I know this sounds odd, but I remember Terry suggesting that a couple of times. I don't find such a post at the moment, but I expect it is available. Standpipes for years were 1.5 inch, but washers started pumping faster. Thus the codes for new stuff were increased to 2 inch.
3. Especially if 1.5 inch, I would go longer on the standpipe because longer will be less prone to overflow. While UPC limits the length to 30 inches, IPC, which MI uses, allows up to 42 inches. So I would try to see how high the WM drain can reach. Thirty would be better than 24 IMO. There are drain extenders if needed, to go as high as 96 inches above the floor.
4. IPC allows AAVs, although a through-the-roof vent is preferred.
5. Bends that transition drainage from vertical to horizontal need to be long sweep. Horizontal to vertical can be medium bends.
6. While you are at it, any thoughts of a water softener in the future? Softener drain lines are often routed into the standpipe via an air gap. WMs have their own built-in air gaps.

 

[John King]

Thanks for the feedback.

The drain line is 1.5” and I was planning to stay consistent with that. Decreasing the drain size seems restrictive, but I definitely like the idea to maximize the standpipe and overall volume.

No plans for a water softener now or in the future.

What are your thoughts on the floor drain? Is it ok to vent as-is or do I need to an something?