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Drain and vent plumbing for addition

Posted by matt_m (My Page) on
Thu, Jun 26, 08 at 18:46

Hello, folks. I'm not a plumber by trade, but I'm trying to learn the trade. On our addition, there will be a laundry room with a front load washer and a large utility sink. A 2" ABS drain/vent pipe is in the wall right behind and between where the two items will be.

For option a, I want to put a double sanitary tee on the main vent stack, about 14 inches off the slab, and then run the vent out of the top of it. Off of one side of the double san tee, I would run 2" ABS about 18 inches to first a p-trap, and then up to my standpipe for the washing machine (top will be at 36"). Off of the other side of the double san tee, I would run about 18" of 2" ABS horizontally, add a 90 degree elbow, and stub it out of the wall where my sink drain will be. This stub out for the sink drain would be located approximately 14" off the slab.

For option b, I would stack two sanitary tees, one on top of the other, starting at about a foot off the slab, and continue my vent out the top of the topmost san tee. Off the top san tee, I would run about 18" of 2" ABS horizontally, put a 90 degree elbow on it, and stub it out for my sink drain (about 14" off the slab). Out of the bottom san tee, I would run 18" of 2" ABS horizontally to a p-trap, and then hook that to my stand pipe for the washing machine. My question here is this: If this is even a good configuration, which device should be hooked up to which san tee (should the sink drain be on the top or bottom san tee, etc.)?

In my inexperienced opinion, the two devices (sink and washing machine) are close enough to the 2" main vent stack that additional venting wouldn't be necessary (like a vent pipe leading from the sink drain pipe, up a few feet, and then cutting over to the main vent stack).

If my explanations are confusing at all, I will gladly draw a picture of the layouts and upload it to photobucket. Thanks for any help that any of you can give me!


Follow-Up Postings:

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RE: Drain and vent plumbing for addition

Go with option b, Washing machine first then sink. Its a minor over a major. The minor being the sink and the major being the washing machine. A lot of people have the impression that a vent line helps drain plumbing fixtures, which it does but thats not the only use for it. A vent when installed correctly will save the trap from being sucked dry when a large voulume of water rushes down the line. If you put a double san tee in the vent line and stub the lines both out at 18" you run a change of having the discharge water from your washing machine to back up in the sink because of the two fixtures being close together. Option b is how I would tackle it.....


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RE: Drain and vent plumbing for addition

Does the sink require 2"?

If it requires 1-1/2 then you're allowed to go up one size but you don't have to. If I am right, and if you use separate tees then I would think you should stay with 1-1/2 to keep the velocity high in the drain line more to your point, to minimize chance of laundry water backing up.


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RE: Drain and vent plumbing for addition

He could arm out for the sink 1 1/2 but the washing machine line needs to be at least 2"


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RE: Drain and vent plumbing for addition

When installing a washing machine supply and drain box I like to consider both the Plumbing Code requirements and the ergonomics of operation.

Typically a top loading automatic washers main body is approximately 36" high, and the rear control panel extends up another 5 or 6" for a total combined height of 41" or 42". I prefer to set the washing machine box so that the top of the standpipe will be 40" to 42" above the finished floor. In this manner the Hot & Cold water shutoff valves can be easily accessed by reaching directly forward instead of trying to contort down behind the control panel. I find those homemakers who prefer to turn the valves off between washing periods especially appreciate this.

From this point we must consider the code requirements. Under both the International Residential Code(IRC) & The Uniform Plumbing Code(UPC) the minimum allowable height for a washing machine standpipe is 18" therefore we must plan on placing the P-trap a minimum of 18" below the top of the standpipe, however I prefer to keep the length of the standpipe as long as practical under the circumstances.

Under the IRC the P-trap may be installed under the floor and we are allowed a maximum of 42" vertical rise from the water level in the P-trap to the top of the standpipe.

Under the UPC we have four code elements that we must be concerned with.
1. Maximum allowable vertical rise from the water level in the P-trap to the standpipe opening may not exceed 30".
2. P-trap must be on the same floor as the standpipe.
3.The horizontal tailpiece from the P-trap must be a minimum of 6" above the floor.
4.The P-trap may be no more than 18" above the finished floor.

I would begin by installing the washer box at 40" to 42" above the floor then attach a 30" length of 2" pipe to the bottom of the washer box. The washer box will add about an inch to the overall vertical height however the bottom of the 2" pipe will be inserted about 2"into the hub of the 2" return bend (return bend is the proper name for the 180deg bend fitting that forms the U of your P-trap.) When the 2" Street 1/4bend is inserted into the opposite side of the return bend to form the tailpiece of the P-trap the water level of the trap will be even with the bottom inside of the 1/4bend, which is slightly higher than the hub of the return bend. Thus your vertical rise will still be within the 30" maximum vertical rise.

The IRC would then allow the horizontal 2" line to run up to 8 without requiring additional venting, while the UPC would limit a 2" horizontal to 5, however in this case your horizontal run is approximately 2 so you are well within the limits.

At this point your P-trap should be approximately 11" above the finished floor and allowing for the required pitch of the horizontal line (2 x "/ft =1/2") your 2" sanitary Tee on the stack will now be about 10-1/2" above the floor.


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