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Plumbing in the Home/Adding a floor drain for a washing machine


Hi Jay, thanks for accepting this question. My home was built 15 years ago. My laundry room is on the second floor. Since i opened up my kitchen ceiling, i have now complete access to the drain pipes required to install a floor drain under the washing machine. I am a bit confused though on how i should rough this new drain.

The existing plumbing is up to code. So i am thinking of doing like in the attached image. But not sure. Can you give me some advice please?

Couple of things:

I have complete access to all the plumbing
I would like the floor drain P trap to remain full of water all the time
From the new drain to the existing drain pipe would be about 48 inches long.

Again any help would be much appreciated.

HI Stephane,
There are several factors that have to be considered for the application. First, the trap should be directly under the drain opening. The pipe size is dependent upon the distance from the stack to the drain grate is. If the center of the drain opening is less that 3 ft you could use 1-1/2" pipe. If it is more than that but less that 5' then 2" must be used. Anything over 5' away from the center of the stack must be 3" pipe. BUT.. if you back-vent the distances can be lengthened. Now, your diagram is not going to work because you are installing a trap on a line that is a combination drain and vent. If done like your diagram shows, the vent would no longer work and you would have problems with your entire system..
The simplest way to do this is install a 2" drain with a trap and run 2" pipe the entire way and keep it within 5 ft of the stack Do not trap it where you indicated. as for keeping it full,a trap primer can be installed that spits water into the drain automatically. This would require a waterline to be installed and the side of the pipe above the trap to be tapped.
If you use a 3" trap, it will take much longer for the water to evaporate but keep in mind that it will eventually and you may have to pour water into the drain from time to time to refill it.(that is if no trap primer is used)

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Jay Nicholas


Plumbing & Heating (warm air, hot water & steam heat)


40 years in the business, 40 years as a lic. master plumber in NY State. Retired

Graduated Magna cum laude at the School of Hard Knocks

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Commercial, residential and light industrial. You name it ...I have probably worked on it

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