Trying to setup plumbing for custom cabinet, help please!

This is probably a pretty noobie post, I apologize in advance. I'm trying to setup plumbing and fixture selections for a new home. I want to do something custom in the bottom of a sink cabinet in my laundry room. The KD has asked me to try and hold plumbing 20" above the floor. In order to do that I need to spec a sink of a proper depth to allow a P trap to be enclosed in the top cabinet section. I've read there is such a thing as a shallow P trap? That said, if I tell my plumber to utilize a shallow P trap, what depth maximum (drop in and alternatively undermount, I know the depth will vary between the two) sink should I spec. The problem I'm running into is I have no idea what the maximum height of plumbing pipes for a shallow P trap configuration is below the sink. Any help would be appreciated!

There are few, if any, "shallow P traps" due to the specifications they have to meet. There are "low inlet" P traps, but mechanically, they are inferior because they are prone to dislodging if they are disturbed by bumping. You have to know the depth of the sink, the "length" of the drain fitting, and the height of the sink rim above the floor, (whether it is above or below the counter top), THEN you put the drain at that location, NOT the other way around.

hj said:

There are few, if any, "shallow P traps" due to the specifications they have to meet. There are "low inlet" P traps, but mechanically, they are inferior because they are prone to dislodging if they are disturbed by bumping. You have to know the depth of the sink, the "length" of the drain fitting, and the height of the sink rim above the floor, (whether it is above or below the counter top), THEN you put the drain at that location, NOT the other way around.

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Alright that is good to know, that answers one of my questions. Now I'm still trying to confirm the vertical height of a "standard" P trap that isn't extended purposely by a tailpiece to reach a lower plumbing inlet. I need to know what depth of sink to spec in order to keep the plumbing 20" or higher from the floor. My guess is 7-8 inches below the sink is standard for a P trap at its lowest dip using the shortest tailpiece possible?

The design of the drain fitting will have to be considered. Some are longer than others which would cause the trap to be installed lower. IF you want the 'tightest' installation, you will have to "mock up" the pieces ahead of time and then do an actual measurement.