Two P traps?

11 years ago

I'm putting in a powder room (toilet and vanity) in the corner of a large (400 sqft), stand-alone studio room that is on a slab; I was thinking of having a shower, too, but changed my mind. The plumbing was all roughed in when the house was built about 10 years ago. All the plumbing comes up through the concrete and the vent/waste pipes are capped off about 2' above the floor level. I have a 3" vent stack, a 1-1/2" waste line, a 4" toilet flange, and three cold water supply lines.

I removed the knockout from the 1-1/2" drain line and shined a flashlight down; it goes about a foot under the concrete. I tested it by pouring about a gallon of water down it, and it flowed easily, but there was no water showing at the bottom after the pour, which means there is no P trap, or it's situated further along. I have three neighbors who have the identical house and their houses came with a small bathroom already installed in this studio room, and each of them also has a bathtub or shower installed in this area, so I am assuming they have no problems with the drains.

In the area I'm adding a powder room to, I either have a P trap that I can't see, or the plumbing was done wrong. I have a walk-in shower on a slab in a different area of the house, and I pulled the drain cover and can see that it has water at the bottom, so it does have a P trap in the usual location.

Since the new sink will have its own P trap right below the bowl, does it matter if there is a second P trap below the concrete somewhere? I assume that it wouldn't make any difference in operation of the sink if there were two P traps, but wanted to check.

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Comments (2)

lazypup
11 years ago
Per code, every fixture must have a trap but code prohibits two traps in series.

You 1-1/2" drain line has no trap under the slab, which is correct, however, when you extend that line up to your fixture arm from the sink, you will need to extend that line up past the fixture arm until it is at least 6" higher than the flood level rim of the sink and attach that to the vent.
kudzu9
Original Author
11 years ago
lazypup-
Thanks...I always appreciate your knowledge and straightforward answers.

I am assuming from what you said that it was intentionally plumbed that way (without a trap) precisely because:
1) that is the way it has to be for a sink to be correctly connected (with one P trap under the bowl), and
2) if someone wanted to hook up a shower or bath to share that drain line with the sink, then there would have to be a P trap at the floor level beneath the shower pan or beneath the tub.

Correct?

The reason I was thinking the drain line might have a P trap under the slab is that none of my neighbors' houses (identical slab construction to mine) have elevated tubs or shower pans, so I couldn't figure out how that would work. All of our houses have radiant heat lines in the slab, so it's always dicey to be jackhammering out concrete as you might have to do to set a P trap lower under a shower or tub. So I'm curious: What is the minimum height one would typically have to have to fit a P trap under a tub or a shower pan if you don't want to break into the slab? Or is there some other solution to this situation that I'm not aware of?
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