Is this P-trap Redundant/Unnecessary?

[-MIKE-]

I have a slow vanity sink drain that's filling up when we're brushing teeth or running water for a while. The under-cabinet trap is clear and so is the horizontal drain pipe going into the wall. The other sink on this cabinet is draining fine. They each go into the wall and down to the crawlspace through separate drain pipes. I noticed this trap in the crawlspace underneath the slow draining sink and suspect it's the source of the clog.

There's a chance I can snake it out from the vent in the roof (single story, shallow pitch roof) as I think it might be a straight shot down. If it's not, I might have to deal with it by cutting this trap off (can't tell yet if that's a threaded coupling).

Here's my question! Am I wrong or is this trap NOT even necessary and just a clog-in-waiting? It's my understanding these under-floor traps are for use with tubs and showers that don't have an incorporated P-trap like a toilet or an under-sink drain trap.

If this is an unnecessary, redundant trap, I will gladly cut it out and put a simple sweep combination in its place. What do you guys think?

 

[Cacher_Chick]

The only trap that should be below the floor is that of a tub, shower, or floor drain.

 

[Sponsor]

 

[-MIKE-]

The only trap that should be below the floor is that of a tub, shower, or floor drain.

That's what I thought. Thank you.

 

[hj]

Unless you also have a "P" trap above the floor in the sink's drain line, then that trap IS necessary.

 

[-MIKE-]

Unless you also have a "P" trap above the floor in the sink's drain line, then that trap IS necessary.

Yes, there are p-traps below each of the two sinks in the double vanity. They are right below each sink like you'd see under just about every bathroom or kitchen sink.

It is also my understanding that the p-trap in my picture isn't correct, anyway, because waste water can fall too far down, building up velocity that can siphon out the water in the trap.

 

[Terry]

Yes, cut out the p-trap under the floor then if you have traps below the lavs.
Somebody got a little carried away there.

And also yes about tailpieces being too long and siphoning.

 

[-MIKE-]

Thanks, guys! I appreciate it.