Is an unreachable P-trap under sink OK?

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Old 05-25-17, 05:10 PM
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Is an unreachable P-trap under sink OK?

I've got a pedestal sink with not much room in the base and the prob is my drain line and supply lines both need to go through the bottom of the sink. I don't think installing a p-trap above the floor is really doable. Is it ok to install the p-trap below the floor in the floor cavity like is done on bathtubs? Or is this so nonstandard for a sink that I should get a vanity sink?
 
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Old 05-25-17, 07:14 PM
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In a single word, no. Pedestal sinks sometimes use what is called a "bottle" trap although most plumbing codes do not allow them.
 
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Old 05-25-17, 10:16 PM
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Every pedestal sink I've had the trap installed between the sink and the wall, where is your drain exiting?
 
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Old 05-26-17, 05:09 AM
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Perhaps a picture of your install will help.

Furd, I agree with you, but are you able to explain why it would be a code violation? It seems like a good solution and the trap would only be a few feet away separated by a floor. Access can still be had. Just curious.
 
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Old 05-26-17, 05:15 AM
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I have found that some pedestal sinks will not work with existing pipe locations. You need to locate the drain and supply lines in the wall so they will work with your sink. Because of the pedestal you have little room to work and little room to waste so plan your piping. It doesn't hurt to have the pedestal nearby to slide into position and test to make sure your piping will work before installing the sink.
 
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Old 05-26-17, 06:28 AM
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I believe code typically specifies 24" maximum tailpiece length before trap, which is probably not enough for you to go below floor. Even if it was, I wouldn't recommend it. That long tailpiece will accumulate "crud deposits" and will start stinking up the place before you know it. I suspect that a long tailpiece also increases the likelihood of trap siphoning because the water is moving faster than normal when it hits the trap due to the long fall. With a bathtub or shower, the drain is right above the floor, and therefore right above the trap.

As the others have said, do what you have to in order to get the drain and vent behind the sink so you can do a p trap or decorative trap under the sink.

I've seen folks do a small bump out (2-3 inches) from floor to ceiling just behind the sink to get room to run the plumbing if they can't get it in the wall for whatever reason.
 
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Old 05-26-17, 07:28 AM
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Paul,

That explanation makes sense to me. Thanks for the follow-up.

I also agree the OP should do the necessary re-plumping to accommodate the pedestal sink.
 
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Old 05-26-17, 08:03 AM
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Some pedestal sinks do not have enough distance between the wall and pedestal so you must use an 1" trap.
 
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Old 05-26-17, 12:01 PM
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Thanks for the advice guys. It's an old house so I can't install pipes in the exterior wall due to difficulty. The drain is currently going into the floor from a 2nd floor bathroom and exits in the dining room ceiling on the first floor.

Trying to get some type of decorative p-trap that sticks out from the pedestal might be doable but it would be real tight either way. I might just use a vanity to make life simpler on myself. I definitely won't install the p-trap below the floor though.

I'm going to probably seal up some plumbing in the dining room ceiling using drywall. I assume it's better to use cemented pvc in there than any of those kits with the twist-type/gasket connections (like a bathtub drain line kit)? Also for supply lines is cemented c-pvc longer lasting and more stable than SharkBite fittings?
 
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