A running trap will save more than 3" in floor height. You must vent the drain downstream or the trap will siphon dry occasionaly.

Can you please explain the difference between a P-trap and a running trap?

The answer is, "a short piece of crooked pipe". How crooked? Just enough down then up to and in same direction as drain is running. Visuialize a long flexiable hose lying across two tables sitting 6 inches apart. Open your hand as if preparing to Karate chop the hose but instead push down until it is curved exactly the diamiter of hose. A friend places his palms on hose both sides of the 6" opening and flattens hose on tables. What you see is a running trap,no mechanical connections with-in the bend.

Thanks. I think I can visualize that. Sort of like a p-trap without such a deep bend. I presume that the only problem with that is that not much water would have to evaporate before sewer gas could get through...Correct?

I think think you understand. It's possible that plumbing supply houses can supply one with more "belly". Might be a challange making connection to dwv but a muffler shop can make one to your specs using stainless steel. If it is a vacation cabin or guest bath,there is a kittchen sink/ tub stopper that is a flat,soft and flexiable piece of rubber. I have supplied those to customers for thier cabin sinks and tubs and never heard a complaint with it not doing the job.

klem-
Thanks for all the info...really helpful.

Have we established whether that pipe from the floor already has a trap under it? If it was intended for a shower, it does. Pour water into it, look down in with flashlight and see if it retains water. I don't know how you go from there, code-wise, except maybe you can use angled pipe to reach it?

homebound-
Thanks...good insights, but I had already tried to pour a lot of water down it and it doesn't retain water. It's in a location that's intended for a powder room or small bathroom. Coming up through the slab is a 4" sewer line for a toilet, a 4" vent stack, and the 2" drain which I think is intended to service a sink with a regular p-trap, and, if there's a shower, you have to come up with a solution for that with the shower sharing the same drain. Any other thoughts are welcome, though.

Probably the easiest way is to just let a plumber open the floor around the drain and do what's necessary. Short work with a hammer drill + sledge (and safety glasses) for that small area. I'd consult one first, though. Drainwork like that is best left to the professionals.

Probably the easiest way is to just let a plumber open the floor around the drain and do what's necessary. Short work with a hammer drill + sledge (and safety glasses) for that small area. I'd consult one first, though. Drainwork like that is best left to the professionals.

homebound-
Thanks...sounds like good advice.

I appreciate that you posted a follow up. It's awesome hearing how people ended up solving the issues they were up against. Thanks.