# Trench drain traps required?



## Mech (Mar 15, 2010)

2009 IBC / IPC

I have an existing auto repair building with a trench drain emptying into a sump.  I believe the sump pump discharges through the oil water separator.  We are designing an addition, with two 15 ft long trench drains.  I know *1002.1 Fixture traps* says each plumbing fixture shall be separately trapped.  So, I planned on adding a separate trap and vent line for each drain.

But after talking with my boss, I had an brain trickle.

Is there a way I can legally connect the new trench drains to the existing trench drain without the need for additional traps / vents?  If the first trench drain empties into the second trench drain, can we omit one trap / vent?  And if we empty the second drain into the existing drain, could we omit that trap / vent as well?

I'm thinking we need separate traps, but I thought I would ask the question(s).

I know the oil water separator is based on area drained, so please feel free to skip that issue.

Thanks


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## Builder Bob (Mar 16, 2010)

Re: Trench drain traps required?

Not the brightest light bulb in the box, but isn't an oil seperator a fancy smancy trap to begin with?


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## Dr. J (Mar 16, 2010)

Re: Trench drain traps required?

For most garage drain installations, to make it legal, you have to trap each drain, but to make it work, the drains should have no trap except for the sand/oil interceptor.  The sand/oil interceptor does the job of the trap.  But just as important, the p-traps do the job of the sand/oil interceptor.  Thus each p-trap will collect gunk.  If each drain simply elbows to the drain line then the drain discharges into the sand/oil interceptor the lines will be kept (relatively) clean and the interceptor will separate sewer gasses from the building.  However this is not in accordance with the code.

Unfortunately, it sounds like in this situation the sump pump and sand/oil interceptor are installed bass ackwards.  The drain into the sump pump is not protected from the sewer gasses, because the inlet can be above the water level.  Also, the intermittent high flow rate of the pump when it cycles on and off may very well stir up the interceptor and pass sand and oil downstream.  It shoulda been sand oil interceptor THEN sump pump.  In this case, the trench drains need to pass through a p-trap before the sump pump.

I am not quite sure how one drains a new drain into an existing drain.  Assuming both the new and old drains are on the same floor level, how does the pipe from the bottom of a new drain make it to the existing drain?  Will these be connected end-to-end?  If so, then maybe you can call this one huge fixture only needing one p-trap.


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## Mech (Mar 17, 2010)

Re: Trench drain traps required?

The two new trench drains are parallel to one another in a proposed addition.  The existing trench drain is in a different room, about 18" below where the new trenches will be.  The sump pump is needed to lift the trench drain water into the gravity drain heading to the oil water separator.


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## Dr. J (Mar 18, 2010)

Re: Trench drain traps required?

Ok, that helps explain it.

I think draining one trench drain to the other will work, and will work better than trapping it (by avoiding gunking up another p-trap) but it is not code compliant.  To justify this situation, you could call it similar to a multiple compartment sink where the individual compartments are trapped by a single p-trap.  Ok, the "compartments" in this case are in different rooms, but the concept is the same.  Yeah - that's pretty much a stretch.


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## peach (Mar 20, 2010)

Re: Trench drain traps required?

I don't think it's that much of a stretch, Dr. J.


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