When I laid out the drains for my new laundry area, i put the p-trap underfloor as it made the most sense connecting to the existing sewer branch nearby. There's also an adjacent sink that drains into the same sewer branch and they share a 2" vent. One of my first inspections was an "underfloor". I filled up a 10 foot pipe, nothing leaked, inspector didn't say anything about my p-trap underfloor.

Our city uses the UPC code. My Code check book says that the trap should be 6-18" above the floor. For my own knowledge, what's the reasoning behind having the trap above floor? A lot of plumbing codes seem to make intuitive sense, but I'm curious to know why this code is the way it is. When my rough inspection comes up, the inspector may point it out and ask me to replumb it. Otherwise I can't seem to think that moving it above the floor will make any difference.

Thanks for any insights.

For a laugh, I left no good way to get the water out of the standing 10 foot pipe and new laundry drains. Luckily it didn't matter because it was raining so hard when I let the water out.