We just moved to a new house, and found some small insects in the tub. We hired an inspector, who believes the root is the pluming issue. Both traps for the tub and the shower are below the main sewage line.

But the builder considers no issue with this design.

In my opinion, if the trap is below the main sewage line, then it is not funcatinoal at all because the waste can easily flow into the trap and gas can get into the house.

But I am no expert. Would you please offer some help? Is there a building code about whether the trap for tub and shower should be above the main sewage line?

Thanks.

Jay

In plumbing drainage is based on one thing. Gravity! The traps MUST be above the sewer main to function properly. If the trap's below the pipe that it drains into then the head pressure at the strainer is all that's forcing the discharge through the drain line. The trap plus the drain line will retain the water that remains in the line. This is called "backfall" and is not acceptable in drainage. Drain flies live in the crud that resides in the trap and raiser. They are a small black fly about a third as large as a house fly. Is that what you have? Regards, Tom