How does the UPC distinguish a building sewer from a building drain?
Not quite. The answer is C: The building drain is the lowest piping inside the building walls, ending 2 feet outside the building wall; the building sewer continues from that point to the public sewer, private sewer or private disposal system
The UPC's definition puts the dividing line 2 feet outside the building wall: the building drain collects soil, waste and other drainage inside the walls and carries it to that point, and the building sewer is the horizontal piping from there to the public or private sewer or disposal system. (Washington does not adopt the UPC's building-sewer installation rules — those fall under other authorities — but the definitions still apply.)
Reference: UPC 2021 Chapter 2 — Building Drain; Building Sewer (Definitions)
Checked against the official text
Checked against the 2021 UPC text. Washington's exam is written against the 2015 edition, which we have not read for this section, so confirm it against your 2015 book.
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