What is the minimum depth of a trap seal for any above-ground plumbing fixture?

What is the minimum depth of a trap seal for any above-ground plumbing fixture?

• A. 1 inch
• B. 2 inches
• C. 3 inches
• D. 4 inches

Answer: (B)

The main idea this tests is how much water a trap must hold to block sewer gases in an above‑ground fixture. The minimum trap seal depth is two inches. This depth provides a reliable barrier against odors even if some water is lost over time or if venting isn’t perfect. A shallower seal, like one inch, wouldn’t be as resistant to evaporation or minor siphoning, so gases could escape. Deeper seals (three or four inches) can meet the requirement and are allowed, but they aren’t the minimum required. Many codes also cap trap seals at a maximum depth of four inches, but the smallest acceptable depth to meet code is two inches.

The main idea this tests is how much water a trap must hold to block sewer gases in an above‑ground fixture. The minimum trap seal depth is two inches. This depth provides a reliable barrier against odors even if some water is lost over time or if venting isn’t perfect. A shallower seal, like one inch, wouldn’t be as resistant to evaporation or minor siphoning, so gases could escape. Deeper seals (three or four inches) can meet the requirement and are allowed, but they aren’t the minimum required. Many codes also cap trap seals at a maximum depth of four inches, but the smallest acceptable depth to meet code is two inches.