In vertical wet venting, the drain between the two fixtures must be at least how large compared to the upper fixture drain?

• A. Equal in size to the upper fixture drain
• B. At least one pipe size larger than the upper fixture drain
• C. At least three inches in diameter
• D. One pipe size smaller than the upper fixture drain

Answer: (B)

In vertical wet venting, the lower fixture’s drain must do double duty: carry waste from the lower fixture and act as a vent for the upper fixture. To avoid restricting that venting path, the drain segment between the two fixtures needs extra capacity. Making it at least one pipe size larger than the upper fixture drain provides enough cross-sectional area to carry the upper waste while still allowing air to vent up the stack. If the between-fixture drain were the same size or smaller, it could create a bottleneck that impedes venting, leading to trap siphonage and poor drainage. For example, if the upper fixture uses a 1-1/2" drain, the between-fixture drain should be at least 2" (and larger if the upper drain is larger). The “three inches” rule isn’t universal because pipe sizes vary with fixture, so the general guideline is one pipe size larger.