Plan Review Discrepancy in Drainage Fixture Units and Pipe Slope
A plan reviewer receives drawings for a three-story office building with restrooms on each floor. The drawings show a 4-inch building drain at 1/8 inch per foot slope, carrying a total of 160 drainage fixture units. The horizontal branch from the third-floor restroom is shown at 1/8 inch per foot on a 3-inch pipe. The vent stack is shown as a 2-inch pipe serving all three floors. The drawings do not show the number of fixture units per branch, only the total. The reviewer must decide whether to approve the plans, issue comments, or reject them, and what specific corrections to require.
1. Evaluate the 4-inch building drain at 1/8 inch per foot for 160 DFUs and state whether it is acceptable.
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A 4-inch building drain at 1/8 inch per foot has a limited capacity. Typical code tables allow a 4-inch pipe at 1/8 inch per foot to carry on the order of 216 DFUs (the exact value depends on the adopted table), so 160 DFUs is within the tabulated capacity and the slope is acceptable for a building drain. However, the reviewer must confirm the table used by the adopted code, because some tables limit 4-inch pipe at 1/8 inch per foot to a lower value. The key point is that capacity must be checked against the specific code table, not assumed, and the total must include all fixtures on all floors. The drain itself appears acceptable if the adopted table permits it, but the reviewer should require a fixture-unit breakdown to verify the 160 total and confirm no branch is overloaded.
2. Evaluate the 3-inch horizontal branch at 1/8 inch per foot and the 2-inch vent stack, and list the comments you would issue.
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A 3-inch horizontal branch at 1/8 inch per foot is generally not permitted for a branch serving water closets; branch slopes are typically required at 1/4 inch per foot for 3-inch and smaller pipe, with 1/8 inch per foot allowed only for 4-inch and larger. The reviewer should require the branch to be shown at 1/4 inch per foot or upsized to 4 inches. The 2-inch vent stack serving three floors of restrooms is likely undersized; vent stack sizing depends on the drainage fixture units connected and the stack height, and a 2-inch stack may not handle the aggregate. The reviewer should require a vent sizing calculation and a fixture-unit breakdown per branch. Comments: provide DFU breakdown per branch and per stack, correct the 3-inch branch slope, verify vent stack size against the code table, and show the building drain capacity calculation. The plans should be returned for correction rather than approved as submitted.