Plan Review: Slope and Fixture Unit Load on a Branch
A plan reviewer receives a drawing for a commercial kitchen remodel. A 4-inch horizontal branch drain serves three floor sinks (2 dfu each), two hand sinks (1 dfu each), and one three-compartment sink (3 dfu). The branch runs 35 feet at 1/8 inch per foot slope to a 4-inch stack. The plan notes that the branch "slopes to drain" without a specific grade. The reviewer must decide whether the branch is acceptable as drawn, and what corrections to require before issuing a permit. The designer has not shown a cleanout on the branch.
1. Evaluate the slope and sizing of the 4-inch branch. What does the reviewer need to confirm, and what is a likely correction?
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First, total the fixture units: three floor sinks at 2 dfu = 6, two hand sinks at 1 dfu = 2, and one three-compartment sink at 3 dfu = 3, for 11 dfu. A 4-inch branch is more than adequate for 11 dfu at any normal slope, so capacity is not the problem.
The problem is slope. A 4-inch horizontal drain must be laid at a grade that produces self-cleansing velocity. Common code values are 1/4 inch per foot for pipes 2-1/2 inches and smaller, and 1/8 inch per foot for 4-inch and larger, though some codes require 1/4 inch per foot for all sizes up to 4 inches. The plan's "slopes to drain" note is not a specification.
The reviewer should require the designer to state the grade explicitly, show the invert elevations at both ends, and confirm the grade meets the adopted code. If the code requires 1/4 inch per foot for a 4-inch branch, the 35-foot run needs about 8.75 inches of fall; if 1/8 inch per foot is allowed, about 4.4 inches. The reviewer should also confirm that the branch does not exceed the maximum developed length for its size and that the stack connection is by an approved fitting (wye or combination), not a sanitary tee on its back.
2. The reviewer notices no cleanout on the 35-foot branch. What should the reviewer require, and why?
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Cleanouts are required at the upper end of a horizontal drainage branch and at intervals along long runs, and at changes of direction greater than 45 degrees. A 35-foot branch without a cleanout is a maintenance defect even if it drains.
The reviewer should require a cleanout at the upstream end of the branch (or at the most accessible point), sized the same as the pipe it serves, and with a clearance for rodding. The reviewer should also confirm that the cleanout is accessible, not concealed behind finished surfaces, and that it is not located where it would spill onto food-contact surfaces in a kitchen.
The reasoning is that a branch serving floor sinks and a three-compartment sink will accumulate grease and solids. Without a cleanout, the only access is by removing fixtures or cutting pipe. The reviewer should condition permit issuance on a revised drawing showing the cleanout location, size, and access. This is a plan-review correction, not a field fix.