Eighteen Inches of Fall on a 180-Foot Building Drain
A five-story, 60-year-old apartment building has a 4-inch cast-iron building drain that runs 180 feet from the base of the soil stack to the public sewer. The basement ceiling provides only 18 inches of fall. The owner wants to add a third-floor bathroom (water closet, lavatory, bathtub) to each of two units, adding 22 fixture units to the existing 96 fixture units. The contractor proposes laying the new 30-foot branch at 1/4 inch per foot and re-laying the building drain at 1/16 inch per foot to fit the 18-inch space. There is one cleanout at the upstream end of the building drain; the line passes through two 45-degree offsets. The owner wants to avoid replacing the 4-inch drain with a larger pipe because the basement is finished.
1. Evaluate the proposed 1/16-inch-per-foot slope for the 4-inch building drain. Show the fall required at the code minimum and explain whether the 18-inch space is adequate.
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For 4-inch horizontal drainage pipe, the adopted code minimum is 1/8 inch per foot (1 percent), not 1/16. At 1/8 inch per foot over 180 feet, required fall = 180 × 0.125 = 22.5 inches. The available fall is 18 inches, so the existing route is already about 4.5 inches short before any new work. At the proposed 1/16 inch per foot, fall would be 180 × 0.0625 = 11.25 inches; that fits the space but is half the required slope and would reduce scouring velocity and increase solids deposition. The 30-foot branch at 1/4 inch per foot is fine for a 3-inch or smaller branch, but it does not fix the building drain. You cannot approve the 1/16 slope just because it fits; the code minimum is not a design suggestion. Options include rerouting or shortening the run to the sewer, lowering the connection, installing a lift station with a properly sized receiving basin and discharge, or obtaining an engineered alternative only if the code official accepts it. Replacing with a larger pipe may allow a different slope only if the adopted table permits it; verify before promising.
2. What cleanout deficiencies exist, and where would you require cleanouts on this 180-foot building drain?
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The single upstream cleanout is not enough. The code generally requires cleanouts at the base of the soil stack, at intervals not exceeding 100 feet for this size of horizontal drain, at changes of direction greater than 45 degrees, and at the junction of the building drain and building sewer. A 180-foot run therefore needs at least one intermediate cleanout so no portion exceeds the maximum interval, plus cleanouts at the two 45-degree offsets if their aggregate change exceeds 45 degrees or if the code treats them as a direction change. Cleanouts must be accessible, gas-tight, and sized to the pipe, with clearance for rodding. If the basement is finished, install access panels or floor boxes rather than concealing them. Do not count a cleanout on a branch as the building-drain cleanout unless it is located and sized to allow the main line to be rodded in both directions.
3. The owner adds 22 fixture units to 96 existing. What must you check before approving the 4-inch building drain, and what alternatives would you present if the slope cannot be corrected?
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First total the load: 96 + 22 = 118 fixture units, then verify the 4-inch building drain capacity at the actual slope using the adopted fixture-unit table; do not assume 4-inch is adequate merely because it served the old load. Check the building sewer, stack, branch sizes, and venting for the added bathrooms; a new bathroom group needs a water closet, lavatory, and bathtub, each with the correct trap and vent. If the 18-inch fall cannot be achieved at the code slope, present these options: (1) reroute the drain to a closer sewer connection or lower the outlet; (2) install a sewage lift station serving the basement or the new fixtures, with a vented receiver and check valve; (3) replace the building drain with a larger size if the adopted code allows a lesser slope and the capacity table supports it; (4) reduce the developed length by relocating the stack or sewer tap. Any alternative must be shown on plans and accepted by the code official before installation.