Office Building Drain Capacity With Two New Restroom Groups
A three-story office building has a 4-inch cast-iron building drain under the slab at 1/8 inch per foot. The owner is combining two suites and adding a new restroom group on each of two floors. The existing load calculated from the approved drawings is 168 drainage fixture units (DFU). Each new restroom group contains four water closets with flush tanks, two urinals, four lavatories, and two floor drains. The contractor proposes connecting both new groups to the existing 4-inch drain without changing slope or pipe size. The building official will require a load calculation and a slope and capacity check for the new work. The existing drain has no reported backup history, but the building has had two tenant changes in the past five years and the drawings are not fully as-built.
1. Using UPC drainage fixture unit values, calculate the added load and the new total load. Does the existing 4-inch drain at 1/8 inch per foot have adequate capacity?
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Apply the UPC fixture-unit table for drainage. For each new restroom group: four flush-tank water closets at 3 DFU each = 12 DFU; two urinals at 2 DFU each = 4 DFU; four lavatories at 1 DFU each = 4 DFU; two floor drains at 1 DFU each = 2 DFU. That is 22 DFU per group. Two groups add 44 DFU. The new total is 168 + 44 = 212 DFU.
The existing 4-inch building drain at 1/8 inch per foot is limited to about 180 DFU under the UPC drainage sizing table. At 212 DFU, the proposed connection is over capacity by 32 DFU, so the answer is no. The 1/8 inch per foot slope does not make the 4-inch drain adequate merely because the building has not backed up. The correct conclusion is that the drain must be upgraded, re-sloped, or supplemented before the new groups are connected.
2. If the drain cannot be re-sloped, what are two compliant options, and what other checks are needed before selecting one?
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Two compliant options are: increase the drain size, or provide a second building drain or separate drainage path for part of the load. Increasing the slope from 1/8 to 1/4 inch per foot would raise the 4-inch capacity to about 216 DFU, but that is only a 4 DFU margin above 212 and may not be achievable under the existing slab. If the contractor can obtain the 1/4 inch per foot slope and verify it by survey, it is technically possible, but the design should be checked carefully because future tenant loads could push it over again.
Before choosing, check the building drain and sewer capacity beyond the first section, the stack capacity, the horizontal branch loads, venting, and the available fall to the sewer. Also check whether the existing cast-iron pipe is deteriorated or has improper fittings that would make re-sloping impractical. The selected option must preserve self-cleansing velocity and be inspected before concealment. A load calculation should be submitted with the permit revision rather than relying on the contractor's field judgment alone.
3. Suppose the calculated new total were 220 DFU instead of 212 DFU. Would changing the slope to 1/4 inch per foot solve the problem? Explain.
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No. At 1/4 inch per foot, the 4-inch building drain is still limited to about 216 DFU, so 220 DFU would exceed the table value. The small difference matters because the code limit is not a suggestion and there is no field discretion to round down the load. The designer would need to upsize the drain, add a second drain, or reduce the connected load. This is exactly the kind of judgment the exam tests: re-sloping is not a universal fix, and a value just above the table limit still fails. The designer should also confirm that the 220 DFU figure includes all future fixtures on the same drain and that the stack and venting are adequate for the revised layout.