A Six-Foot Offset in a Four-Story Soil Stack
A 36-unit, four-story apartment building is being renovated. Each unit has one bathroom group (water closet, lavatory, bathtub) and a kitchen sink; the engineered drainage load is 9 drainage fixture units per unit, 324 total, and the approved 4-inch cast iron soil stack is within the tabulated capacity for the stack height. To clear a new structural beam at the third-floor ceiling, the design team proposes a 6-foot horizontal offset in the stack, with the upper 12 units reconnecting above the offset. The existing vertical chase is 10 inches wide. The contractor wants to use two 45-degree bends with a short horizontal section, keep 4-inch pipe throughout, and omit any additional venting because "the stack is already vented at the top." The building has a single 4-inch vent through the roof and no separate vent stack. The inspector is scheduled for the rough-in on Friday.
1. Will the stack drain and vent correctly with the proposed 6-foot offset and no added venting? Explain the flow mechanism and the remedy.
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No. A vertical stack carries a mixture of water and air, and a horizontal offset interrupts that pattern. At the offset the falling water decelerates, backs up, and then re-accelerates as a slug. The slug compresses air in the lower section (positive pressure that pushes trap seals out) while the upper section drains faster than the single roof vent can replace air (negative pressure that siphons traps). The taller the building and the longer the offset, the worse the effect; a 6-foot offset serving 12 units above is well past the length threshold in the model codes, which is generally in the 3- to 4-foot range depending on edition.
Remedy: install a relief vent at the base of the upper section, connecting the stack immediately above the offset to the vent system. The relief vent is sized from the drainage fixture units on the stack, so the plumber must check whether the existing 4-inch stack vent can carry the additional load; if it cannot, a new vent stack and roof penetration are required. Two 45-degree bends are the right fitting choice, but they do not substitute for the relief vent.
2. The offset must stay inside a 10-inch-wide vertical chase. What constructability and sizing issues must be resolved before the inspection?
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The geometry does not work as drawn, and this is the judgment the field team must surface now rather than at inspection:
- A 10-inch vertical chase cannot contain a 6-foot horizontal run. The offset needs either a soffit below the beam, a new chase, or a redesign that shifts the stack or resizes the beam opening. Two 45-degree bends with a "short horizontal section" will not clear a beam that requires 6 feet of lateral travel.
- Slope and support. The horizontal portion is now a drain, not a stack: it needs the code slope (typically 1/4 inch per foot for 4-inch pipe), hangers at the required spacing, and fittings of drainage pattern. Short-radius 90s and sanitary tees on their backs are not acceptable.
- Size. The offset must not be smaller than the stack. If the horizontal run is repiped, verify the capacity of the offset section against the 324 fixture units above and below it.
- Cleanout and access. An offset of this length should have a cleanout at the base of the upper section, and it must remain accessible after the soffit is closed.
The plumber should submit a sketched detail and a written RFI to the designer of record before Friday, because the inspector will not approve a chase detail that cannot be built.
3. What must be complete and exposed for Friday's rough-in inspection to pass?
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The inspector is looking at the drainage and vent system before concealment, so the plumber should have:
- The tested system. Complete the water or air test required by the adopted code (typically a 10-foot head of water or the specified air pressure, held for the required time) with the test visible and documented. Test the system with the offset and relief vent in place, not planned.
- The relief vent installed and connected to the vent system, sized from the drainage fixture units, with the connection visible at the offset.
- Supports and fittings visible: hanger spacing on the horizontal offset, proper no-hub couplings with clamps correctly positioned and torqued, and drainage-pattern fittings.
- The approved revision on site. If the offset required a plan revision, have the stamped revision and any RFI response available; an inspector who has not seen the revised drawing is likely to red-tag the work regardless of quality.
Passing the rough-in is largely about sequencing: nothing gets covered, tested pipe stays exposed, and the paperwork matches the pipe. If the relief vent is missing, the plumber should expect to be back for a second inspection and to pay a re-inspection fee.