Stack Vent Offset Below the Highest Fixture
A four-storey office building is being converted from storage to washrooms. The existing 100 mm (4 in.) soil stack runs vertically through a chase and is offset 1.5 m horizontally at the third-floor ceiling to clear a new structural beam, then continues to the roof. The topmost fixture on the stack is a water closet on the fourth floor, and the offset occurs about 2.4 m below that closet's drain connection. The contractor proposes keeping the existing stack size and simply adding 45-degree bends at each end of the offset. The inspector flags the arrangement because the offset is located below the highest fixture branch but above the next lower branch. No relief vent is currently shown on the drawings.
1. Evaluate whether the proposed offset is acceptable as drawn, and state what must be added if it is not.
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The critical fact is the location of the offset relative to the highest fixture. An offset in a vertical drainage stack that occurs below the highest fixture branch but still within the upper portion of the stack is treated differently from an offset above the highest fixture or in the lower, building-drain portion.
Because the offset is roughly 2.4 m below the topmost water closet branch, the upper section of stack above the offset becomes, in effect, a vent for the lower section through a horizontal offset. A horizontal offset in a vent path is a poor vent unless it is properly sized and arranged, and the code requires that where a stack is offset more than 45 degrees below the highest fixture, the portion above the offset be installed as a vent (correctly sized as a vent stack) and the lower portion be provided with a relief vent or the offset be vented so the lower stack is not dependent on the offset for relief.
So the proposal to simply keep the stack size and add 45-degree bends is not acceptable as drawn. Required corrections:
- Treat the stack section above the offset as a vent stack sized per vent-sizing rules, not as a drainage stack carrying the full stack load.
- Provide a relief vent for the lower stack section, connected below the offset, or re-vent the offset per the adopted code's offset provisions.
- Confirm the horizontal offset length and slope meet vent-offset limits; excessive horizontal vent runs must be increased in size.
If the offset can be relocated above the highest fixture branch, the relief-vent complication disappears and the simpler arrangement is preferred.
2. The designer argues that adding 45-degree bends makes the offset 'equivalent to vertical' and therefore exempt. Assess that argument.
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The argument confuses fitting geometry with code classification. Two 45-degree bends produce a 90-degree change in direction overall; the intervening segment is a horizontal run, not a vertical one. Code offset provisions are triggered by the change in direction and the length of the horizontal segment, not by whether individual fittings are 45 or 90 degrees.
A 1.5 m horizontal offset is a substantial horizontal segment in a drainage/vent path. In the drainage mode it is a place where waste can decelerate, and in the vent mode it is a place where the air path is restricted and where condensate or suds can collect. Neither condition is cured by using 45-degree bends; the bends only smooth the transition and reduce turbulence at the ends.
The correct analysis is:
- Above the highest fixture: offsets are generally permitted with proper sizing and support.
- Below the highest fixture: the upper part must function as a vent and the lower part must be relieved; the offset must meet vent-offset length and sizing limits.
- In the building drain/soil branch below: offsets are drainage, not vent, and are judged on slope and cleanout access.
Therefore the 'equivalent to vertical' claim fails. The designer should either relocate the offset above the top fixture branch, shorten it within permitted vent-offset limits, or add the relief vent and resize the upper section as a vent.