Sizing a Branch for Added Fixtures in an Older Building
You are renovating a 1960s two-storey commercial building. The existing 3-inch cast-iron soil stack serves two water closets (2.5 fixture units each), three lavatories (1 FU each), and two floor drains (2 FU each) on the upper floor. The owner wants to add a mop sink (2 FU) and a second water closet on that same branch. The branch to the stack is 4-inch cast iron at 1/8-inch-per-foot slope and is roughly 40 feet long to the stack. The building official has asked for a drainage fixture unit (dfu) check before issuing the permit. The existing stack vent is 3-inch and appears undersized for the new load. You must decide whether the existing branch and stack can handle the added load or whether upgrades are required.
1. Calculate the existing and proposed drainage fixture unit loads and determine whether the 4-inch branch is adequate.
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Existing load: 2 WCs x 2.5 = 5 FU; 3 lavatories x 1 = 3 FU; 2 floor drains x 2 = 4 FU. Total = 12 FU.
Proposed additions: mop sink 2 FU + one WC 2.5 FU = 4.5 FU. New total = 16.5 FU.
A 4-inch horizontal branch at 1/8-inch-per-foot slope is permitted to carry well over 100 dfu under typical code tables (a 4-inch building drain at 1/8 slope handles roughly 180 dfu). So the branch itself is not the limiting factor at 16.5 FU. The branch diameter is adequate; the concern shifts to the stack and vent capacity, not the branch.
2. The 3-inch soil stack now carries 16.5 FU. What is the governing code concern, and what would you check before signing off?
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A 3-inch stack is limited by code (commonly around 30-40 dfu for a 3-inch stack depending on the table and whether it is a stack or a stack vent). At 16.5 FU the stack is likely still within its dfu allowance, but the vent must be checked independently.
Key checks:
- Stack dfu allowance vs. 16.5 FU (verify against the adopted code table).
- Vent sizing: a 3-inch stack vent serving this load is generally adequate, but confirm the developed length and the fixture-unit-to-vent-diameter table.
- Whether the added WC requires a 3-inch or 4-inch closet bend and a minimum 2-inch vent.
- Trap arm lengths and vent distances for the new mop sink.
Conclusion: the branch is fine; the stack and vent must be verified against the adopted code table, and the mop sink needs its own vented trap arm. Do not approve on branch size alone.