Carport Conversion: Vent Sizing and a 14-Foot Trap Arm

Count the load the stack actually carries, not just the branch you are adding. Two bathroom groups at roughly 6 fixture units each (water closet 3, lavatory 1, tub/shower 2) is 12 FU; the laundry sink at 2 FU plus the washer standpipe at 2 FU adds 4 FU, so the single stack would serve about 16 FU. In the UPC stack/vent sizing tables a 1-1/2 in. stack tops out in the single digits (on the order of 8 FU), so 16 FU is roughly double its limit: the stack needs to be at least 2 in., and it should be checked as a common vent for the two bathroom groups as well. Verify against the edition your county adopts rather than the sketch. Field checks: which fixtures actually discharge into that stack, the number of offsets and developed length, and the tie-in elevation. A vent connected below the flood-level rim of any fixture it serves is a drain, not a vent, so the 4 ft above the highest rim is fine only if nothing on that stack sits higher. If the stack cannot be upsized inside the wall, the fallback is a separate 1-1/2 in. vent to the roof.

A trap arm is protected only until it has dropped one pipe diameter of fall; past that point the arm is draining rather than venting, and the trap seal goes by siphonage or back pressure. At 1/4 in. per foot, 14 ft of arm falls 3.5 in., well over the 1-1/2 in. allowed, and the trap-arm length table caps a 1-1/2 in. arm at about 6 ft developed length regardless of fall. Corrections: first, relocate the vent connection to within the table distance of the trap; going to a 2 in. arm buys about 8 ft, which still does not reach 14 ft, so one vent alone will not solve it. Second, run an individual 1-1/2 in. vent from the standpipe trap up through the wall and terminate it through the roof or a side wall at the clearances the code sets. Third, an approved air-admittance valve on the laundry branch, if the county's amendment permits AAVs, installed accessible and above the flood-level rim per the listing; or wet-vent the laundry to the lavatory only if the wet-vent rules and the branch layout genuinely support it.

Capacity is not the concern: a 3 in. drain at 1/8 in. per foot carries well over 40 fixture units, and 4 FU is noise. The concerns are condition and geometry. First, does the 1976 slab drain actually still have 1/8 in. per foot? A settled or bellied run under a Hilo slab may be flat or back-pitched, so camera the line and shoot elevations with a laser before tying in; if it is holding water, the new load is not the problem to solve, the drain is. Second, the fitting: a wye or combination, drainage pattern, installed in the direction of flow, with the branch at the required slope to the wye. Third, cleanouts: check the developed length and direction changes from the new wye to the nearest existing cleanout, because adding a branch can trigger a required cleanout. Fourth, material and support: cast iron to ABS transitions need shielded couplings, and the wye must be supported, not hanging off the concrete patch. Any of these findings can turn a small patch into a bigger scope, so price it that way.