Reading our P2.1 riser for a 12-unit slab, we’ve got a 2" shower trap arm at 1/4" per foot before the vent takeoff — what’s the IPC max developed length there, straight shot on the blueprint? Settling a code-compliance bet in the trailer.
I keep a 2" drop stick on slab jobs — if the trap weir to the vent takeoff drops past that when dry-fit at 1/4" per foot, I move the vent upfield; one stray 22.5° in the pour can eat your margin. Straight shot on your P2.1 you’re at about 8’ by the math, but I’d aim 7’–7’6" to cover field slope and fitting depth — @OP any jogs around beams on that 12‑unit?
Straight run on that P2.1 riser? You’re effectively capped around 8’ at 1/4-in pitch (2-in fall limit), but on a slab I spec the vent takeoff within 7’-6" to cover a hub or tiny roll; measure the centerline, not the line-of-sight. @peter51’s drop stick is smart — I use a laser and check the trap outlet to the vent mark before we pour; any fitting in there and you’ve gotta pull it closer.
Call it 8 ft on a 2-in arm at 1/4-in/ft, but lay it out weir-to-vent takeoff centerline, not trap center; on slabs I mark about 7’-8" to leave room for hubs and keep the drop under 2 in. If you need a tad more, bumping the trap and arm to 3 in under IPC buys more run (one‑diameter fall rule), but clear it with your inspector. Ref: IPC 1002.4 https://up.codes/viewer/ipc-2021/chapter/10/traps-interceptors-and-separators#1002.4 — nobody wants to lose a bet to gravity.
At 1/4-in per foot, you’re capped at 8’-0" under IPC; on slab I paint 7’-7" and pull a string so a coupler or a sneaky 22.5 doesn’t push you past two inches of fall. > buys more run (one‑diameter fall rule), but clear it with your inspector. Ref: IPC 1002.4 https://up.codes/viewer/ipc-2021/chapter/10/traps-interceptors-and-separators#1002.4 — nobody wants to Agree — that bump to the next arm size has bailed me out on long shower runs. You got any offsets before the stack?
8 ft is the cap under IPC; a slightly out-of-level trap steals fall, so I laser the slope and shim the trap dead level, then keep the vent connection a touch short so the fitting centerline stays under 2 in. > bump to the next arm size has bailed me out on long shower runs. You got any offsets before the I’ve seen that work, but some inspectors won’t allow the arm larger than the trap — any offsets between the trap and the vent, @dmorales91?
But straight shot on slab, you’re limited to 8’-0" on a 2-in arm because ‘one pipe diameter of fall max’ after the trap; measure from the trap weir to the vent fitting’s centerline. I mark 7’-6" on the P2.1 and carry a 2-in spacer as a quick drop gauge so a coupler or a hair of dish in the pipe doesn’t push you over; see IPC 1002.3 here: https://codes.iccsafe.org/content/IPC2021P1/chapter-10-traps-interceptors-and-separators. You on 2018 or 2021?