Garage Construction Project

Loft DWV / Plumbing Rough-In Plan

5 min read

Loft DWV / Plumbing Rough-In Plan

How to run drain-waste-vent (DWV) for the loft bathroom (and future-apartment kitchen/laundry) given that the loft floor is built from pre-engineered attic trusses whose bottom chords cannot be drilled (Letherer Job #123907, 24” OC, 40 PSF; IRC R502.11.3 — no cutting/notching/drilling truss members without the engineer’s approval). Companion to the electrical truss-routing decision that established the same constraint for wiring.

The core strategy

The truss rule does not force the plumbing down into the garage wholesale. Two moves cover almost everything:

  1. Land every vertical drop in a truss bay, not through a chord. At 24” OC there’s ~22.5” of clear bay — ample for a 3” (or 4”) closet bend, a 2” shower trap, or any branch. Vertical penetrations between chords need no drilling. Nudge fixture locations a few inches as needed to center each drop in a bay.
  2. Do all horizontal collection below the trusses, in a dropped soffit, sloped to the mechanical room. Lateral travel (perpendicular to trusses) can’t happen in the floor without crossing a chord every 24”. Below the trusses there’s unlimited slope/routing. The 10’ garage ceiling easily absorbs an 8–10” soffit.

So the only things going down are the lateral collector + the fixture drops. Everything vertical or with-the-grain (vents, short trap arms) stays up.

What stays in the loft vs. drops through the ceiling

Stays in loft (walls/partitions — studs drill freely)Must drop through a bay
All vents (rise to the through-roof vent stack)Toilet — floor-level 3” closet bend
Short horizontal trap arms of wall-connected fixtures (lav, kitchen sink, laundry standpipe) — enter the wall 16–24” up, slope to a drop pointShower — P-trap sits below the pan

Wall-connected fixtures still ultimately drop through a bay into the collector; keeping them up just consolidates penetrations and keeps slope easy.

Minimize penetrations — stack on a wet wall

Put the bathroom group (toilet + shower + lav) on a common wet wall and drop a single 3” stack in one bay, wet-vented per code, so the whole bathroom is ~one penetration instead of three. Group kitchen + laundry the same way where layout allows. Fewer holes through the fire-rated garage ceiling = less firestopping and a cleaner soffit.

Fixture specifics

  • Toilet: 3” closet bend dropping into a bay → soffit collector. Center the flange in a bay with room for the bend’s sweep; verify bay clearance before the subfloor goes down.
  • Shower: the 2” P-trap depth is the issue, not drilling. Either set the shower on a raised base/curb so the trap lives in the floor buildup above the subfloor, or drop the trap into the bay below. Locate the drain over a bay either way.
  • Lav / kitchen sink / laundry: traps in cabinet/wall, drain into the wall, run to the nearest stack/drop.

Routing & code constraints

  • Soffit route: run the collector along a perimeter/back wall — not across the lift bay or where a raised vehicle or the garage-door tracks need clearance. Map it against the 2-post lift envelope before committing.
  • Keep it in conditioned space: DWV in the garage ceiling is fine (garage is heated — radiant + mini-split), but don’t tuck the collector or traps into an uninsulated chase against an exterior wall. Michigan winters — keep everything inside the thermal envelope.
  • Fire separation (IRC R302.6): the garage ceiling is 5/8” Type X fire separation. Every drop that penetrates the membrane needs a listed firestop / maintained rating. Building the collector soffit below the drywall keeps the horizontal pipe out of the membrane — only the fixture drops penetrate it (another reason to minimize/group drops).
  • Slope & sizing: 3” building drain (toilet sets the minimum), 1/4”/ft preferred (1/8”/ft allowed on 3”+). A ~30’ run to the mech room is only ~4–8” of fall. Confirm fixture-unit load if the future apartment adds kitchen + laundry to the same drain.
  • Venting: vents rise in loft walls to the through-roof stack (a sewer vent stack was already routed through the roof, Jan 2026). Maintain trap-arm-to-vent distances; check whether Michigan code permits AAVs before relying on any.
  • Connection to the sewer: the collector drops in a wall to the mechanical-room 3” PVC sewer stub-out (3” PVC to the house stack per the utility plan). Confirm invert depths so the soffit slope lands at the right elevation.

Open items

  • Final fixture layout (wet-wall grouping; confirm each drop lands in a bay).
  • Soffit route vs. the lift envelope, garage-door tracks, and conditioned-space boundary.
  • Confirm mech-room sewer invert elevation against the soffit slope budget.
  • Verify AAV acceptability vs. all-vented-to-roof under local code.
  • Coordinate firestop details for each ceiling penetration with the drywall fire-separation plan.

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