Manuall Trade Library Run your shop on Manuall

How to Rough In a Lavatory Drain, Trap Arm and Supplies

How-to Guide ServiceDelivery Updated Oct 28, 2026 ยท Plumbing

Why this matters

A lavatory rough-in gets judged twice: once by the inspector with the wall open, and once by the finish plumber who discovers the drain stub sits behind the vanity's back rail or the stops collide with the trap. Both failures trace to the same habit, roughing to "the usual 18 and 22" without the fixture's spec sheet in hand. The other failure is invisible until the house is lived in: a trap arm that falls too far before it meets its vent siphons the trap, and the customer calls about a sewer smell from a brand new vanity.

This card covers the drain stub height, the trap arm run to a sanitary tee, the vent above it, and the hot and cold stubs. Trap arm length limits by trap size belong to Trap Arm Lengths by Trap Size Under IPC and UPC; this card uses them, it does not re-derive them.

Before you start

Have the fixture and its paper. The spec sheet for the lavatory and the faucet, and the vanity or pedestal cut sheet, decide the drain outlet height, the faucet supply centers and where the cabinet back or pedestal column will hide or expose the stubs. A vessel sink, a wall-hung lav and an undermount in a 36 in counter put the drain in three different places. If the customer has not bought the fixture, rough to the conventional heights in Standard Rough-In Heights and Clearances for a Bathroom and write on the work order that the rough-in is generic.

Know the wall. Find out whether the wall is bearing and whether it is 2x4 or 2x6 before you plan a horizontal arm through studs (see the worked example for why). Before cutting into an existing finished wall, shut the breaker for any circuit in that cavity and confirm absence of voltage at the nearest receptacle with a tester proven on a known live source before and after, per NFPA 70E Article 120 (current edition). In a house built before 1978, treat painted plaster and trim as lead-containing until tested, and keep dust controlled with plastic, a HEPA vacuum and wet wiping rather than sweeping; the EPA's RRP rule (40 CFR 745 Subpart E) governs the work where it applies, with a certified renovator on the job.

Materials. 1-1/2 in DWV for the arm and the tee (most lavs drain 1-1/4 in at the tailpiece, and the arm is commonly run a size up; the sibling card covers why that buys length), a sanitary tee for the vent connection, drop-ear elbows for the supplies, wood blocking, steel shield plates, test caps.

Steps

1. Mark the fixture centerline and the drain stub height on the framing. Snap or plumb a line for the fixture center, then mark the drain stub center at the height the spec sheet gives for the stub, which for a counter-mounted lav in a standard vanity usually falls between 16 and 20 in above finished floor. Take the finished floor height into account: a tile floor over a backer adds most of an inch you must measure from. Why: the stub has to sit below the tailpiece end so the P-trap fits between them, and high enough that the vertical drop from fixture outlet to trap weir stays inside the code limit (24 in under the IPC, in IPC Chapter 10 of the 2021 edition, as adopted; check the current edition). Skip it and the finish plumber either adds a tailpiece extension that puts the trap low enough to strike the cabinet floor, or cannot reach the stub at all.

2. Lay out the trap arm and its fall. The trap arm is the pipe from the trap weir to the vent connection. Set the sanitary tee's branch so the arm falls 1/4 in per foot toward it, never back toward the trap. Two rules decide where the tee can go. The arm's developed length must stay within the adopted code's table for its size. And the total fall of the arm must not exceed one pipe diameter, so the vent opening at the tee never ends up below the trap weir (IPC 909.2 in the 2021 edition, as adopted; the UPC carries the same rule in substance). Why it matters: once the vent connection drops below the weir, the arm runs full like a siphon tube on discharge, pulls the trap seal down after it, and the trap stops being a seal.

                           vent plumb to 6 in min
                           above lav flood rim
                                 |
  lav bowl                       |
  \______/                       |
     |  tailpiece                |
     |                           |
    _|_  trap arm, 31 in         |
   |   |weir------------------->-+ sanitary tee,
   |___|  falls 1/4 in/ft        | branch center
   P-trap stub center 18 in AFF  | 17-1/2 in AFF
                                 |
  H stub 22 in AFF, 4 in left    | 1-1/2 in drain
  C stub 22 in AFF, 4 in right   v continues down
  of lav centerline

3. Set the tee on the vent riser with the branch facing the fixture. A sanitary tee, not a wye or a straight tee, is the fitting for a horizontal arm entering a vertical drain; the curve turns the flow down without throwing it across the stack. Which fitting goes on each change of direction is covered in DWV Fittings for Each Change of Direction. The vent above the tee has to rise vertically until it is at least 6 in above the lav's flood level rim before it turns horizontal (IPC 905.4 in the 2021 edition, as adopted), because a vent that goes flat below the rim becomes a drain the day the lav overflows, and fills with debris it can never clear.

4. Bore and run the arm. Drill on the layout line with the fall built in, cut and dry-fit the run end to end, check the fall with a level at each joint, then prime and cement. Hole saws and self-feed bits in a right-angle drill bind hard when they break through; brace the drill handle against a stud and keep your wrist out of the handle's arc. Solvent cement and primer give off vapor: open the room to outside air, keep ignition sources away, and cement with the can lidded between joints. Steps for a PVC joint are in How to Cut, Prime and Glue a PVC DWV Joint. Fit a steel shield plate wherever the pipe passes within 1-1/4 in of the stud face (IRC P2603.2.1 in the 2021 edition, as adopted), because drywall screws go through PVC without any warning.

5. Stub the supplies with the stops in mind. Hot on the left as you face the fixture, cold on the right: that is the code's flow rule (IPC 607.4 in the 2021 edition, as adopted), and a reversed rough-in becomes a reversed faucet handle the customer lives with. Use drop-ear elbows screwed to blocking set flush with the stud face so the stub cannot push back into the wall when someone tightens a stop. Set them at the spec sheet's height, typically about 4 in either side of center and a few inches above the drain stub, and offset them sideways from the trap so the stops and supply risers do not land on top of the P-trap. Leave enough straight, round, undamaged pipe past the finished wall for the stop you will use: a compression stop needs clean pipe for its ferrule, and a press or push stop has its own marked insertion depth.

6. Cap, test and label. Cap the drain stub with a test cap and test the DWV as described in How to Water or Air Test a DWV Rough-In. Cap the supply stubs and pressure-test the water lines per the adopted code and the joining method's instructions. Write the stub heights and the fixture they were roughed for on the work order and on the framing beside the stubs in marker. If you opened a breaker for the wall cavity, close it once no tool is in the cavity and confirm the receptacle you tested is live again.

Worked example: a lav moved 24 in sideways in a bathroom remodel

The customer wants the vanity centered on a new wall section. The existing 1-1/2 in vent riser stays where it is, so the new lav centerline lands 24 in to the right of it. The wall is a non-bearing 2x4 partition. The vanity cut sheet calls for the drain stub center at 18 in above finished floor and supplies at 22 in, 8 in apart.

Developed length. From the trap weir to the wall face is about 5 in through the trap outlet and stub. The stub turns 90 degrees inside the cavity, about 2 in more through the fitting, then runs 24 in horizontally to the tee. Developed length: 5 plus 2 plus 24 is 31 in. Check against the tables: under the UPC as adopted, a 1-1/4 in arm is limited to 30 in, so 31 in fails at 1-1/4 in, and the 1-1/2 in arm's 42 in limit passes with 11 in to spare. Under the IPC, 31 in sits well inside both sizes. The arm is run in 1-1/2 in pipe either way, and the code jurisdiction did not change the build.

Fall. 31 in at 1/4 in per foot: 31 divided by 12 is 2.58 ft, times 1/4 in is 0.65 in, about 5/8 in. The horizontal run itself falls 24 in times 1/48 in per in, which is 1/2 in, so the tee's branch center sits at 18 minus 1/2, or 17-1/2 in above finished floor. The rest of the 5/8 in falls inside the trap outlet and stub fitting.

The weir check. One pipe diameter for this arm is 1-1/2 in, and the arm falls 5/8 in, so it passes with 7/8 in of margin. Recomputed from the other side: the crown of the pipe at the tee sits about one diameter above its invert, so the crown is at the weir minus 5/8 plus 1-1/2, which is 7/8 in above the weir. The vent opening stays above the trap seal, which is exactly what the one-diameter rule protects.

The framing. 1-1/2 in PVC has an outside diameter of 1.90 in and needs about a 2 in hole. In a 2x4 non-bearing stud, 3.5 in wide, the IRC limit for a bored hole in a stud is 60 percent of its width (IRC R602.6 in the 2021 edition, as adopted), which is 2.1 in. A 2 in hole passes with 1/10 in to spare (the 2-1/8 in bit many techs reach for would not), and the hole has to be centered to keep the edge distance the section requires. Had this been a bearing or exterior 2x4 wall, any hole over 40 percent (1.4 in) is allowed only in a doubled stud, with no more than two successive doubled studs bored, so a run across several studs is out; the answer there is a furred-out or 2x6 wall (40 percent of 5.5 in is 2.2 in), or relocating the vent, decided before the drywall comes off rather than after a framing inspector flags it.

Supplies. Hot drop-ear at 22 in, 4 in left of the lav centerline; cold at 22 in, 4 in right. The stub at 18 in is centered, so each stop sits 4 in sideways and 4 in up from the drain stub center, which leaves the trap's J-bend clear of both stops.

Verify you got it right before the wall closes

  • Drain stub height measured from the finished floor line, not the subfloor: 18 in to center on this job, matching the cut sheet.
  • Fall on every horizontal piece, read on a level, toward the tee; no flat or back-pitched joint. Total fall over the arm 5/8 in, under the 1-1/2 in one-diameter limit.
  • Vent plumb from the tee to at least 6 in above the lav flood rim before any horizontal offset.
  • Supply stubs hot left, cold right, anchored to blocking so they do not move when pushed, with clean pipe past the finished wall line.
  • Shield plates on every bored stud the pipe passes within 1-1/4 in of the face.
  • DWV test and supply pressure test passed, and the result written on the work order with the date.

If the fixture changes after rough-in, recheck step 1 against the new spec sheet before the drywall goes up; a vessel sink swapped in for an undermount can move the drain stub up several inches.

References

  • IPC 2021, as adopted: Section 909.2 (vent connection not below the weir), 905.4 (vent rise above the flood level rim), 607.4 (hot on the left), and Chapter 10 trap requirements; UPC 2021, as adopted, for the trap arm table. Check the current edition.
  • IRC 2021, as adopted: R602.6 (boring and notching studs) and P2603.2.1 (protection of pipes near framing faces).
  • NFPA 70E, Article 120, current edition; EPA 40 CFR 745 Subpart E where pre-1978 paint is disturbed.
  • See related: Trap Arm Lengths by Trap Size Under IPC and UPC
  • See related: Standard Rough-In Heights and Clearances for a Bathroom

Related articles