Bathroom Plumbing Rough-In: Dimensions, Pipe Sizes, and Layout

A bathroom plumbing rough-in sets every drain, vent, and supply line in place before the walls and floor close up. The standard numbers anchor the whole job: a 12-inch toilet flange, a 3-inch closet drain, 1.5-inch lavatory and tub traps, a 2-inch shower drain, and 1/2-inch supply stubs at fixed heights. Hit these and the finish trim drops in clean. Related: bathroom remodel plumbing.

What is a bathroom plumbing rough-in?

The rough-in is the stage where you run and secure all pipe inside the framing, then cap and pressure-test it, without connecting any fixtures. Drain, waste, and vent (DWV) lines go first, then the 1/2-inch supply lines. Finish plumbing comes later, after drywall and tile, when you set the toilet, sink, and valves.

Every dimension is measured to a centerline or to the finished floor and wall, not to bare studs or subfloor. Add the thickness of drywall, backer board, and tile before you commit a stub location. A drain that looks centered on the framing can end up an inch off once the wall is finished.

If you are new to reading DWV layouts, our guide to residential plumbing diagrams shows how supply, drain, and vent lines stack together across a house. This page zooms in on the bathroom group specifically.

What are the standard bathroom rough-in dimensions?

The table below gives the working numbers for a standard full bath: drain size, trap size, vent size, and the rough-in height or offset for each fixture. These reflect common International Plumbing Code (IPC) and Uniform Plumbing Code (UPC) minimums, but fixture spec sheets and local code override them, so verify before you cut.

AFF means “above finished floor.” A 1.6-gallon toilet counts as 3 drainage fixture units under IPC Table 709.1, a bathtub or shower counts as 2, and a lavatory counts as 1, which is why the toilet gets the 3-inch line and the sink gets 1.5 inches.

What pipe sizes does a bathroom drain and vent need?

Bathroom pipe sizing follows the fixture load. The toilet needs a 3-inch drain and a 2-inch vent. The lavatory, tub, and standard shower drain through 1.5 to 2-inch pipe with matching traps. Every fixture takes a 1/2-inch supply. Vents can shrink to half the drain size they serve but never below 1.5 inches.

Drain and trap size are not the same as vent size. A shower runs a 2-inch drain and 2-inch trap because standing water drains slower than a sink, while its vent can drop to 1.5 inches. The same P-trap and vent logic that governs a lavatory also covers kitchen sink plumbing, though the kitchen drain is usually 1.5 or 2 inches for the disposal.

Supply pipe is commonly 1/2-inch PEX or copper at each fixture, branching off 3/4-inch trunk lines. PEX has taken over most new bathroom rough-ins because it flexes through studs with fewer fittings and resists freeze bursts better than rigid copper.

Sink numbers shift with the mounting style. A standard vanity hides the trap in the cabinet, so the drain roughs in at 18 to 20 inches. A wall-mount sink exposes the trap, so the drain moves up to 30 to 36 inches and the supplies to 28 to 32 inches, all set to the fixture spec sheet. Pedestal sinks fall between the two and demand blocking in the wall for the mounting bracket.

How far apart should bathroom fixtures be spaced?

Fixture spacing protects clearance and passes inspection. Code sets a minimum, and comfort usually calls for more. The toilet drives most of the layout because it needs the widest clear zone on both sides and in front. Plan these gaps to finished surfaces, then confirm against your local code, which may exceed the IPC minimums below.

• Toilet: 15 in minimum from centerline to any side wall, tub, or vanity (IPC), 18 in preferred
• Toilet: 21 in minimum clear space in front of the bowl (IPC), 24 in or more preferred
• Two fixtures side by side: 4 in minimum between edges, more for elbow room
• Shower: 30 in by 30 in minimum interior (IPC), 36 in by 36 in recommended
• Vanity: 30 in clear width and a 21 in front approach for the user

What order do you rough in bathroom plumbing?

Rough-in runs largest pipe to smallest, drains before supplies. Locking in the DWV path first means the sloped waste lines and vents claim their space in the framing, then the flexible half-inch supply lines route around them. Working in this order avoids tearing out a supply run to fit a drain that had nowhere else to go.

1. Mark every fixture centerline on the subfloor and studs, measured to finished surfaces.
2. Tie into or set the main soil stack and locate the 3-inch toilet drain first.
3. Run branch drains to tub, shower, and sink at 1/4-inch-per-foot slope back to the stack.
4. Install each trap and trap arm, keeping the arm within its maximum length to the vent.
5. Run vents up from each fixture and tie them above the flood-level rim before they join the vent stack.
6. Route the 1/2-inch hot and cold supply lines to each stub-out and secure them.
7. Cap all stubs, then air-test or water-test the DWV and pressure-test the supply before inspection.

The toilet drain and flange set the tone for the whole group, so get that closet bend and 12-inch dimension exact before anything else locks in around it. Our walkthrough on setting the toilet covers the finish side once the rough-in passes.

What does code require for slope, venting, and trap arms?

Code controls three things that make or break an inspection: drain slope, vent connection height, and how far a trap can sit from its vent. Get slope and trap-arm length wrong and the trap siphons dry, letting sewer gas into the room. These IPC figures are typical, but the UPC and local amendments differ, so confirm with your inspector.

Drains 2.5 inches and smaller pitch at 1/4 inch per foot. Pipe 3 inches and larger can drop to 1/8 inch per foot. Support horizontal drain pipe every 4 feet for small lines and vertical pipe at each floor. Vents must rise above the fixture’s flood-level rim before they run horizontal.

The maximum developed length from a trap to its vent, called the trap arm, grows with pipe size. Exceed it and the fixture needs a closer vent or a rework.

What are the most common bathroom rough-in mistakes?

Most failed rough-in inspections trace back to a handful of repeat errors, and nearly all of them are cheaper to fix before the walls close than after. The pattern is almost always a measurement taken to framing instead of finished surfaces, a vent set too low, or a trap arm run too long. Watch these before you call for inspection.

• Measuring the 12-inch toilet dimension to the stud instead of the finished wall, which pushes the bowl a full inch off.
• Running a trap arm past its maximum length to the vent, so the trap siphons dry and vents sewer gas.
• Tying a vent in below the fixture’s flood-level rim, which fails inspection and lets waste back into the vent.
• Under-sloping a branch drain below 1/4 inch per foot, so solids settle and the line clogs early.
• Forgetting in-wall blocking for a wall-mount sink, grab bar, or valve, leaving nothing solid to fasten to.
• Skipping the pressure test, then finding a leak after the tile is set.

Do you need a permit to rough in a bathroom?

Most jurisdictions require a plumbing permit any time you add, move, or replace drain, vent, or supply pipe, and the rough-in must pass a pressure-tested inspection before you close the walls. Straight fixture swaps on existing connections often skip the permit. Fees commonly run 50 to 500 dollars, and the rules vary widely by state and local code.

The rough-in inspection is where an inspector checks pipe sizes, slope, venting, trap arms, and your air or water test on the open walls. Passing it is the green light to insulate and drywall. Skipping the permit can surface later as a failed home-sale inspection or a denied insurance claim. Check whether your work needs a permit before you open a wall, not after.

Frequently asked questions

What is the standard rough-in for a bathroom toilet?

The standard toilet rough-in is 12 inches from the finished back wall to the center of the closet flange, which sits on a 3-inch drain. Older homes sometimes measure 10 or 14 inches, so confirm before you buy. The cold supply stub roughs in about 8 inches above the floor and 6 inches left of the toilet centerline.

What size pipe is used for bathroom plumbing?

A full bath uses a 3-inch drain and 2-inch vent for the toilet, a 1.5-inch drain and trap for the lavatory sink, a 1.5-inch tub drain, and a 2-inch shower drain and trap. Every fixture supply line is 1/2-inch. Vents cannot drop below 1.5 inches or half the drain they serve, whichever is larger.

What order do you rough in bathroom plumbing?

Run the largest drain, waste, and vent lines first, starting at the main stack and working out to each fixture at 1/4-inch-per-foot slope. Set vents next and tie them above the flood rim. Then run the 1/2-inch hot and cold supply lines. Cap, then pressure-test both systems before the inspector signs off and walls close.

How high should bathroom sink supply lines be?

Rough in vanity sink supply stubs about 20 to 22 inches above the finished floor, spaced 4 inches apart center to center and 8 inches off the drain centerline. Set the drain stub at 18 to 20 inches. Wall-mount and pedestal sinks change these numbers, so match the fixture spec sheet before you cut the wall.

What is the difference between rough-in and finish plumbing?

Rough-in is the in-wall stage: running, securing, capping, and testing all drain, vent, and supply pipe inside open framing, with no fixtures attached. Finish plumbing happens after drywall and tile, when you set the toilet, mount the sink, install valves and trim, and make the final connections. A rough-in inspection separates the two.

Do you need a permit for a bathroom rough-in?

In most jurisdictions, yes. Adding, moving, or replacing drain, vent, or supply pipe almost always requires a plumbing permit and a rough-in inspection before walls close. Simple fixture swaps that reuse existing connections often do not. Permit fees commonly run 50 to 500 dollars, and rules vary by state and local code, so check with your building department first.

Last reviewed: September 2026.