Bathroom rough-in is the plumbing phase that happens before walls close — when drain, waste, and vent pipes are installed through the framing and supply lines are run to fixture locations. Getting it right matters more than almost any other rough-in phase, because mistakes are expensive and invasive to fix once drywall is up. Here’s how it works and what to expect.
What Bathroom Rough-In Includes
A complete bathroom rough-in covers three systems:
- Drain, waste, and vent (DWV) pipes — the large-diameter pipes that carry waste to the septic or sewer connection and vent sewer gases out through the roof
- Supply lines — hot and cold water supply pipes from the water heater or pressure tank to each fixture location
- Stub-outs — the short pipe ends that extend through the wall or floor at each fixture location, capped until fixtures are installed at the finish stage
The Rough-In Sequence
- DWV layout first. Drain pipes are large (3 to 4 inches for the toilet drain, 1.5 to 2 inches for sink and shower), difficult to route around other systems, and must maintain a continuous downhill slope to drain properly.
- Vent stack. Every drain in a bathroom needs to be vented to prevent sewer gas from entering the living space. The vent stack runs vertically through the wall framing and exits through the roof.
- Supply lines. Hot and cold supply pipes are run after DWV because they’re smaller and more flexible to route around the existing drain and vent work.
Critical Dimensions: Why Rough-In Measurements Matter
Fixtures have fixed rough-in dimensions that must be hit precisely during the rough-in phase. Key dimensions to know:
- Toilet rough-in: The center of the toilet drain is typically 12 inches from the finished wall behind the toilet
- Sink drain height: Typically 18 to 20 inches above the finished floor
- Shower drain: Centered in the shower floor, location set by your shower pan or custom tile design
- Supply stub-out height for sink: Hot and cold supply lines typically stub out at 21 to 24 inches above the finished floor, 8 inches apart
Your architectural plan set specifies fixture locations — use these dimensions to set your rough-in correctly. Designs like our 22’ x 28’ Lean Cottage and 24’ x 34’ Large Adirondack Cabin include complete plumbing fixture location plans as part of the full drawing set, so your plumber has a clear reference from day one.
Drain Pipe Slope Requirements
Drain pipes must slope continuously downhill from fixture to main drain. The standard requirement is 1/4 inch of drop per foot of horizontal run. Too little slope and waste doesn’t drain; too much slope and liquid outruns solids, causing clogs.
Venting: The Most Commonly Misunderstood Part
Every fixture drain requires venting — an air supply that allows water to flow freely without creating a vacuum. Options include:
- Traditional wet venting: The vent stack runs vertically through the wall framing and exits the roof. This is the standard approach and what most plans assume.
- Air admittance valves (AAVs): A mechanical device that allows venting without a roof penetration. Permitted in many jurisdictions for sink and fixture venting, not always for toilet drains. Check local code before specifying.
What Bathroom Rough-In Costs
Plumbing rough-in for a single bathroom typically runs $1,500 to $4,000 depending on complexity, fixture count, and how far the bathroom is from the main drain stack. A simple one-fixture half bath costs less; a full bath with tub, shower, toilet, and double sink costs more.
Permits and Inspections
Plumbing rough-in requires a permit in virtually every US jurisdiction and is subject to a rough-in inspection before walls can be closed. The inspection typically includes a pressure test of the supply lines and a visual check of drain slope, venting, and stub-out locations. Don’t close walls before the inspection is passed.
Bottom Line
Bathroom rough-in is a sequence-dependent, code-governed phase that sets the physical location of every fixture in your cabin permanently. Work from your plan’s fixture location drawings, verify rough-in dimensions against your specific fixtures before cutting, and schedule your inspection promptly.