Every sink, toilet, shower, and dishwasher in your home depends on pipes you will never see once the walls are closed up. That hidden network is the rough in plumbing — the phase of construction where supply lines, drain pipes, and vent stacks are installed inside walls and under floors before drywall, tile, and fixtures go in. Getting the rough-in right is critical because mistakes buried behind finished walls cost thousands to fix later. Whether you are building new or remodeling a bathroom or kitchen, understanding this phase helps you communicate with your plumber and avoid costly surprises.
What Happens During Rough-In
Rough-in plumbing includes three interconnected systems that must be planned and installed together.
Supply Lines (Water In)
Hot and cold water supply lines run from the main water line to each fixture location. Modern residential supply lines use either copper pipe ($3 to $8 per linear foot installed) or PEX tubing ($0.50 to $2 per linear foot installed). PEX has largely replaced copper in new construction because it is cheaper, faster to install, freeze-resistant, and requires fewer fittings. Supply lines are typically 1/2 inch in diameter for individual fixtures and 3/4 inch for the main trunk line.
Drain-Waste Lines (Water Out)
Drain lines carry wastewater from fixtures to the main sewer line or septic system by gravity. They are made of PVC (most common), ABS, or cast iron (older homes). Drain pipe sizing is dictated by code and varies by fixture: 1-1/2 inch for bathroom sinks, 2 inch for shower drains and bathtubs, 3 inch for toilets (with a 4-inch main sewer line). All drain pipes must slope downward at 1/4 inch per foot toward the main drain to ensure proper flow.
Vent Stack (Air Balance)
Every drain needs a vent pipe that connects to a vertical vent stack extending through the roof. Vents allow air into the drain system so water flows freely and sewer gases exhaust above the roofline rather than backing up into the house. Without proper venting, drains gurgle, flow slowly, and trap seals can be siphoned out — allowing sewer gas into living spaces. Vent pipes are typically 1-1/2 to 2 inches in diameter and connect to the drain line within a code-specified distance from the fixture trap.
Rough-In Dimensions for Common Fixtures
Plumbers work from standard rough-in dimensions that position pipes precisely for fixture installation later. Knowing these numbers helps you verify the work before walls are closed.
- Toilet — The drain (closet flange) is centered 12 inches from the finished wall (some older toilets use 10 or 14 inches). The cold water supply stub-out is typically 6 inches to the left of center, 6 inches above the floor.
- Bathroom sink — Drain stub-out centered at 18 to 21 inches above the floor. Hot and cold supply lines at 20 to 22 inches above the floor, spaced 8 inches apart.
- Bathtub — Drain centered on the tub waste opening. Supply lines roughed in at 28 inches above the floor for a standard tub/shower combo, with the showerhead stub-out at 72 to 80 inches.
- Kitchen sink — Drain centered at 16 to 18 inches above the floor. Supply lines at 20 to 22 inches, spaced to match the faucet spread.
Rough in Plumbing for a Bathroom Remodel
Bathroom remodels are the most common rough-in projects for homeowners. If you are keeping fixtures in their existing locations, the rough-in work is minimal — a plumber may only need to update old galvanized supply lines with PEX and replace corroded drain fittings. Moving fixtures to new locations is where costs escalate, because it means cutting into floors and walls to reroute drain lines, supply lines, and vents.
Moving a toilet more than a few feet is the most expensive relocation because the 3-inch drain pipe must maintain its slope to the main stack. In slab-on-grade homes, this requires cutting and jackhammering the concrete slab ($500 to $2,000 for the concrete work alone). In homes with crawl spaces or basements, rerouting under the floor is far simpler and cheaper.
Code Requirements and Permits
Rough-in plumbing must comply with local building codes, which are typically based on the International Plumbing Code (IPC) or Uniform Plumbing Code (UPC). Key code requirements include minimum pipe sizes for each fixture type, maximum distances between fixtures and vent connections, proper slope on all horizontal drain lines, the use of cleanout access fittings at required intervals, and fire-stopping where pipes penetrate walls and floors between stories.
A plumbing permit ($50 to $250 in most municipalities) is required for new rough-in work or significant modifications. The permit triggers an inspection — typically performed after pipes are installed but before walls are closed. The inspector checks pipe sizing, slope, venting, fitting types, and hanger spacing. Passing this inspection is mandatory before drywall can begin.
Cost of Rough-In Plumbing
Rough-in costs depend heavily on the scope of work and local labor rates.
- Single bathroom rough-in (new construction) — $1,500 to $4,000 for supply, drain, and vent lines to a toilet, sink, and tub/shower.
- Kitchen rough-in — $500 to $1,500 for sink supply and drain, plus dishwasher and refrigerator water line connections.
- Full house rough-in (new construction) — $8,000 to $15,000 for a typical 3-bed, 2-bath home including all fixtures, hose bibs, and water heater connections.
- Bathroom remodel with fixture relocation — $2,000 to $7,000 depending on how far fixtures move and floor/wall construction.
Labor accounts for 60 to 70 percent of rough-in costs. Materials (PEX, PVC, fittings, hangers) are relatively inexpensive compared to the skilled labor required to install them correctly.
Common Rough-In Mistakes to Avoid
Even experienced DIYers can make errors during rough in plumbing that cause problems months or years later. Drain pipes with insufficient slope create slow-draining fixtures and recurring clogs. Missing or undersized vents cause gurgling drains and siphoned traps. Supply lines run through exterior walls without insulation freeze in cold climates. Failing to pressure-test supply lines before closing walls means a pinhole leak can soak insulation and drywall for weeks before detection. And skipping the permit and inspection might save $200 upfront but can create serious problems at resale when a buyer’s inspector flags unpermitted plumbing work.
Should You DIY Rough-In Plumbing?
Running PEX supply lines with push-fit fittings is within reach for a handy homeowner. The tools are simple and affordable, and mistakes are easy to fix before walls close. Drain-waste-vent work is more challenging because pipe sizing, slope, and vent connections must meet code precisely. Most jurisdictions allow homeowners to pull their own plumbing permits and do the work themselves, but the installation must still pass inspection. If you are comfortable reading a plumbing code book and working with PVC solvent cement, a bathroom rough-in is a manageable project. If you are unsure about any aspect, hiring a licensed plumber for the rough-in protects you from hidden mistakes that are expensive to correct once the walls go up.
