Residential Rough-In Plumbing

"Rough-in" is the plumbing done in the walls, floors, and ceilings before the drywall goes up. It is the skeleton of every water, waste, and vent line in the house. Get it right and the finish work (fixtures, faucets, trim) drops in quickly weeks later. Get it wrong and the drywallers, tile crew, and cabinet installers are all working around a mistake that will cost ten times as much to fix once the walls close in. This guide walks a new apprentice through the stages, the design rules, the dimensions, the tests, and the inspector's checklist on a typical single-family residential rough-in.

The Two Stages

Every plumbing job breaks into two phases:

• Rough-in - Water lines (supply) and drain/waste/vent lines inside the walls, floors, and ceilings. Ends with a pressure test and an inspector's sign-off.
• Finish (or "trim-out") - All visible fixtures: faucets, toilets, tubs, showers, hose bibs, dishwashers, garbage disposals. Happens after drywall, tile, flooring, and cabinets.

The rough-in is usually done in two passes on a new build:

• Underground rough - Sewer main from the house connection point out to the septic/city tap, plus any drains in slab-on-grade floors. Backfilled and compacted before the concrete slab pours.
• Top-out rough - Everything above the slab / first floor: supply, waste, vent, tub/shower drains, stub-outs. Done in parallel with framing.

On new construction, the typical sequence of a single project:

1. Sewer tie-in and underground drains (before slab)
2. Slab placement (concrete crew)
3. Framing complete
4. Top-out DWV rough (waste and vent stacks in walls and roof)
5. Water service and manifold/trunk
6. Pressure test and inspection
7. Drywall, tile, cabinets, finish flooring
8. Fixture finish / trim-out
9. Final inspection

DWV Design - The Big Picture

The DWV system moves waste out of the house and keeps sewer gas from coming back in. Two networks, wrapped together:

• Drain (waste) lines - Everything downstream of a fixture trap, sloped to fall by gravity. Connect to the sewer main.
• Vent lines - Air connections that let sewer gas escape up through the roof and let atmospheric pressure equalize behind flushing fixtures so the traps do not siphon dry.

Key Terms

• Trap - The U-bend under every fixture. Holds about 2 inches of water to block sewer gas.
• Trap arm - Horizontal drain pipe from the trap to the vent takeoff. Length is limited by code (usually 3.5 feet for 1.5 inch, 5 feet for 2 inch, etc.).
• Vent - Dry pipe that connects the drain line to outside atmosphere.
• Stack - Vertical pipe that drains multiple floors. A "waste stack" carries waste; a "vent stack" carries air; a "soil stack" carries toilet discharge. A single stack often does both waste and vent duty.
• Cleanout - A threaded plug in the drain line for snaking blockages. Required at the base of every stack and at 90-degree direction changes.

Vent Rules - Short Version

Every fixture must be vented. The vent can be:

• Individually vented - A dedicated vent pipe off the trap arm, going up through the roof.
• Wet vented - The drain of an upstream fixture doubles as the vent for a downstream fixture, within strict code limits (kitchen sink venting a dishwasher, lavatory venting a toilet in a bathroom group).
• Common vented - Two back-to-back fixtures share a single vent (double-bowl kitchen sink, back-to-back bathrooms).
• Studor / AAV (air admittance valve) - A one-way check valve that allows air INTO the drain but not out. Used where running a vent to the roof is impractical - an island sink, a basement bathroom, a remodel into an existing wall. NOT allowed in all jurisdictions. Check your local code. Some states ban AAVs on all drainage fixtures; others allow them only on secondary branches.

Island Sinks and Kitchen Islands

An island sink is the trickiest drain in a typical house. There is no wall behind the sink to run a vent up. Two legal options:

• Island loop vent (Chicago loop) - A vent pipe rises from the trap arm to the underside of the counter, loops over, and drops down through the floor before connecting to the vent stack elsewhere. Code-compliant in most jurisdictions. Ugly to engineer.
• Studor AAV under the cabinet - Mechanically simple. Code-legal only where AAVs are allowed. Some inspectors require specific brand/model approvals.

Confirm island venting method with your journeyman and the local inspector BEFORE roughing in.

Tub/Shower Drain Rough

Showers and tubs are special because the drain is below the floor.

• Tub: 1-1/2 inch drain with a trap 14-18 inches below the finished floor, rough-centered on the tub end. The overflow fitting is a plastic tee built into the tub waste-and-overflow assembly.
• Shower: 2 inch drain (not 1.5 inch; 1.5 is outlawed for showers in the IPC). The trap arm is limited to 5 feet; a tile shower with a linear drain often pushes that limit.

The subfloor gets a hole cut for the drain, the trap glued in underneath, and a test cap or riser left for the finishers. Do NOT finalize the drain height until you know the floor stack-up: plywood + mortar bed + thinset + tile can be 2-3 inches deep. Leave the trap slightly low and plan to raise it with an adjustable riser.

Water Supply Stub-Outs

Stub-outs are the short pieces of supply pipe that stick out of the wall or floor where a fixture will later connect. Heights and locations matter - cabinet installers and tile setters will not move their work to accommodate a stub-out 2 inches out of place.

| Fixture                  | Stub-out height above finished floor | Horizontal offset from centerline |
|--------------------------|---------------------------------------|-----------------------------------|
| Lavatory (bathroom sink) | 18-22 inches (hot left, cold right)   | 4-8 inches either side of drain   |
| Kitchen sink             | 18-24 inches                          | 8 inches either side of drain     |
| Toilet                   | 8-12 inches, behind fixture           | 6 inches left of rough center     |
| Shower/Tub               | Behind tile backer, at valve height   | Manufacturer template             |
| Washing machine box      | 38-48 inches to box centerline        | Standard dual-box                 |
| Hose bib                 | 18-24 inches AFG, outside              | -                                 |
| Dishwasher               | Hot only, 8-12 inches above subfloor  | Behind cabinet                    |
| Ice maker                | 6-12 inches above subfloor            | Behind fridge location            |

Always check the fixture manufacturer's rough-in template for tubs, showers, and toilets. Kohler, Delta, Moen, and others all publish a rough-in sheet that shows exact dimensions from the finished wall and finished floor. Hang the rough-in sheet on the stud next to the fixture valve during install.

Stub-Out Support

Stub-outs must not move when a finisher wrestles a compression fitting onto them. Support options:

• Copper stub-out brackets - Soldered or press-fit 90 degrees off a short copper stub, screwed to a stud.
• PEX stub-out elbows with nailing ears - Plastic 90-degree fitting with screw tabs, nailed to a stud. A short piece of PEX comes out of the wall with the elbow hidden.
• Tub/shower drop-ear elbow - Threaded drop-ear with two screw flanges, for tiled showers.

A loose stub-out that rotates in the wall when a finisher pushes on it is a callback guaranteed. Screw it down.

Pressure Testing

Inspectors require two separate tests at rough-in:

DWV Test

Fills the drain/vent system with water or pressurized air to prove no leaks. Two flavors:

• Water test - Cap every opening below the top of the highest vent. Fill the system with water from the roof vent down. Hold for 15 minutes minimum with no drop in water level visible at the highest cap. Messy, heavy, but definitive - any leak shows up wet.
• Air test - Seal every opening. Pressurize the DWV system to 5 psi. Hold for 15 minutes with no pressure drop. Quicker but requires a test plug on every opening.

Water Supply Test

Cap every supply stub-out. Pressurize the whole supply system to 100 psi (or 150 psi in some jurisdictions; check local code). Hold for 15 minutes with no pressure drop.

Document the test: photograph the gauge at the start and end of the test window with a timestamp. Many inspectors accept photo documentation in addition to witnessing.

Inspection Gotchas

A punch list of the most common rough-in failures:

• Missing nail plates - Any supply or drain pipe within 1-1/2 inches of the face of a stud must have a steel nail plate over it to stop a drywaller or trim carpenter from driving a nail through the pipe. Missing nail plates are the single most common red-tag item.
• Slope errors - Back-pitched drain, too little slope on a long horizontal, or too much slope on a 3 inch sewer main. Always measure fall directly.
• Too-tight bend radius on PEX - A 1/2 inch PEX bent inside the 3-inch minimum radius cracks the wall. Visible as a kinked, whitened section.
• Untested system - Inspector shows up, no gauge on the test manifold, no pressure in the system. Automatic fail.
• Wrong pipe material - Copper type M used underground where type L was specified; PEX run outdoors; ABS glued with PVC cement.
• Fittings installed backwards - A sanitary tee with the sweep facing up (should face down, in the direction of flow) will fail. A 22-degree fitting used where a 45 was called for.
• Unsupported horizontal drain runs - Sagging between hangers causes water to pool and sludge to build up.
• Missing cleanouts - Required at the base of every stack, at every 90-degree direction change on a main drain, and at intervals not exceeding the code max (usually 100 feet on a 4 inch sewer).
• Vent tied in below the fixture flood rim - A vent must rise vertically to at least 6 inches above the flood-level rim of the fixture it serves before it can run horizontal. Low horizontal vent tie-ins are a classic rookie mistake.
• Missing expansion provision on PEX or CPVC - Long straight runs need a slight serpentine or an expansion loop to absorb heating/cooling movement.

Day 1 Checklist

• Drawings or plans with fixture rough-in dimensions printed and on the truck
• Fixture manufacturer rough-in templates for every non-standard fixture
• Rotary hammer drill with bits, hole saws (2 inch, 3 inch, 4 inch) for floor and top plate penetrations
• Reciprocating saw with wood and metal blades for tear-outs and notching
• Box of nail plates in 1.5, 2, and 3 inch sizes
• Torpedo level, 4-foot level, laser level for long horizontal slope
• Test caps, test balls, and a pressure gauge with a Schrader valve adapter
• Sharpie, chalk line, framing square for marking stub-out centers
• Code book (UPC or IPC, per local jurisdiction) in the truck cab

Expert Tips

• "Slope every pipe as you set it." Do not dry-fit a run, cement it, then check slope. Every joint, torpedo level, check fall.
• "Six inches above the flood rim, then horizontal." Memorize this vent rule. Inspectors test it.
• "Nail plate every penetration." If you are in doubt, plate it. A 40-cent plate is cheap; a new drywall patch is not.
• "Test before the drywallers show up." Pressure test the system while the walls are still open. Finding a leak during the water test saves you from a Sawzall party later.
• "Photograph every test with a timestamped gauge." One extra minute of documentation wins a dispute with an inspector every time.
• "When in doubt, call for rough inspection early." A short pre-inspection conversation with the AHJ catches problems before they become framed-in failures.