Plumbing Rough-In: Complete Guide

Overview

Plumbing rough-in installs all water supply and drain-waste-vent (DWV) pipes before walls are closed. This is one of the most code-intensive phases, requiring knowledge of proper sizing, slopes, and venting requirements.

I've seen many owner-builders successfully DIY their plumbing rough-in, but it requires studying code requirements and planning carefully. Mistakes here mean cutting open walls later - expensive and frustrating.

When This Phase Happens

Plumbing rough-in happens after house is weathertight.

Must be complete first:

• Roof complete
• Windows and doors installed
• House is weathertight
• Under-slab plumbing complete (if slab foundation)

Can happen in parallel:

• Electrical rough-in
• HVAC rough-in
• All three rough-in trades can work simultaneously

What comes after:

• Rough-in inspection (all three trades)
• Insulation
• Drywall

Should You DIY This Phase?

DIY If:

• Comfortable with PEX or CPVC (easier than copper)
• Can learn code requirements (IRC P2900 series)
• Have access to code-required tools (pressure tester, etc.)
• Simple layout with straightforward runs
• Have time to study and plan carefully
• Local code allows owner-builder plumbing

Hire Out If:

• Complex multi-story plumbing
• Copper piping required (harder to work with)
• Uncomfortable with code requirements
• Want liability protection
• Need speed (pros do in 3-5 days vs 1-2 weeks DIY)

Materials Needed

Water Supply System (PEX example - 2,000 sq ft home)

Drain-Waste-Vent (DWV) System

Tools Required

Essential:

• PEX crimping tool or expansion tool
• Tubing cutter (for PEX and CPVC)
• Hacksaw or reciprocating saw
• Drill with hole saw kit (1/2" to 2")
• Tape measure
• Level (torpedo and 4-foot)
• Marker and pencil

Nice to have:

• Inspection camera (see inside walls)
• Pressure test pump (required for inspection)
• Drain test equipment
• Pipe threading equipment (if using threaded pipe)

Code-required:

• Pressure test gauge (test to 50+ PSI)
• Smoke or peppermint test for DWV

Step-by-Step Process

Days 1-2: Planning and Layout

Review plans and code

• Study fixture locations on plans
• Calculate fixture units (determines drain sizing)
• Plan vent routing (every fixture needs venting)
• Identify main stack location (straightest path to roof)
• Plan water supply routing (minimize long runs)
• Mark all penetrations on framing

Fixture unit calculations determine pipe sizing:

Days 3-5: DWV Rough-In (Do This First!)

Always rough-in DWV before water supply. DWV pipes are larger and less flexible in routing.

Main drain installation:

1. Install cleanouts at base of stacks and every 100 feet
2. Slope horizontal drains toward sewer/septic: 1/4" per foot for pipe under 3"; 1/8" per foot is permitted for 3" and larger (IPC Table 704.1 / IRC P2904—verify your adopted edition and local rules)
3. Support every 4 feet with proper hangers
4. Keep drain lines straight (no sags or bellies)
5. Test slope with level before cementing

Toilet drain installation:

1. Position flange at correct height (flange bottom rests on the finished floor surface)
2. Connect 3" or 4" drain to toilet flange
3. Water closets are self-siphoning and have no maximum trap-to-vent distance under the IPC (IPC 909.1)—you still must vent the toilet, but the trap arm can run as far as needed (verify your adopted IPC/IRC edition and local rules, and watch the drop limit)
4. Secure flange to floor framing

Vent stack installation:

1. Run 3" main stack from main drain through roof
2. Keep stack as straight as possible (no more than 45° offsets)
3. Extend 12" above roof penetration
4. Flash at roof penetration (critical!)
5. Connect branch vents to main stack

Vent requirements (critical for passing inspection):

• Main vent stack: 3" minimum
• Branch vents: 1.5" minimum
• Wet venting allowed in some configurations (check code)
• The maximum trap-to-vent distance is set by the trap-arm (drain) size, not the fixture type
• These figures vary by code—verify your adopted IPC/IRC edition and local rules

Shower/tub drain:

1. Install drain at correct depth (varies by fixture - check specs)
2. P-trap should be accessible for service
3. Keep the trap arm within the maximum distance for its size (e.g., 8 feet for a 2" arm, 6 feet for a 1.5" arm—verify your adopted IPC/IRC edition and local rules)
4. Test drain before installation (fill and check for leaks)

Days 6-8: Water Supply Rough-In

Main supply planning:

1. 3/4" main from water heater to distribution point
2. Branch to 1/2" for individual fixtures
3. Keep hot and cold parallel (easier for fixture installation)
4. Maintain proper spacing (6" between hot and cold)

PEX installation (easiest for DIY):

1. Run 3/4" trunk lines along main routes
2. Branch 1/2" lines to each fixture
3. Use proper fittings (crimp or expansion - not both!)
4. Support every 32" with plastic hangers
5. Protect from freezing in exterior walls
6. Leave loops at fixtures for flexibility during finish

Installation tips:

• Use red PEX for hot, blue for cold (not required but helpful)
• Label lines at panel for future reference
• Install shut-off valves for each fixture
• Install zone shut-offs for easier maintenance
• Keep lines away from heat sources (recessed lights, exhaust)
• Protect pipes where passing through framing (nail plates required within 1-1/4" of edge)

Water heater connection:

• 3/4" supply and return lines
• Install expansion tank on cold side
• Install shut-off valves for service
• Install drain valve at lowest point
• Follow manufacturer spacing requirements

Days 9-10: Testing and Inspection Prep

The supply system and the DWV (drainage) system are tested separately, with different methods. Don't confuse the two.

Supply (water) test (required):

1. Cap all outlets with test caps
2. The supply test holds pressure: typically the system's working water pressure, or for non-plastic pipe an air test around 50 PSI, held for at least 15 minutes
3. Pressure should not drop—any drop means a leak
4. Identify and fix any leaks, then re-test until the system holds
5. Confirm your jurisdiction's exact pressure and duration—these vary (verify your adopted IPC/IRC edition and local rules)

DWV (drainage) test (varies by jurisdiction):

1. The most common is the water/head test: plug the lowest opening, fill the drain and vent piping with water to produce at least a 10-foot head, and hold for the required time
2. Water level should not drop—check every joint for leaks
3. Some jurisdictions accept an air test, smoke test, or peppermint test instead
4. Confirm which DWV test your inspector requires

Inspection preparation:

• All lines properly supported
• All penetrations have nail plates
• Pressure test complete and holding
• DWV test complete
• No kinked or damaged lines
• Proper slopes on drains verified
• All vents properly routed and sized
• All cleanouts accessible

Code Requirements

Key IRC/IPC plumbing requirements. Code numbers and figures change between editions—always verify your adopted IPC/IRC edition and local rules before relying on any of these:

• IRC P2603 - Water supply: 3/4" minimum service, sized per fixture units

• IRC P2904 / IPC Table 704.1 - Slope of horizontal drainage: 1/4" per foot for pipe under 3", 1/8" per foot permitted for 3" and larger

• IRC P3104 / IPC 909.1 - Vent required and distance: Every fixture requires venting; the maximum trap-to-vent distance depends on trap-arm size (e.g., 1.5"→6 ft, 2"→8 ft, 3"→12 ft). Water closets are self-siphoning and have no distance limit

• IRC P3103 - Vent size: Based on fixture units and developed length

• IRC P2503.5 - Support: Horizontal piping every 6 feet for copper, about 3 feet for 1/2"-3/4" CPVC, every 32" for PEX, every 4 feet for DWV (spacing varies by material and pipe size—check the support table for your material)

• IRC P2503 / P2601 - Testing: The supply system holds pressure (working pressure, or ~50 PSI air for non-plastic pipe, for at least 15 minutes with no loss); the DWV system is tested separately, most often a 10-foot head of water

• IRC P2503.6 - Nail plates: Required where pipes pass within 1-1/4" of framing edge

Subcontractor Considerations

If hiring plumbers:

Timeline: 3-7 days for typical house

What to look for:

• Licensed plumber (required in most jurisdictions)
• Experience with residential new construction
• Familiar with local code requirements
• Will handle inspection
• Provides warranty on work
• Carries liability and workers comp insurance

Red flags:

• Not licensed (you're liable for code violations)
• Won't pull permit or handle inspection
• Can't explain vent requirements
• Pressure to skip testing

Common Mistakes

1. Inadequate Venting

Why it's a problem: Slow drains, gurgling, sewer gas, fails inspection. #1 plumbing rough-in failure. How to avoid: Study IRC P3100 venting requirements. Every fixture must be properly vented. Cost if you don't: $1,000-$5,000 to cut walls open and add vents.

2. Wrong Drain Slope

Why it's a problem: Standing water, slow drains, clogs, fails inspection. Too little slope leaves solids behind; too much can let water outrun solids. How to avoid: Maintain 1/4" per foot for pipe under 3"; 1/8" per foot is permitted for 3" and larger. Check with a level. No sags or bellies. Cost if you don't: $500-$2,000 per drain to re-pipe.

3. Improper Pipe Support

Why it's a problem: Sagging lines, noise, leaks, fails inspection. How to avoid: Support every 32" for PEX, every 4 feet for DWV. Use proper hangers. Cost if you don't: Failed inspection, must expose and add supports.

4. Forgetting Cleanouts

Why it's a problem: Can't clear clogs, fails inspection, code violation. How to avoid: Install cleanout at base of each stack and every 100 feet of horizontal drain. Cost if you don't: $300-$800 per cleanout to add later (cut walls/floors).

5. No Pressure/Leak Testing

Why it's a problem: Leaks hidden in walls discovered after drywall. Expensive water damage. How to avoid: Test both systems before inspection—the supply side holds pressure (working pressure, or ~50 PSI air for non-plastic pipe, 15 min), and the DWV side gets its own drainage test (commonly a 10-foot head of water). Cost if you don't: $1,000-$5,000 per leak to cut walls, repair, patch, repaint.

6. Wrong Pipe Sizing

Why it's a problem: Insufficient flow, pressure loss, fails inspection. How to avoid: Size pipes per code based on fixture units. When in doubt, go larger. Cost if you don't: Re-piping, failed inspection.

7. Forgetting Nail Plates

Why it's a problem: Drywall screws puncture pipes, leaks behind walls. How to avoid: Install 1/16" nail plates wherever pipes pass within 1-1/4" of framing edge. Cost if you don't: Leaks, water damage, emergency repairs.

8. Improper Toilet Flange Height

Why it's a problem: Toilet rocks, leaks at base, wax ring fails. How to avoid: Set the flange so its bottom rests on the finished floor surface (not the subfloor). Account for tile or other flooring thickness. Cost if you don't: $300-$600 to reset toilet, repair subfloor damage.

9. Mixing Pipe Types Incorrectly

Why it's a problem: Incompatible materials, leaks, corrosion, code violations. ABS and PVC are a classic trap—they're not interchangeable and use different solvents (ABS = one-step cement, no primer; PVC = primer + cement). How to avoid: Use proper transition fittings between different materials (PEX to copper, etc.). For DWV, pick one material—ABS or PVC—and use its matching solvent throughout. Cost if you don't: Leaks, corrosion, failed joints.

10. No Expansion Tanks

Why it's a problem: Water hammer, damage to fixtures, shortened appliance life, code violation in many areas. How to avoid: Install expansion tank on cold water side near water heater. Cost if you don't: Fixture damage, noisy pipes, code violation.

Quality Checkpoints

Before rough-in inspection, verify:

• [ ] All fixtures have proper drain size (1.5", 2", or 3")
• [ ] All drains slope correctly (1/4" per foot for pipe under 3"; 1/8" per foot for 3" and larger)
• [ ] All fixtures properly vented within code distances
• [ ] Main vent stack 3" minimum through roof
• [ ] Cleanouts installed at base of stacks
• [ ] Toilet flanges set so the flange bottom rests on the finished floor surface
• [ ] All DWV joints properly glued (one DWV material—ABS or PVC—with the matching solvent)
• [ ] All supply lines properly crimped/connected
• [ ] Proper pipe support (every 32" PEX, ~3' for 1/2"-3/4" CPVC, 6' copper, 4' DWV)
• [ ] Nail plates installed where required
• [ ] Supply test passed (holds working pressure, or ~50 PSI air for non-plastic pipe, 15+ min)
• [ ] DWV test passed (10-ft head of water, or air/smoke/peppermint per local rules)
• [ ] No kinked or damaged lines
• [ ] All lines labeled (hot/cold)
• [ ] Shut-off valves installed appropriately
• [ ] No leaks detected

Budget Breakdown

Example for 2,000 sq ft home, 2.5 baths:

Timeline Tips

Best season: Any time (interior work)

Scheduling:

• Coordinate with electrical and HVAC (all can work simultaneously)
• Schedule inspection for all three trades together
• Allow 1-2 weeks before inspection (time to test and fix issues)

What Comes Next

After plumbing rough-in inspection passes:

1. Complete all rough-in trades (electrical, HVAC)
2. Combined rough-in inspection
3. Insulation installation
4. Drywall installation

Link to: Electrical Rough-In Phase

Related Resources

• Plumbing Rough-In Inspection Guide

Need Plumbing Help?

Plumbing rough-in is complex with strict code requirements. If you're unsure about venting or sizing, a consultation can save you from failed inspections.