Rough Plumbing Installation
Rough plumbing establishes your ADU's water supply and waste removal systems before walls are closed. This includes running supply lines to fixtures, installing drain pipes with proper slope, and connecting vent stacks to prevent sewer gas. Getting it right now prevents costly repairs and ensures reliable operation for decades.
Quick Summary
Time needed
2-4 days
Cost range
$3,000-$7,000
Professional
Licensed plumber
Why Rough Plumbing Matters
Plumbing is one of the most critical ADU systems—problems here lead to water damage, mold, and expensive repairs. Proper rough plumbing ensures adequate water pressure, complete waste removal, and code compliance. Common issues from poor installation include:
- Slow drains from improper slope
- Sewer gas odors from missing or improper venting
- Water damage from leaky joints discovered after drywall
- Low water pressure from undersized supply lines
Step-by-Step Rough Plumbing Guide
Connect to Main Water Supply
Your ADU needs a dedicated water line from the main house supply or a new meter (depending on local requirements). Typical options:
- • Tap existing supply: Connect to main house line (most common)
- • Separate meter: Some jurisdictions require separate utility connection
- • Line size: 3/4" minimum for most ADUs, 1" for larger units
- • Shut-off valve: Install dedicated ADU shut-off near connection point
Pro tip: Install a pressure regulator if incoming pressure exceeds 80 PSI. High pressure damages fixtures and wastes water.
Run Water Supply Lines (PEX Recommended)
PEX (cross-linked polyethylene) is now the standard for residential supply lines. It's flexible, freeze-resistant, and easier to install than copper:
- • PEX-A: Most flexible, best for tight spaces, highest cost
- • PEX-B: Good balance of flexibility and cost (most common)
- • Line sizing: 3/4" main trunk, 1/2" to fixtures
- • Hot/cold: Red PEX for hot, blue for cold (code requirement)
- • Manifold system: Consider home-run manifold for individual shut-offs
Note: Some jurisdictions still prefer copper. Check local code and your plumber's recommendation. Copper costs more but may be required.
Install Drain Lines with Proper Slope
Drain lines rely on gravity, so slope is critical. The standard is 1/4 inch drop per foot of horizontal run:
- • Standard slope: 1/4" per foot (1/8" minimum for 3"+ pipes)
- • Too little slope: Solids settle, causing clogs
- • Too much slope: Water outruns solids, also causing clogs
- • Drain sizing: 1-1/2" for sinks, 2" for showers, 3" for toilets
- • Material: ABS or PVC (check local code for approved types)
Verification: Use a level to check slope. A 4-foot level should show 1" drop across its length for proper 1/4" per foot slope.
Install Vent Stack and Branch Vents
Proper venting is essential for drain function and preventing sewer gas entry. Every fixture needs a vent path:
- • Main vent stack: 3" or 4" vertical pipe through roof
- • Branch vents: Connect fixtures to main stack
- • Air admittance valves: May be allowed for some fixtures (check code)
- • Wet venting: Some fixtures can share vents if properly sized
- • Distance limits: Vents must be within specific distance of trap
Without proper venting, drains gurgle, empty slowly, and traps can be siphoned (allowing sewer gas to enter the living space).
Set Fixture Rough-In Locations
Ensure supply and drain locations match your fixture specifications:
- • Toilet: 12" rough-in is standard (measure from wall to center of flange)
- • Shower: Drain centered, mixing valve at 48" height typically
- • Vanity: Drain at 18-21" from floor, supplies at 20-24"
- • Kitchen sink: Drain at 18" from floor, supplies at 20-22"
- • Dishwasher: Supply and drain adjacent to sink location
Pro tip: Have your specific fixtures selected before rough-in. Different manufacturers have different rough-in requirements.
Install Water Heater Connections
Plan your water heater location and connections:
- • Tankless: Cold supply, hot out, gas line (if gas), condensate drain
- • Tank: Cold supply, hot out, T&P relief drain, gas line (if gas)
- • Heat pump water heater: Same as tank plus condensate drain, space for air flow
- • Location: Minimize distance to kitchen/bath for faster hot water
Pressure Test All Lines
Before inspection and drywall, all plumbing must be tested for leaks:
- • Supply lines: Air or water pressure test, typically 40-60 PSI for specified time
- • DWV system: Air test or water fill test
- • Duration: Hold pressure for 15-30 minutes with no drop
- • Visual inspection: Check all joints and connections
Critical: Never skip pressure testing. A small leak hidden in a wall can cause thousands in water damage and mold remediation.
ADU Fixture Plumbing Requirements
| Fixture | Supply Size | Drain Size | Trap Required |
|---|---|---|---|
| Toilet | 3/8" supply | 3" or 4" | Built-in (integral) |
| Bathroom sink | 1/2" hot & cold | 1-1/4" or 1-1/2" | P-trap |
| Shower/tub | 1/2" hot & cold | 2" | P-trap (below floor) |
| Kitchen sink | 1/2" hot & cold | 1-1/2" or 2" | P-trap |
| Dishwasher | 3/8" hot only | Connects to sink | Air gap or high loop |
| Washer | 1/2" hot & cold | 2" standpipe | P-trap at standpipe |
Rough Plumbing Inspection Checklist
The inspector will verify code compliance before drywall installation:
Frequently Asked Questions
Should I use PEX or copper for supply lines?
PEX is now the standard for most residential applications. It's less expensive, easier to install, freeze-resistant, and doesn't corrode. Copper is still preferred in some areas and by some plumbers—it's more durable and has a longer track record. Either works well; follow your plumber's recommendation and local code requirements.
How much does rough plumbing cost for an ADU?
Expect $3,000-$7,000 depending on ADU size and fixture count. A basic 1-bath ADU might cost $3,000-4,500 for rough plumbing. A larger unit with 2 bathrooms, washer hookup, and more fixtures could run $5,500-7,000+. The connection to existing sewer/septic can add significant cost if long trenching is required.
Why is proper venting so important?
Vents serve two critical functions: they allow air to enter the drain system so water flows freely (like removing your thumb from a straw), and they route sewer gases above the roofline instead of into your living space. Without proper venting, drains gurgle and empty slowly, and you risk sewer gas exposure—which is both unpleasant and potentially dangerous.
Can I do rough plumbing myself?
Most jurisdictions require licensed plumbers for permitted work. Even where homeowner work is allowed, plumbing requires specific knowledge of code requirements, proper techniques, and pressure testing. Mistakes can cause severe water damage. Unless you have plumbing experience, hiring a licensed professional is strongly recommended.
Ready for the Next Step?
Once rough plumbing passes inspection, you'll install the HVAC system to provide heating and cooling for your ADU.