How to Rough-In Plumbing Drain Lines
Planning and installing DWV (drain-waste-vent) piping below the slab and through walls during rough-in, before concrete or finishes are in place.
Step-by-Step
- 1
Review the plumbing drawings
Identify all fixture rough-in dimensions (toilet flange location, sink drain height, shower drain location), pipe sizes, and routing. Drain sizing is set by the DFU (drainage fixture unit) load — do not reduce pipe sizes without a licensed plumber's review.
- 2
Layout fixture locations
Snap chalk lines and mark the exact location of every drain in the floor. For toilets, the center of the flange is typically 12 inches from the finished wall (rough-in dimension). Confirm this matches your toilet specification.
- 3
Plan the slope
Drain lines must slope toward the main stack. Required slope is 1/4 inch per foot for 3-inch and smaller pipe, 1/8 inch per foot for 4-inch pipe. Calculate the invert elevations at every connection point before cutting anything — flat or reverse-sloped drain is the #1 plumbing failure.
- 4
Excavate under-slab trenches
For under-slab plumbing, trench from the main stack location to each fixture. Keep trenches as shallow as possible while maintaining slope. Trench walls should be clean and stable — cave-ins after pipe is installed shift your carefully set grades.
- 5
Cut and fit pipe dry
Dry-fit all pipe and fittings before gluing. Mark each piece with a pen line across the joint so you can confirm correct alignment when gluing. PVC primer and cement set in seconds — dry-fitting catches mistakes before they're permanent.
- 6
Glue fittings
Apply purple primer to both the pipe and socket, then immediately apply cement and push the pipe home with a slight twist. Hold for 5–10 seconds. The cement does not allow re-positioning after contact — work fast and correct. Wipe excess cement immediately.
- 7
Verify slopes
After installation, verify slope at every run with a level. A 4-foot level should show a bubble just off center at 1/4-inch-per-foot slope. Check every horizontal run — not just spot-checking.
- 8
Install cleanouts
Install accessible cleanouts at the base of every vertical stack, at every change of direction more than 90 degrees, and every 100 feet on horizontal runs. Cleanouts must remain accessible — never pour concrete over a non-extended cleanout.
- 9
Pressure test before closing in
Plug all openings and pressurize the system with air to 5 psi (or water column per code). Hold for a minimum of 15 minutes with no drop. Call for inspection — most jurisdictions require a witnessed pressure test before the slab is poured or walls are closed.
Pro Tips
- → Set your toilet flange after the floor tile is in — it should sit on top of the finished floor, not below it.
- → Use long-sweep elbows (not standard 90s) on drain lines whenever possible — they reduce clogs and improve flow.
- → Photograph all under-slab pipe before the slab is poured. A photo with a measuring tape visible is worth its weight in gold during future renovations.
Watch Out
- ! Insufficient slope or reverse slope is not self-correcting. A slab pour over improperly sloped drain will require jackhammering to fix.
- ! Every fixture trap requires a vent. An unvented trap will siphon dry and allow sewer gas into the building.