How to Rough In a Toilet Drain and Closet Flange

Why this matters

A toilet rough-in is the one drain in the house where a half-inch mistake cannot be hidden. Short of 12 in from the finished wall, and the tank of a standard toilet will not fit. A flange set below the tile, and the wax ring is asked to bridge a gap it was never made for, so it seeps sewer water into the subfloor for a year before anyone notices the soft spot. A closet bend notched through a joist to make the layout work turns a plumbing call into a framing repair.

This card is new work and remodel rough-in: layout, the floor opening, the bend and its run, and setting the flange. Replacing a flange under an existing toilet is in Toilet Flange Replacement Reference, and setting the bowl is in Replace a Toilet.

The layout, in plan

Two numbers fix the flange center. The rough is the distance from the finished back wall to the flange center: 12 in for the standard toilet, with 10 in and 14 in models made for the rooms where 12 will not work. The side clearance is a code minimum: at least 15 in from the flange center to any side wall, partition, vanity or other obstruction, and 30 in center to center between two water closets, with a clear space in front of the bowl (21 in under the IPC and 24 in under the UPC, as adopted; IPC 405.3.1 and UPC 402.5 in the 2021 editions; check the current edition).

  finished back wall face (tile or drywall)
  =============================================
  |                    |                     |
  |                    | 12 in to            |
  |                    | flange center       |
  |                    v                     |
  |<---- 15 in min ---( O )--- 15 in min --->|
  |                  flange                  |
  side wall    bolt slots parallel     vanity side
  |            to the back wall              |
  |                                          |
  |   clear space in front of the bowl:      |
  |   21 in IPC / 24 in UPC, as adopted      |

Measure from the finished wall, not the stud. On a framed wall with 1/2 in drywall and nothing else, 12 in finished is 12-1/2 in from the stud face. Tile on a backer adds its own thickness; ask what the wall finish will be before you mark. Aim for 12 in and accept a little over, never under: a 12 in toilet set at 12-1/4 in leaves a small gap behind the tank, while one at 11-3/4 in does not go on.

Steps

1. Mark the flange center and check it against the framing below. Mark the center from the finished-wall line and the side clearances, then go below (basement, crawl or by opening the ceiling) and find every joist, duct, wire and pipe within a few inches of it. Why: the flange opening is the cut you cannot move after it is made, and the framing decides where it can be. Before cutting near any cable below the floor, shut its breaker and confirm absence of voltage with a tester proven on a known live source before and after, per NFPA 70E Article 120 (current edition). Close that breaker again once the opening is cut and the bend is set, and confirm a device on the circuit works.

2. Cut the floor opening. Drill a pilot at the center, confirm from below it landed where you expected, then cut the opening with a hole saw or jigsaw sized to the closet bend's outside diameter plus a little clearance, not to the flange. A reciprocating saw blade or hole saw that hits a nail kicks: hold the tool with both hands and keep your feet clear of the cut. Wear eye protection and a dust mask rated for wood dust at minimum; old subfloor in a pre-1978 house can carry painted or glued layers, and resilient flooring and its mastic over a subfloor may contain asbestos, which you do not cut until it is tested.

3. Plan the bend's run without weakening a joist. A 3 in closet bend (a 4x3 bend where the flange needs a 4 in connection) drops through the floor and turns horizontal toward the stack. Run it between joists where you can. Where it must cross one, the IRC limits a bored hole to one-third of the joist's depth with at least 2 in to either edge, and limits notches by location and depth (IRC R502.8 in the 2021 edition, as adopted). 3 in PVC has a 3.5 in outside diameter: a 2x10 is 9-1/4 in deep and allows a 3.08 in hole, so a 3 in drain cannot be bored through it; a 2x12 at 11-1/4 in allows 3.75 in and can. Engineered I-joists and trusses follow their maker's hole chart, not R502.8, and many forbid any cut in a flange. If the run cannot be made without exceeding those limits, the answer is a header or a different route, decided with the builder or an engineer, never a notch the inspector finds later.

4. Set the closet bend and its horizontal run. Join the bend to the horizontal drain with the fittings the change of direction calls for (a long-pattern fitting or a combination wye and eighth bend where a horizontal drain turns or enters another horizontal line; see DWV Fittings for Each Change of Direction). Give the run a steady 1/4 in per foot of fall to the stack; slope and venting for the toilet are in Drain Pipe Slope + Venting Quick Reference. Support the bend directly so the flange cannot sag: a strap or a block under the heel of the bend, fastened to framing. A bend hanging on its own joints rocks every time someone sits down.

5. Leave the stub long and test before the floor goes on. Cut the bend stub a few inches above the subfloor and cap it with a test cap, then run the DWV test in How to Water or Air Test a DWV Rough-In. Leaving it long is deliberate: the finished floor height is not known yet, and you cut once.

6. Set the flange on top of the finished floor and anchor it. After the finished floor is down, dry-fit the flange on the stub so its ring rests on the finished floor, mark the pipe, and cut so the pipe bottoms in the flange socket with the ring still resting on the floor. Turn the flange so its bolt slots run parallel to the back wall, cement it, and fasten it through every hole with corrosion-resistant screws into the subfloor (on tile, through the tile into the subfloor with a masonry bit for the tile layer). Why on top: the wax ring or foam gasket is made to seal between the bowl's horn and a flange at floor level. A flange below the finished floor leaves a gap the seal cannot close, and the usual field fix, stacked wax rings, is the start of the next callback.

Worked example: an alcove toilet over 2x10 joists

A second-floor bath remodel. The toilet sits in a 33 in wide alcove between a wall and a new vanity side. The back wall gets 1/2 in drywall and a 3/8 in tile wainscot. The floor will be tile over a backer, 3/4 in total above the subfloor. Joists are 2x10 at 16 in on center, running perpendicular to the back wall, toward the stack in the opposite wall about 5 ft away.

Rough. Finished wall face is 1/2 plus 3/8 in, or 7/8 in, off the stud face. 12 in finished is 12-7/8 in from the stud face, marked on the subfloor.

Side clearance. Centered in 33 in, the flange center is 16-1/2 in from each side, inside the 15 in minimum with 1-1/2 in to spare on each side.

The joist. Below, the nearest joist face sits 1-3/4 in from the planned center. The opening for the 3 in bend, cut 4-1/2 in across (radius 2-1/4 in), would bite 1/2 in into the joist, since 2-1/4 minus 1-3/4 is 1/2 in. Shifting the center 1 in away from that joist puts its face 2-3/4 in out, clearing the 2-1/4 in radius by 1/2 in. The side clearances become 15-1/2 in on the vanity side and 17-1/2 in on the wall side, both over 15 in. The customer was shown the 1 in offset on the floor with tape and accepted it; it is written on the work order. Crossing the joist was never on the table: R502.8 allows a 3.08 in hole in a 2x10, against a 3.5 in pipe.

Run. The bend's heel sits between the joists, and the horizontal leg runs 4 ft toward the stack in the joist bay, falling 1/4 in per foot, which is 1 in over the 4 ft. A block screwed between the joists carries the heel. The stub was left 4 in above the subfloor, capped, and the DWV test held.

Flange. After tile, the flange dry-fit on the stub with its ring on the tile, 3/4 in above the subfloor; the pipe was marked and cut so it bottomed in the socket at that height. Bolt slots set parallel to the back wall, cemented, and fastened with four stainless screws through the tile and backer into the subfloor.

Finish test: level, fastened, at the right height

• Height. The flange ring rests on the finished floor all the way around. A ring sitting proud of the tile on one side, or a visible gap between ring and tile, fails.
• Level. A torpedo level across the flange ring, laid both parallel and perpendicular to the back wall, reads with the bubble centered in both directions. A flange out of level makes the bowl rock and the seal load unevenly.
• Fastened. Push and twist the flange by hand; it does not move. Every screw hole that lands on subfloor carries a screw.
• Position. Re-measure from the finished wall to the flange center: 12 in on this job (or 12 in to a little over, never under), and 15 in or more to each side; here 15-1/2 and 17-1/2 in.
• Bolt slots parallel to the back wall, so the bowl lands square.

Cap the flange opening with a rag-free cover or a test plug until the bowl is set; a bathroom under remodel collects drywall mud, and the drain is the nearest hole.

When the rough-in changes

On a slab, the bend sits in a box-out or is cut in after the fact with a concrete saw, and cutting concrete releases respirable crystalline silica: wet cutting with the saw's water feed, plus a respirator under a written program where 29 CFR 1926.1153 Table 1 calls for one (it does for a handheld saw used indoors), is the control, not a dust mask.

A 10 in or 14 in toilet solves the room where 12 in will not fit, but the customer must buy the matching model. Write the rough on the work order so the fixture order matches the floor.

Offset flanges move the outlet an inch or two to dodge a joist, at the cost of a tighter turn under the bowl. Use one only where it is listed and the inspector accepts it, and never as a substitute for laying out against the framing first.

References

• IPC 2021 Section 405.3.1 and UPC 2021 Section 402.5, as adopted, for water closet clearances; check the current edition.
• IRC 2021 Section R502.8, as adopted, for drilling and notching joists; the I-joist or truss maker's hole chart.
• NFPA 70E, Article 120, current edition; 29 CFR 1926.1153 for silica when cutting concrete.
• See related: Toilet Flange Replacement Reference
• See related: Replace a Toilet