A back outlet toilet — also called a rear discharge toilet or rear outlet toilet — connects its drain to a pipe in the wall rather than a floor flange. This design is the solution for bathrooms where a floor drain is impossible: concrete slab floors, finished basements, renovations where cutting the floor is not an option, and apartments where floor penetration is not permitted.
Installing a back outlet toilet correctly requires a fundamentally different rough-in than a standard floor-mount toilet. There is no wax ring. There is no floor flange. The toilet connects to the wall drain via a rubber gasket and a P-trap connector — and the anchor bolts go into the wall, not the floor. Getting the rough-in measurements wrong by even ½ inch means the toilet won’t align with the drain and the installation fails.
This guide covers the complete installation procedure including rough-in requirements, wall flange positioning, gasket sealing, anchor bolt placement, and final connection. It also covers the key differences between back outlet toilet types to confirm you have the right components before starting.
- Tools required: Adjustable wrench, drill with masonry or wood bits (depending on wall type), torpedo level, measuring tape, silicone caulk, channel-lock pliers, bucket, sponge.
- What this covers: Installing a back outlet toilet (rear discharge) on an existing in-wall drain connection. Applies to standard residential back outlet toilets including Saniflo rear discharge models, Caroma, and other rear-exit bowl designs.
- What this does NOT cover: Installing the in-wall drain rough-in from scratch (plumber required for new drain work), upflush macerating toilet systems (Saniflo SaniPLUS — different installation), or back outlet toilet installation on exterior walls (thermal bridging and freeze risk concerns apply).
- Experience level: Intermediate DIY. Requires comfort with drilling into walls and making watertight connections. The procedure is more precise than a standard floor-mount toilet installation.
Back outlet toilets are designed for specific drain centerline heights from the finished floor — typically 6 inches (standard) or 7 inches, depending on model. The drain rough-in in the wall must match the toilet’s outlet height exactly. Most homeowners purchase a rear outlet toilet first, then discover the wall drain rough-in is at a different height and the toilet outlet won’t align.
Measure the existing wall drain centerline height from the finished floor before purchasing any toilet. Then select a toilet model with a matching outlet centerline height. The measurement tolerance is ±¼ inch — the gasket and P-trap connector have no ability to compensate for a significant height mismatch.
Back Outlet Toilet Rough-In Requirements
Before purchasing a toilet or beginning installation, confirm all rough-in measurements against the existing wall drain position.
| Measurement | Standard Value | Where to Measure |
|---|---|---|
| Drain centerline height | 6″ from finished floor (most models) | Center of wall drain pipe to floor |
| Drain pipe diameter | 3 or 4 inches | Inside diameter of wall drain opening |
| Horizontal rough-in | Varies by model (measure from toilet spec) | Finished wall to center of drain opening |
| Anchor bolt height | Varies by toilet model (from spec sheet) | Bolt holes in toilet base to floor |
| Side-to-side centerline | 15″ minimum from side wall (ADA: 18″) | Drain center to nearest side wall |
Back Outlet Toilet Installation: 8 Steps
Step 1 — Shut Off Water and Remove the Old Toilet (If Replacing)
Turn the shut-off valve fully clockwise. Flush to empty the tank. Use a sponge to remove remaining tank and bowl water. Disconnect the supply line. On an existing back outlet toilet, disconnect the P-trap connector from the wall drain. On a standard floor-mount toilet being replaced with a back outlet model, the floor flange and drain will be decommissioned — cap the floor drain pipe with a test plug or permanent cap after removal. Clean all wall and floor surfaces at the installation area.
Step 2 — Install the Wall Drain Flange (If Not Already Present)
If the wall drain connection is an open pipe stub, a rear outlet toilet flange must be installed to provide a sealing surface for the gasket. The flange slides or glues into the wall drain pipe opening (use ABS or PVC cement appropriate for the pipe material). The flange face should be flush with the finished wall surface — not recessed or protruding. A recessed flange reduces gasket compression; a protruding flange prevents the toilet from sitting flush against the wall.
Step 3 — Mark and Drill the Anchor Bolt Holes
Position the toilet against the wall temporarily — do not connect to the drain yet. Using a torpedo level to confirm the toilet is plumb side-to-side, mark the anchor bolt hole locations through the base onto the floor (some back outlet toilets bolt to the floor; others bolt to the wall — check your model’s spec sheet). For floor anchor models: mark 4 points corresponding to the toilet base bolt holes. For wall anchor models: mark the bolt positions on the wall at the toilet’s specified height.
Drill the anchor holes using the appropriate bit for the floor material: carbide masonry bit for concrete slab, standard wood bit for wood subfloor. Install concrete anchors (Tapcon or equivalent) for concrete floors. Drive lag screws with wall anchors for stud-wall bolt patterns.
Step 4 — Install the Rear Outlet Gasket
The rear outlet gasket — a rubber or foam ring that creates the watertight seal between the toilet outlet and the wall drain flange — is the equivalent of the wax ring in a standard floor-mount installation. Unlike wax rings, most rear outlet gaskets are reusable and allow repositioning before final compression. Press the gasket onto the wall drain flange face (or onto the toilet outlet, depending on the model’s design). Confirm the gasket is centered on the flange opening with no gaps or folds.
Some back outlet toilets use a P-trap connector instead of or in addition to a gasket — a rubber coupling that connects the toilet outlet stub to the wall drain pipe, clamped on both ends. If a P-trap connector is specified, ensure both clamps are on the appropriate sides of the connection before tightening.
Step 5 — Position and Set the Toilet
Slide the toilet outlet stub into the gasket or P-trap connector while aligning the anchor bolt holes with the drilled positions. The toilet outlet must insert fully into the gasket/flange connection — a partial insertion will leak. Push the toilet firmly back against the wall until the outlet is fully seated and the toilet base is flat on the floor.
Confirm alignment: the toilet should be parallel to the finished wall with equal gaps on both sides, plumb from front to back, and level side to side. Use a level on the toilet rim to verify. Insert toilet shims under the base if needed to eliminate any rocking before bolting — the bolt tightening step cannot compensate for an unlevel base.
Step 6 — Secure the Toilet to the Floor or Wall
Thread the anchor bolts or lag screws through the toilet base into the drilled holes. Hand-tighten all fasteners first to bring the toilet base down evenly. Then tighten with a wrench — alternating sides, ¼ turn per side — until the toilet has zero movement when pushed in any direction. Stop immediately if the base begins to flex or crack. The toilet base should feel rigid, with no rocking on any side. Tighten the P-trap connector clamps (if applicable) to 15–20 inch-pounds — firm but not overtightened.
Step 7 — Connect the Tank and Water Supply
For two-piece models, set the tank onto the bowl and secure with tank bolts — hand-tight plus ¼ turn with a wrench, alternating sides. Attach the flush valve connection between tank and bowl per the manufacturer’s instructions. Connect the water supply line to the fill valve inlet at the bottom of the tank — hand-tight plus ¼ turn with a wrench. Do not use thread tape on compression fittings; use it on threaded male connections only.
Step 8 — Test for Leaks and Confirm Drainage
Open the shut-off valve slowly. Allow the tank to fill completely (60–90 seconds). Inspect all connections for drips: the supply line connection at tank bottom, the tank-to-bowl connection, and — critically — the rear outlet gasket/P-trap connection at the wall. The wall connection is the highest-risk leak point; inspect it for weeping or dripping during the first 3 flushes. Flush 5 times total and observe the wall connection throughout. No moisture at the wall connection after 5 flushes confirms a correct seal.
Red Flags: Stop and Reassess
If the toilet outlet centerline is more than ¼ inch off from the wall drain centerline (horizontally or vertically), do not force the connection. Forcing a misaligned outlet into the gasket creates a partial seal that will leak within days. If the height is wrong, the toilet model is incompatible with the existing rough-in — a different model with the matching outlet height must be sourced, or the wall drain rough-in must be modified (plumber required).
Any moisture at the toilet outlet-to-wall connection after flushing indicates a failed seal. Do not continue using the toilet. Shut off the water supply, remove the toilet, inspect the gasket for damage or misalignment, and reinstall with a new gasket. A leaking rear connection routes sewage-contaminated water directly into the wall cavity — causing structural damage and mold growth that remains hidden for months.
Back outlet toilets require a P-trap in the drain system — either built into the toilet itself (most modern rear outlet designs include an integral trap) or in the wall drain connection. If the wall drain stub has no P-trap and the toilet has no integral trap, the installation will allow sewer gas to enter the bathroom directly. Confirm trap presence before proceeding. If neither the toilet nor the wall drain includes a trap, a trap must be added to the wall connection — a plumber repair.
What Most Back Outlet Toilet Guides Miss
Most installation guides describe back outlet toilets as if all models have the same outlet height. They don’t. American Standard back outlet models typically have a 6-inch outlet centerline from finished floor. Caroma models vary from 5½ to 7 inches depending on series. Some European-specification rear outlet toilets designed for S-trap connections have outlet heights of 9–11 inches. Installing a toilet with the wrong outlet height for the existing rough-in requires either a different toilet model or a plumber to modify the wall drain rough-in. This single measurement — the drain centerline height — must be confirmed before any toilet purchase.
A back outlet toilet’s drain exits horizontally through the wall rather than vertically through the floor. The horizontal drain then connects to a vertical stack or runs horizontally to an existing drain system. Horizontal drain runs for back outlet toilets require the same ¼-inch-per-foot slope as any horizontal drain — but the direction of slope is away from the toilet (toward the stack), not toward the toilet. If the wall drain rough-in was installed with insufficient slope or with the pipe running uphill toward the stack, the toilet will drain poorly regardless of how well it’s installed. Confirm the drain slope before installation.
Standard ADA requirements specify toilet seat height at 17–19 inches from finished floor. Most standard floor-mount ADA-compliant toilets achieve this naturally. Back outlet toilets have a slightly different base geometry — the toilet sits on a floor-level base with the drain exiting at the rear. ADA-compliant back outlet models exist (most Caroma and American Standard rear outlet models are ADA height) but must be specifically selected. A standard residential back outlet toilet may have a seat height of 15–16 inches — below ADA requirements. If ADA compliance is required, confirm seat height on the toilet specification sheet before purchase.
A back outlet toilet requires an existing wall drain rough-in at the correct height — or the installation of one by a plumber. If no wall drain exists and the floor cannot be cut (slab, finished basement), the correct solution is not a back outlet toilet. It’s a macerating upflush toilet system (Saniflo SaniPLUS or equivalent) that pumps waste upward through a small-diameter pipe to an existing drain connection above. Macerating toilets can connect to any existing drain within 15 feet horizontally and 9 feet vertically — making them the practical choice when no rear drain rough-in exists.
Back outlet toilets are the right choice when the wall drain rough-in already exists at the correct height. Macerating systems are the right choice when no suitable drain is accessible. Confusing the two leads to purchasing an incompatible toilet.
Frequently Asked Questions: Back Outlet Toilet Installation
What is a back outlet toilet?
A back outlet toilet — also called a rear discharge or rear outlet toilet — is a toilet where the drain exits horizontally through the back of the toilet base into a wall drain connection, rather than vertically downward through a floor flange. This design is used in installations where a floor drain is not accessible or not permitted: finished basements, concrete slab floors, apartment renovations, and renovations where cutting the finished floor is not feasible. The installation requires a wall drain rough-in at the correct height and a rear-specific gasket connection instead of a wax ring.
Does a back outlet toilet need a wax ring?
No. Back outlet toilets do not use a wax ring. The wax ring is specific to floor-mount toilets that seal a vertical toilet outlet to a floor flange. A back outlet toilet seals its horizontal outlet to a wall drain flange using a rubber gasket or a P-trap connector with compression clamps. These components are included with most back outlet toilet models. If they’re not included, they must be purchased separately based on the specific outlet diameter and wall drain fitting type of your toilet model.
Can I install a back outlet toilet myself?
Yes — if the wall drain rough-in already exists at the correct height and the installation does not require new drain work. Connecting an existing back outlet toilet rough-in to a new toilet requires intermediate DIY skills: comfort drilling into walls or floors for anchor bolts and making a watertight gasket connection. The measurement precision required (outlet height matching to ¼ inch) is the most common challenge. If the installation requires new drain rough-in work — cutting into the wall to add a drain connection — a licensed plumber must perform that portion, as drain work requires permits and inspection in most jurisdictions.
What is the rough-in height for a back outlet toilet?
The rough-in height for most residential back outlet toilets — measured from the finished floor to the center of the wall drain opening — is 6 inches. Some models specify 7 inches. European-specification S-trap rear outlet toilets may specify 9–11 inches. The toilet manufacturer’s installation specification sheet lists the required rough-in height. Measure the existing wall drain centerline height from your finished floor before purchasing a toilet to confirm compatibility. A ½-inch mismatch between toilet outlet and rough-in height cannot be corrected without either a different toilet model or drain modification.
How do you seal a back outlet toilet to the wall?
The seal between a back outlet toilet outlet and the wall drain is made with a rear outlet gasket — a rubber or neoprene ring that compresses between the toilet outlet face and the wall drain flange face when the toilet is pushed against the wall and secured with anchor bolts. Some models use a P-trap connector — a rubber coupling sleeve that slides over both the toilet outlet stub and the wall drain pipe stub, then is clamped watertight on both ends. The gasket approach requires the toilet outlet and wall flange to be at the same height. The P-trap connector approach allows a small amount of height and horizontal offset tolerance (typically ±½ inch).
Where can I buy a back outlet toilet flange?
Rear outlet toilet flanges are available from plumbing supply houses and online retailers. They are specified by the pipe diameter (3-inch or 4-inch) and material (PVC, ABS, or cast iron for cast iron pipe connections). Standard plumbing supply houses stock PVC rear outlet flanges for PVC drain systems. For cast iron existing drain connections, a rubber fernco coupling to a PVC flange extension is the most practical approach. Most home improvement stores (Home Depot, Lowe’s) carry basic versions; plumbing supply houses carry a wider range of fitting types for specific drain configurations.
- If a wall drain rough-in exists at the correct height — a back outlet toilet installation is an intermediate DIY project. Measure the drain centerline height first, match it to the toilet specification, use the correct gasket or P-trap connector, and test for leaks at the wall connection after every flush for the first 5 uses.
- If no wall drain exists and the floor cannot be cut — a back outlet toilet is not the correct solution. A macerating upflush system is the right approach for that situation.
- If the wall drain rough-in needs to be created or modified — this requires a licensed plumber and a permit. Budget $400–$900 for the rough-in work before the toilet installation cost.
Back Outlet Toilet vs. Floor Outlet Toilet: Which Is Right for Your Situation
The choice between a rear outlet and floor outlet toilet is determined by what drain connections exist or can feasibly be created in your bathroom — not by aesthetic preference. In most residential installations, a standard floor outlet toilet is simpler, less expensive, and easier to maintain. The back outlet design is the solution when a floor drain is genuinely inaccessible.
If you’re choosing between the two options for a new build or renovation with an open floor, choose a floor outlet toilet. Floor drain connections are simpler to install, use standard wax ring seals that every plumber knows, and have wider toilet model selection. Rear outlet toilets are worth the additional complexity only when the floor truly cannot be penetrated.
For basement bathroom installations specifically, compare the cost of a back outlet toilet rough-in ($400–$900 plumber work) versus an upflush macerating system ($800–$1,200 installed). In many basement situations, the macerating system costs less despite the higher unit cost because it eliminates the drain rough-in labor entirely. See best Saniflo toilet guide for macerating system options evaluated for residential basement use.
Back Outlet Toilet Gasket: Selection and Replacement
The rear outlet gasket is the component most likely to need replacement over the toilet’s life — typically every 10–15 years, or whenever the toilet is removed for any reason. Unlike a wax ring, which permanently deforms on installation, most modern rear outlet gaskets can be reused if the toilet is carefully removed without damaging the gasket. However, best practice is to replace the gasket whenever the toilet is removed from the wall connection.
Gasket selection requires matching three dimensions: the toilet outlet outer diameter, the wall flange inner diameter, and the mounting face dimensions. Most US-specification back outlet toilet gaskets are standardized for 3-inch connections. European-specification rear outlet toilets often use metric dimensions that require manufacturer-specific gaskets. Always purchase the gasket from the toilet manufacturer’s parts system when replacing — third-party gaskets that claim universality frequently have dimensional inconsistencies that create leaks within 6–12 months of installation.
Back Outlet Toilet Installation Cost: DIY vs. Plumber Breakdown
The total cost of a back outlet toilet installation depends on whether the wall drain rough-in already exists and whether new drain work is needed:
| Component | DIY Cost | Plumber Cost |
|---|---|---|
| Back outlet toilet (mid-range) | $200–$500 | $200–$500 (part only) |
| Gasket / P-trap connector | $15–$45 | Included with install |
| Installation labor (existing rough-in) | $0 (DIY) | $150–$300 |
| New drain rough-in (if needed) | Not DIY — permit required | $400–$900 additional |
For a typical replacement of an existing back outlet toilet on an existing rough-in, total DIY cost is $215–$545. Professional installation on an existing rough-in runs $350–$800. The DIY savings ($135–$255) are worthwhile for anyone comfortable with the measurement precision and watertight connection requirements described in this guide. See the toilet installation cost guide for a complete breakdown of all toilet installation types and current labor rates by region.
