Introduction

Converting a WC from floor mounted to back outlet toilet looks simple on paper—swap the pan and shift the pipe. In the real world, it’s about alignment, gradients, venting, and finishing without a recall. This guide shows you exactly how to deliver a clean, compliant conversion that flushes right first time and looks like it was always there. We’ll cover planning, drainage, wall outlet build-out, installation, testing, and handover. Capture site notes and photos, then generate a sharp proposal in minutes with Donizo.

Quick Answer

Quick Answer: Converting from floor mounted to back outlet toilet means relocating the WC discharge from the floor (S-trap) to the wall (P-trap), aligning a 110 mm wall stub at the manufacturer’s set-out height, ensuring correct gradient and venting, capping the floor outlet, and installing the new back-outlet pan to spec.

Table of Contents

• Quick Answer
• Key Takeaways
• Planning the Conversion
• Drainage, Venting and Compliance
• Building the Wall Outlet and Closing the Floor
• Installing the Back-Outlet Pan and Cistern
• Testing, Handover and Making Good
• FAQ
• Conclusion

Key Takeaways

• Most back-outlet centres land between 180–210 mm above finished floor—confirm from the pan’s spec sheet before cutting.
• Keep the 110 mm branch at a fall around 1:40 to 1:80; steeper often causes solids to stall.
• Dry-fit the pan, connector, and cistern together and mark fixings before you glue anything.
• Choose the right connector (rigid or flexi-rated) and avoid stacking tight 90s; use two 45s where possible.
• Photograph set-outs and gradients; build your proposal and get e-signature approval via Donizo to reduce back-and-forth.

Planning the Conversion

Switching from floor mounted to back outlet toilet starts with a tight survey. If you get the set-out wrong by even 10–15 mm, you’ll fight the seal and risk leaks.

Scope and Assessment

• Identify existing WC type: floor outlet (S-trap) vs wall outlet (P-trap). You’re moving to a back-outlet P-trap pan.
• Trace the soil route. Can you run a 110 mm branch to a vertical stack in the same wall, or do you need to chase/box-out?
• Check wall construction: solid, stud, or service duct. Rear discharge pans don’t need a carrier frame (that’s for wall-hung), but you do need a solid fix and clear set-out.
• Verify space: Many back-outlet pans need 20–40 mm additional rear clearance for the connector.
• Water supply: Plan isolation and a 15 mm supply point at the cistern inlet height (commonly 150–300 mm left or right of centre—follow the cistern spec).

Tools and Materials

• 110 mm soil pipe, bends (two 45s preferred), adjustable pan connector rated for back outlets
• Hole saw/core bits for masonry; multi-tool/diamond blades for tile; SDS if required
• Pan floor fixings, silicone, fast-set mortar/levelling compound for floor patch
• Compression or push-fit couplers, 15 mm isolation valve, PTFE tape, WRAS-approved fill valve
• Test plugs, dye, spirit level (600 mm), angle finder, laser or tape, marker

Quick Comparison

Drainage, Venting and Compliance

A neat pan is worthless if it gurgles or backs up. Get the drainage right.

Gradients and Bends

• Set fall on the 110 mm horizontal run between 1:40 and 1:80. Too flat clogs; too steep lets water outrun solids.
• Use two 45-degree bends instead of a single tight 90 where possible. Keep total changes of direction minimal.
• Keep the outlet stub length short enough to prevent sag but long enough for a full connector seal—often 40–80 mm engaged.

Venting and Traps

• Ensure the system is vented per Building Regulations Part H. If the run is long or has multiple bends, consider an air admittance valve (AAV) where permitted.
• Maintain trap seal depth as specified by the pan; avoid negative pressures that can pull the seal.

Standards and Water Fittings

• Use pans and cisterns conforming to BS EN 997 and WRAS-approved valves.
• Part G: Dual-flush performance and water efficiency (commonly 6/4 litre or 4.5/3 litre systems).
• Water Supply (Water Fittings) Regulations 1999: Backflow protection, isolation valves, and correct materials.

Tip: Many contractors find that marking the intended outlet centre at, say, 185 mm, then adjusting +/- 10 mm during dry-fit avoids costly rework.

Building the Wall Outlet and Closing the Floor

This is the make-or-break stage when going from floor mounted to back outlet toilet.

Set-Out and Cutting

• Mark finished floor level (FFL) and the pan centreline on the wall.
• From the spec sheet, mark the outlet centre height—commonly in the 180–210 mm range above FFL.
• Cut or core for the 110 mm wall penetration. Allow for tile thickness (often 8–12 mm) and adhesive (3–5 mm).

Pipework and Connection

• Install the 110 mm branch to the stack with the correct gradient. Measure fall over the run (e.g., 10–25 mm per metre depending on chosen ratio).
• Offer up the rear outlet stub. Dry-fit the pan connector to check depth. Aim for a straight, unstressed connection.
• If the old floor outlet ties into the same branch, cap and seal at the floor level with a solvent-weld cap or permanent blank. Mortar infill typically 25–50 mm deep, then level.

Closing the Floor

• Remove the floor flange and patch the penetration. Use a suitable backer, fast-set mortar, and leveller. Aim for less than 3 mm deviation under the pan footprint.
• Watch out for underfloor heating loops—scan if unsure. Many contractors have drilled straight into a pipe here. Don’t be that story.

Installing the Back-Outlet Pan And Cistern

Now you turn a plan into a working, tidy installation.

Dry-Fit First

• Position the pan, connector, and cistern. Confirm outlet alignment and that the connector isn’t in torsion.
• Mark fixing holes. Typical pan fixings sit 180–220 mm apart; check the actual pan.
• Verify the water supply tail reaches the fill valve without strain; add a 300–500 mm braided hose if needed.

Fixing and Sealing

• Drill fixings with the correct bit for tile/stone. Use sleeves and stainless screws. Tighten evenly—snug, not over-torqued.
• Fit the connector per manufacturer’s guidance. Lubricate seals if specified; avoid petroleum greases on rubber.
• Silicone the pan perimeter with a neat 3–5 mm bead; leave a small unsealed gap at the rear to spot leaks during testing.

Cistern and Controls=

• Fit a WRAS-approved inlet valve. Set fill height per spec to achieve the designed flush (e.g., 6/4 L or 4.5/3 L).
• Level the cistern, connect the flush, and check for weeps at 1–3 bar typical mains pressure. Isolate if boosted systems exceed this.

Finishing Clearances

• Maintain 10–15 mm clearance behind the pan for service if the design allows.
• Ensure front projection complies with client needs and room layout—avoid door clashes.

Pro move: Save 20–30 minutes on documentation by snapping final set-out photos and drop-in voice notes, then producing a branded PDF for sign-off via Donizo. Convert it to an invoice in one click when the client approves.

Testing, Handover and Making Good

A leak-free, quiet flush is your best marketing.

Functional Tests

• Flush dye test: Add food dye to the cistern and check the connector and wall joint for colour traces after 5–10 minutes.
• Multiple flushes: Run 6–10 flushes back-to-back to verify clear discharge and stable trap seal (no gurgling).
• Paper test: Use a realistic wad. Watch for slow clears—recheck gradient if sluggish.

Compliance and Documentation

• Record outlet centre height, gradient (e.g., 1:60), and materials used. Note any AAV placement.
• Provide user guidance on dual-flush and isolation valve location.
• Include product datasheets and warranties in your handover pack. Many contractors attach these to the client portal via Donizo for easy access.

Making Good

• Seal the remaining perimeter once tests pass.
• Touch-up walls or skirting; box out any new service runs with removable panels where appropriate.
• Clean down thoroughly—first impressions matter when the lid opens.

FAQ

Can I convert a floor outlet WC to a back outlet toilet without moving the stack?

Often, yes. If you can run a 110 mm horizontal branch with correct fall to an existing vertical stack in the same wall or adjacent void, you can convert from floor mounted to back outlet toilet. The key is set-out height, gradient (around 1:40 to 1:80), and maintaining venting per Part H. Always verify manufacturer dimensions before cutting.

What height should I set the wall outlet for a rear discharge pan?

Most back-outlet pans expect the centre of the 110 mm outlet between 180–210 mm above finished floor. Always use the pan’s specification—some brands sit slightly higher or lower. Dry-fit the pan and connector, account for tile and adhesive thickness, and adjust before fixing. A 10 mm misalignment can compromise the seal.

Do I need a carrier frame for a back outlet toilet?

Not for a floor-mounted back-outlet toilet. Carrier frames are for wall-hung pans that need structural support. For a floor-mounted rear discharge pan, you anchor to the floor and align the wall outlet. You still need a solid wall penetration, correct gradient, and a reliable pan connector.

How long does the conversion take on site?

In general, a straightforward conversion from floor mounted to back outlet toilet without major rerouting takes about 1–2 days: half a day for demolition and pipework, half for set-out and installation, and the remainder for testing and making good. Add extra time for structural alterations, retiling, or access constraints.

What’s the most common mistake in these conversions?

Misaligned outlet height or an over-steep branch. Both cause sealing issues and poor flush performance. Dry-fit the pan and connector, confirm the centre height from the spec, and set the fall between roughly 1:40 and 1:80. Avoid stacked tight 90s—use two 45s and keep runs supported.

Conclusion

Delivering a clean conversion from floor mounted to back outlet toilet comes down to five things: the right outlet height (typically 180–210 mm), correct fall (around 1:40–1:80), minimal bends, solid fixings, and thorough testing. Photograph set-outs, note gradients, and lock down approvals fast. Capture site details by voice, photos, and text, then generate a branded proposal and e-signature-ready PDF with Donizo. When it’s accepted, convert to an invoice in one click and move on to the next job.