The Plumbing Rough-In: What to Lock In Before the Walls Close

What the rough-in actually covers

Rough-in is the first fix. It installs everything that ends up hidden: hot and cold supply, the drainage and its venting, gas where a hot water unit or appliance is affected, and the in-wall components that people forget are components at all. Concealed cistern frames, in-wall shower mixer bodies, bath filler bodies and wall breeches are all rough-in items, and they are all brand-specific.

It is also the stage where the work gets tested. Supply is pressure tested and the drainage is tested before anything covers it, because a leak found behind a sheet of villaboard costs an hour and the same leak found behind finished tiling costs a fortnight.

Fit-off is the second fix, after tiling. Pans, vanities, tapware, shower fittings, wastes and seals go on, everything is connected and tested again, and the room becomes usable. Fit-off is flexible. Rough-in is not, and the difference between the 2 stages is where nearly every bathroom renovation dispute starts.

The sequence, and where projects come off the rails

1. Strip out and demolition, back to structure.
2. Mark the layout in chalk on the bare floor and walls, then stand in it. Doors, screens and drawers all need to open.
3. Confirm the finished floor build-up with the tiler and the waterproofer, as a number in millimetres.
4. Order the fixtures and collect the manufacturer set-out sheets for every one of them.
5. Rough-in: drainage and venting first because gravity dictates them, then supply, then gas.
6. Noggins and blocking for wall-hung pans, vanities, grab rails and the shower screen.
7. Pressure test and drainage test. Photograph every run with a tape measure in frame before anything covers it.
8. Sheeting, waterproofing to AS 3740, screed, then tiling.
9. Fit-off: pans, vanities, tapware, wastes and seals, then a final test.
10. Certificate of Compliance issued and filed with the house papers.

Steps 3 and 4 are the ones that get skipped, and they are the ones that cause the damage. A rough-in cannot legitimately begin until the fixtures are chosen, because half the dimensions come off the product sheets rather than out of a book. Ordering the vanity in week 6 because the plumber has already been and gone is how a project ends up with a waste 90mm from where the new vanity needs it.

Finished floor level: the mistake that hurts the most

Every set-out height in a bathroom is measured from finished floor level, which means the top of the finished tile. It does not mean the slab, and it does not mean the sheeting. Between those 2 datums sits the screed laid to fall, the waterproofing membrane, the tile adhesive bed and the tile itself, and that build-up commonly runs from 25mm to 60mm depending on how much fall has to be created.

Set a floor waste to the slab instead of to finished level and it finishes buried under the screed. Set a shower drain high and water ponds against the screen for the life of the room. Set a toilet connector at the wrong height and the pan either rocks or never seals. None of those are fixable without lifting tiles, which is why the build-up figure needs to be agreed in writing between the tiler, the waterproofer and the plumber before the rough-in starts.

Falls, venting and the numbers the standard sets

Drainage grades are set by AS/NZS 3500.2, not by preference. A DN100 sanitary drain is graded at roughly 1 in 60, and smaller branches at DN65 and DN80 at roughly 1 in 40. Those are minimums and they exist for a reason that surprises people: too flat and solids strand in the pipe, but too steep and the water outruns the solids and leaves them behind. Both errors block, and the second one blocks in a way that is very hard to diagnose later.

Venting is the half of drainage nobody discusses at the design stage. Every trap needs air behind the flow. Without it, a discharge upstream pulls the trap seal out of a nearby fixture, and the room then smells of sewer whenever the laundry runs. The gurgle people report from a new bathroom is almost never a blockage: it is a trap being siphoned because the venting was compromised to fit the pipework around something else.

Air admittance valves have a legitimate place where a conventional vent cannot be run, but they are not a universal substitute for one, they must be accessible, and they must sit in a ventilated space rather than sealed inside a wall cavity forever. A design that leans on them at every fixture usually means the drainage layout needs another look.

Slab or timber floor: what it changes

Adelaide splits cleanly on this, and it shapes a renovation more than the tile selection does. The pre-1950s inner-south housing stock, the villas and bungalows through Goodwood, Unley, Malvern and Norwood, generally sits on bearers and joists with a suspended timber floor. The postwar brick-and-tile belt from the 1960s onwards is concrete slab on ground.

On a suspended timber floor, moving a toilet or a shower means working from underneath and re-routing the waste between joists. It is real work, but it is not destructive and it does not stop the program. On a slab, the same move means saw-cutting the concrete, breaking it out, excavating, re-laying the drain to grade, backfilling, re-pouring and waiting for the pour to be workable. That is a different job, at a different price, on a different timeline.

If the slab is being opened regardless, take the opportunity that comes with it. Adelaide sits on reactive clay soils that swell and shrink seasonally, and slabs and the drains beneath them move with them. With the drain exposed, a camera run through the rest of the line is the easiest inspection that property will ever offer, and it is the moment to find out whether the section you are not touching is about to become a problem.

When you do not need a rough-in at all

Plenty of bathroom renovations do not need a rough-in at all. If every fixture stays in its current position and the pipework behind the wall is sound, there is no rough-in. The job is a strip out, waterproofing, tiling and a fit-off, and it costs a fraction of a relocated layout.

• Swapping a vanity for another of the same footprint: fit-off.
• Replacing tapware with the same brand and body: fit-off.
• Replacing a toilet with a pan of the same set-out and trap type: fit-off.
• New shower screen, new mirror, new floor and wall tiles with the fixtures unmoved: no plumbing rough-in, but the waterproofing still has to be redone properly.

There are 4 situations where the rough-in becomes worth doing even though nothing is moving, and all of them turn on what is already in the wall. Original galvanised supply pipe, because the wall will never be this accessible again. Lead-jointed or old clay waste pipework. A bathroom with no floor waste. Fixtures that were never properly vented, which is the cause of a room that has always smelled and always gurgled.

Compliance, inspection and the paperwork to keep

Sanitary plumbing and drainage work in South Australia is licensed work. It has to be carried out by a licensed plumber, and a Certificate of Compliance is issued for it at completion. That certificate is not a formality: it is the document that answers a building inspector, an insurer or a buyer's conveyancer years later, and it belongs with the house papers rather than in a drawer in the shed.

Development approval is a separate question and the answer is usually reassuring. An internal bathroom renovation kept within the same footprint, with no structural alteration, generally does not require development approval, though your council is the authority on your specific property and is worth a phone call. The plumbing remains licensed work either way, whether approval is needed or not.

• Sheeting, screed and tiles go on only after the pressure test and the drainage test are done.
• Photograph every pipe run with a tape measure visible before it is covered. That set of photos is worth more than the rest of the file combined the first time someone drills into a wall.
• Keep the waterproofing certificate and warranty. Waterproofing to AS 3740 is a separate trade with separate paperwork, and it is the most common source of a failed bathroom.
• Keep the tapware model numbers with the certificates, because the cartridge that fails in 12 years has to match a product that may no longer be on any shelf.

Questions we get asked

References

• Australian Building Codes Board: National Construction Code, Plumbing Code of Australia
• Standards Australia: AS/NZS 3500 Plumbing and drainage, AS 3740 Waterproofing
• Consumer and Business Services SA: plumbing licensing and compliance