Planning a Bathroom Renovation? The Plumbing and Waterproofing Decisions to Make First
Most of what goes wrong in a bathroom renovation is decided before the first tile goes on. Once the floor is screeded and the membrane is down, moving a toilet or resizing a pipe means pulling the room apart again. The plumbing and waterproofing stages are the two you cannot revisit cheaply, and they both happen early.
This guide covers what to settle before demolition, which parts of the job legally need a licensed plumber, the waterproofing rules that apply in Victoria, and the order the work has to run in.
In short
- Fixture positions, pipe sizing and floor falls are locked in at rough-in. Decide them before anyone starts demolition.
- Regulated plumbing work must be done by a plumber licensed or registered with the Victorian Building Authority.
- Plumbing work totalling $750 or more comes with a compliance certificate, and your plumber has to issue it to you.
- A shower needs a floor waste, and the fall to it has to sit between 1:80 and 1:50.
What to Settle Before Demolition Starts
Three decisions set the cost and the timeline of the whole job. Make them on paper, with your plumber in the room, before anything comes off the wall.
Where the Fixtures Go
Keeping the shower, bath, basin and toilet roughly where they already are is the cheapest version of a bathroom renovation. Every metre you move a fixture adds pipework, and in some cases it adds floor work as well, because waste pipes need a continuous fall towards the drain.
Measure the room and mark the positions on the floor in chalk before you commit. A layout that looks fine on a plan can leave a door swinging into a vanity or a shower screen landing on a floor waste.
Whether You Are Moving the Toilet
This one deserves its own decision because it is the single most expensive change in a bathroom. A toilet needs a large-diameter waste with fall to the sewer, so relocating it usually means cutting the slab or lifting the floor, then reinstating both.
On a ground-floor slab in a Melbourne home the pipe is often set in concrete. On a timber-floored period home there is more room to work, but the joists set where the pipe can run. Either way, ask your plumber what the new position costs before you fall in love with the layout.
Pipe Material and Sizing
If the bathroom is being stripped back anyway, this is the one chance to replace old supply pipework at a fraction of what it would cost later. Galvanised steel pipe in homes built before the 1980s corrodes from the inside and narrows, which is a common cause of poor shower pressure that no new tapware will fix.
Sizing matters if you are adding a second shower head or a bath filler. Two outlets running off a pipe sized for one will leave both of them weak. That is a conversation to have at the planning stage, not after the tiles are on. Our page on water pressure problems covers how undersized pipework shows up day to day.
Who Is Allowed to Do What
A bathroom renovation is two regulated trades sitting next to each other, and they have separate rules.
Plumbing Work
Regulated plumbing work in Victoria has to be carried out by a plumber licensed or registered with the Victorian Building Authority. That covers water supply, sanitary drainage, the toilet connection, and anything involving gas if the bathroom has a gas appliance in it.
Licensed and registered plumbers carry a photo identification card that lists the classes of work they are eligible to do. Asking to see it takes ten seconds and tells you whether the person in your bathroom is allowed to do the work they are quoting on.
The Compliance Certificate
Where plumbing work on your bathroom totals $750 or more, a compliance certificate has to be issued. This is the plumber’s obligation rather than something you have to request, but confirming it is coming is worth doing at the quote stage rather than afterwards.
Why the Certificate Is Worth Having
It records that the work met the standards, it identifies who did it, and it carries warranty protection on the plumbing. If a connection fails in two years, the certificate is what makes that someone else’s problem. It also matters at resale, when a buyer’s conveyancer asks what work has been done to the property.
If One Is Not Offered
Treat it as a warning about the quote rather than a piece of paperwork to chase later. A plumber who does not mention a certificate on a job over the threshold is either not licensed for the work or not intending to lodge it, and both of those are reasons to get another quote.
Building Work
The tiling, the waterproofing and any changes to walls sit under building rules rather than plumbing rules. Work costing more than $10,000 that involves structural changes needs a registered building practitioner, and moving a wall to enlarge a bathroom can put you in that category quickly.
The Victorian Building Authority keeps a public directory of registered practitioners, so you can check a builder the same way you check a plumber.
The Waterproofing Rules in Victoria
Waterproofing is where bathroom renovations fail years after they finish, and the rules exist because the repair is so disruptive. A membrane that leaks means removing tiles, re-waterproofing the recess and re-tiling, which is most of the original job again.
The Standard That Applies
Domestic wet areas are waterproofed to AS 3740, Waterproofing of domestic wet areas, used alongside the Australian Building Codes Board Housing Provisions. Membrane products themselves are classified under AS/NZS 4858. Those two documents decide what goes where, how high it goes up the walls, and which materials are acceptable behind the tiles.
What this means in practice is that a membrane is not simply a coat of something applied before tiling. The product has to be right for the location, and the detailing at junctions, penetrations and the shower hob is where compliance is won or lost.
The Shower Floor Waste and the Fall
Two requirements worth knowing, because they are easy to check and they are often where problems start.
A floor waste has to be installed in the shower area. And the fall of the floor towards that waste has to be at least 1:80 and no steeper than 1:50. Too flat and water sits on the tiles instead of draining, which is the usual explanation for a shower that never dries out and a grout line that keeps going dark. Too steep and the floor feels wrong underfoot and the screed can crack.
You can check this yourself once the screed is down and before tiling begins. Pour a litre of water in each corner of the recess and watch where it goes. Anywhere it pools is somewhere it will pool for the next twenty years.
Where Membranes Actually Fail
Rarely in the middle of a wall. Almost always at a junction: the floor to wall corner, around the tap and shower penetrations, at the hob, and where the membrane terminates at a door. Those are the points to watch being done, and to photograph before the tiler arrives.
Membranes also have a service life, which is why bathrooms renovated once in the 1990s are leaking now. If you are dealing with that rather than planning new work, our guide on why a shower leaks into the room below covers the diagnosis, and the leaking balcony guide covers the same failure in an apartment setting.
The Order the Work Has to Run In
The sequence is not flexible. Each stage covers the one before it, which is why a change of mind at stage four costs so much more than the same change at stage one.
Stage by stage
- Plan and quote. Fixture positions marked, pipe sizing agreed, compliance certificate confirmed, and the question of a building permit settled.
- Demolition. Tiles, fixtures and old pipework out. This is often where surprises appear, such as a rotted floor or a pipe nobody knew ran through the wall.
- Plumbing rough-in. Supply pipes and waste set in their final positions. After this point, moving a fixture means opening walls again.
- Screed and floor falls. The fall to the shower waste is set here. Check it before anyone moves on.
- Waterproofing. Membrane applied and left to cure. Photograph the junctions now, because this is the last time anyone will see them.
- Tiling. Everything underneath is now permanent.
- Fit-off and testing. Tapware, shower, toilet and vanity connected, pressure tested, and the compliance certificate issued.
What Actually Drives the Cost
Two bathrooms of the same size can differ by a large margin, and it is rarely the tapware that explains it. These are the factors that move the number.
Moving the toilet
A new waste position with the right fall, which on a slab means cutting and reinstating concrete.
Replacing supply pipework
Worth doing while the walls are open on an older home, but it adds to the rough-in stage.
What demolition uncovers
Rotted flooring, a failed older membrane, or pipework that is not where the plan assumed it was.
Adding a second outlet
A twin shower head or a bath filler can mean upsizing the pipe feeding the room.
Hot water capacity
A bigger bathroom with more outlets can outgrow the existing hot water system.
Tapware and fittings
Visible, easy to compare, and the part most people focus on first. It moves the total least.
The Mistakes That Cost the Most
- Buying fixtures before the layout is final. A vanity bought on sale can dictate a waste position that does not suit the room.
- Letting the tiler start before the screed is checked. The fall is far easier to correct while it is still screed.
- Skipping photographs of the membrane. If a leak appears later, those photos are the only evidence of how the junctions were detailed.
- Not confirming the compliance certificate at quote stage. It is much harder to obtain once the job is finished and paid for.
- Keeping old supply pipework to save money. Replacing it later means opening the new tiling you just paid for.
- Treating the exhaust fan as optional. Moisture that cannot leave the room shortens the life of everything in it, membrane included.
Talk to a Licensed Plumber in Melbourne
If you are planning a bathroom renovation and want the plumbing scoped before demolition, Plumb Construct can walk the room with you, tell you what each layout option costs, and handle the rough-in and fit-off. We are licensed, we work across Melbourne, and every job over the threshold comes with its compliance certificate.
Related Plumbing Services
Frequently Asked Questions About Bathroom Renovation Plumbing
Do I need a licensed plumber for a bathroom renovation?
Yes, for the plumbing. Regulated plumbing work in Victoria has to be carried out by a plumber licensed or registered with the Victorian Building Authority, which covers the water supply, the waste and drainage, the toilet connection and any gas work. Licensed and registered plumbers carry a photo identification card listing the classes of work they are eligible to do, so you can check before the job starts.
Will I get a compliance certificate for my bathroom plumbing?
You should, where the plumbing work totals $750 or more. Issuing it is the plumber’s obligation rather than something you need to ask for, but it is worth confirming at the quote stage. The certificate records that the work met the standards, identifies who carried it out, and carries warranty protection. It also answers the question a buyer’s conveyancer will ask if you sell.
What waterproofing standard applies to a bathroom in Victoria?
Domestic wet areas are waterproofed to AS 3740, Waterproofing of domestic wet areas, used alongside the Australian Building Codes Board Housing Provisions, with membrane products classified under AS/NZS 4858. Those documents set what membrane goes where, how far up the walls it extends, and how junctions and penetrations have to be detailed.
Does a shower need a floor waste, and how much fall should it have?
A floor waste has to be installed in the shower area, and the fall of the floor towards it has to be at least 1:80 and no steeper than 1:50. Too flat and water sits on the tiles rather than draining, which is the usual reason a shower never dries out. You can check it yourself once the screed is down by pouring water into each corner and watching where it goes.
What is the most expensive change in a bathroom renovation?
Moving the toilet. It needs a large-diameter waste with continuous fall to the sewer, so a new position usually means cutting into a concrete slab or lifting a timber floor and then reinstating it. Keeping the toilet where it is and rearranging everything else around it is almost always the cheaper layout.
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