Bathroom renovation plumbing: the decisions, in the order you have to make them

A bathroom renovation goes wrong in a predictable way. Everything is chosen on looks, the walls get closed up, and then the plumbing has to fit around decisions that were made months earlier by someone who was not thinking about pipes. The tiles are on by the time anyone finds out.

The order below is roughly the order the decisions actually arrive. If you get them in this sequence, most of the expensive surprises never happen.

When does the plumber turn up, and how many times?

Three visits, usually, and they are not close together.

The first is the cap-off. Everything old comes out, the pipes are capped safely, and the room is left ready for the builder to strip and rebuild. This is a short visit and it is what lets demolition happen.

The second is the pre-line. The room is framed and open, and every pipe and waste is run and fixed into position: shower mixer at the right height, waste in the right place, toilet pan connector where the pan will actually sit. Everything gets tested while it is still visible. This visit decides the bathroom. Once the linings and tiles go on, moving a mixer means cutting them off again.

The third is the fit-off, after tiling and painting. Tapware, toilet, vanity, shower, everything connected and tested. This is the one that looks like plumbing to most people and it is the least consequential of the three.

What has to be chosen before pre-line

This is the part worth reading twice. At pre-line, we need to know the actual products, not the general idea of them.

The shower mixer and its rough-in body, because different brands sit at different depths and the wall build-up has to suit. The vanity, because its waste and its tap holes decide where the pipes come out of the wall or floor. The toilet, because a back-to-wall pan, a wall hung pan and a standard close-coupled suite have three different waste positions and one of them needs a frame in the wall. The bath or shower tray, because tray wastes are not in the same place.

Buying the tapware later and hoping is the single most common cause of a bathroom costing more than it was quoted.

Where can the toilet and shower actually go?

Water can be run almost anywhere. Waste cannot, because it moves by gravity.

Every waste needs fall, and it needs somewhere to fall to. On a concrete floor that usually means the toilet stays close to where it is, unless you are prepared to cut the slab. On a timber floor there is more freedom, but the joists still run in one direction and there are limits to what can be cut out of them.

This is why “can we swap the shower and the toilet” is a question to ask at the design stage rather than after the walls come down. Sometimes the answer is yes and it costs almost nothing. Sometimes it means a saw cut through a slab, a day of drainlaying, and a concrete pour.

Does the hot water need dealing with first?

Often, and it is easier before the tiles.

If the house is still on a low pressure cylinder, the new bathroom will look modern and behave like 1974: a thermostatic shower mixer needs mains pressure to do what it is sold to do, and pairing one with a low pressure system gives a disappointing trickle. Changing the cylinder while the walls are open, and while the plumber is already on site, costs a fraction of doing it as a separate job afterwards.

Size matters too. A new bathroom often means a bigger shower head, and sometimes a second shower where there was one. A cylinder that just coped before will not cope now.

Waterproofing, and who is responsible for it

Waterproofing a wet area is its own trade with its own sign-off, and in most jobs it belongs to the builder or a specialist waterproofer rather than the plumber. It matters here because it sits between our two visits: it goes on after pre-line and before tiling, and it has to be done properly and left to cure.

The failures we get called to years later are almost always at the junctions, where the membrane meets the waste or the shower tray. Take photos of the membrane before the tiles go on. It costs nothing and it is the only record that will ever exist.

What you end up with on paper

If the work is under a building consent, the plumbing and drainage will be inspected by the council and we provide the producer statement for our part of it. If we have changed or extended drainage, that gets inspected too.

Keep whatever you are given, along with the tapware manuals and the receipts. When something needs a warranty claim in four years, the brand and the model number are what the supplier asks for first, and nobody ever remembers them.

Common questions

How long is the bathroom out of action?

Two to four weeks for a full renovation in most houses, and the plumbing is only a few days of that. Most of the calendar is waterproofing curing, tiling, and waiting on trades. If you have one bathroom, plan for where you are showering before demolition day, not after it.

Can I keep my existing toilet or vanity to save money?

You can, and it often is not worth it. Old pans and old wastes rarely line up neatly with new finishes, and the labour spent making them fit tends to exceed what a mid-range replacement costs. The exception is a good quality suite that is only a few years old.

Replacing fixtures in the same positions generally does not. Moving drainage, adding a bathroom where there was not one, or altering structure generally does. Your builder or designer will know for the whole job, and we will tell you what our part of it needs.

Why does the plumber need to come back three times? Can it be one visit?

Not on a real renovation. Cap-off has to happen before demolition, pre-line has to happen while the walls are open, and fit-off cannot happen until tiling and painting are finished. Anyone offering to do it in one visit is planning to be somewhere else when it does not work.

The new shower is weaker than the old one. What went wrong?

Usually pressure rather than the shower. Either the house is on low pressure and the fitting expects mains, or a flow restrictor in the new mixer is doing exactly what it was designed to do. Both are worth checking before anyone starts pulling tiles off.

Planning a bathroom and want the plumbing scoped before the walls come down? Call Nexus Plumbing or email [email protected]. Send us the plan and the tapware list and we will price it properly.