First-fix plumbing, explained
First fix is the hidden half of a bathroom — the pipework, feeds and waste runs that get set out and buried before the walls are boarded and the pretty stuff goes in. You never see it once the room’s finished, but it’s the part that decides whether your shower runs properly, your waste clears cleanly and your bathroom stays leak-free for decades. This guide explains exactly what first-fix plumbing involves in a full bathroom installation, why it can’t be rushed, and how we get it right on every job across Plymouth and the South West.
What “first fix” actually means
Plumbers and fitters split a bathroom into two phases. First fix is everything that happens before the walls and floor are closed up — running the pipes, setting the wastes, positioning the feeds and fixing the things that need to sit inside the wall. Second fix is everything that happens after the tiling: hanging the suite, connecting the taps, fitting the shower head and screen. First fix is the skeleton; second fix is the skin.
The reason the two are separate is simple. Once a wall is boarded, tanked and tiled, you can’t easily get back inside it. So anything that lives in the wall or under the floor — a hot feed, a waste pipe, a shower-valve body, the recess for a concealed cistern — has to be run and pressure-tested while the structure is still open. Miss something at first fix and putting it right later means lifting tiles you’ve just paid to lay. That’s why a careful first fix is one of the best-value stages of the whole job: it’s cheap to do right and expensive to do twice.
First fix follows straight after strip-out in the installation process, and it usually runs alongside the electrical first fix, because both trades want the walls open at the same time. On most jobs it takes a day or two, depending on how much pipework is being moved.
What first-fix plumbing covers
Every bathroom is different, but the first-fix plumbing on a full installation almost always comes down to the same handful of runs. Here’s each one, what it does, and why it matters that it’s done properly rather than bodged.
Hot & cold supply feeds
New feeds are run to every outlet — basin, bath, shower, WC and any bidet or towel rail that needs one. We use the right pipe for the run (usually copper or a quality push-fit and plastic combination), size it so flow isn’t choked, and set the tails so they emerge exactly where the tap or valve will sit. Get the position wrong by an inch and the whole suite fights you at second fix.
Waste runs & falls
Every basin, bath and shower needs a waste pipe run back to the soil stack, and — crucially — set to the correct fall. Too shallow and it won’t clear; too steep and the water outruns the solids and it blocks. Getting the falls right is the single most important thing in first-fix waste work, and it’s where quick jobs cut corners because nobody sees it until it smells.
Soil connection
The WC waste and the main branch tie into the soil stack. This has to be watertight, properly supported and, where the toilet is moving, re-routed at the right fall — the most involved bit of first-fix plumbing. Our guide to moving bathroom plumbing and the soil pipe covers when this is worth doing.
Shower-valve body
On a concealed or built-in shower, the valve body sits inside the wall and only the trim shows at second fix. It has to be set at the right depth for the finished tile face and plumbed to hot and cold before boarding. Get the depth wrong and the trim either stands proud or won’t reach.
Concealed cistern & frame
Wall-hung WCs and back-to-wall pans hide their cistern in a supporting frame behind the wall. The frame is fixed, levelled and plumbed at first fix, with the flush plate position set, so the finished wall is flat and the pan hangs solidly. It’s a cleaner look, but it’s all decided at this stage.
Isolation & testing
We fit isolation valves so individual outlets can be shut off later without draining the house, then pressure-test the whole first fix before anything is closed up. Finding a weeping joint now, with the wall open, is a two-minute fix. Finding it after tiling is a nightmare — so we never skip the test.
Why first fix decides how your bathroom performs
Homeowners tend to judge a finished bathroom on the tiling and the taps, but how it actually performs — the shower that runs strong, the bath that fills fast, the waste that clears silently — is decided at first fix. Two things matter most here.
Pipe sizing and flow
A shower fed by an undersized or over-long pipe run will always disappoint, no matter how good the head is. At first fix we size the feeds to the demand, keep runs as direct as sensible, and match the plumbing to your actual water pressure and flow. If your supply won’t run the shower you want, first fix is when we’d design in a pump or rethink the spec — not after it’s all tiled in and disappointing.
Falls and venting on waste
Slow-draining basins, gurgling traps and the faint drain smell that never quite goes away are nearly always first-fix faults: waste run too flat, too long, or not vented. We set every waste to the right fall, keep runs short where we can, and make sure traps hold their water seal. It’s invisible work, but it’s the difference between a bathroom that’s a pleasure to use and one that quietly annoys you every day.
This is also why we check the existing pipework carefully at survey and strip-out. Plenty of older Plymouth homes — the 1960s–90s stock in Plympton, Plymstock and Peverell especially — have tired feeds, undersized runs or waste that’s been added to piecemeal over the years. First fix is our chance to put all of that right while the walls are open, so the new bathroom performs like new rather than inheriting the last one’s problems.
How first fix fits the wider job
First fix doesn’t happen in isolation — it’s one link in a chain, and it has to hand over cleanly to the stages either side of it. Here’s how it connects.
After strip-out
First fix can only start once the old room is out and the subfloor and walls are checked and sound. Any rot or damp uncovered at strip-out is put right first, so the new pipework is fixed to solid structure.
Alongside electrical first fix
The plumber and electrician usually work the walls together, co-ordinating so pipes and cables don’t clash and everything sits clear of the tile face. The electrical first fix for lights, the extractor and any electric shower happens now too.
Before boarding & tanking
Only once the first fix is set out and pressure-tested do the walls get boarded and the wet areas tanked. After that, the pipework is sealed inside the structure — which is exactly why it all has to be right before this point.
Wet rooms need extra care
In a wet room, first fix includes setting the floor drain and former to a precise gradient so the whole floor drains to one point. There’s no margin for error once it’s tanked, so the first fix is even more exacting.
Then second fix
When tiling’s cured, second fix connects everything the first fix set out — taps to feeds, wastes to traps, the shower trim to its valve. A tidy first fix makes second fix quick and clean.
All on one fixed price
Because we survey the pipework before quoting, the first-fix work is priced into your fixed written quote. No mid-job surprises unless strip-out genuinely reveals something hidden — and if it does, you hear it straight away.
The signs of a poor first fix — and what we do differently
We get called out to put right other people’s bathrooms often enough to know exactly how a bad first fix shows itself. Almost every one of these was cheap to avoid at the time and expensive to fix afterwards, because putting it right means opening a wall or floor that’s already tiled. Here’s what to watch for, and how a proper first fix designs each fault out from the start.
A shower that never runs right
The tell-tale sign: weak flow or temperature that swings when another tap opens. The cause: feeds undersized or run too far at first fix. What we do: size the pipe to the demand, keep runs direct, and match the shower to your measured supply before a single tile goes on.
Slow drains and drain smells
The tell-tale sign: a basin that empties sluggishly, a gurgle, or a faint smell that won’t shift. The cause: waste run too flat, too long, or a trap that’s lost its seal. What we do: set every waste to the correct fall, keep runs short, and make sure traps hold water.
Fittings that don’t line up
The tell-tale sign: a shower trim standing proud of the tiles, or tails emerging in the wrong place. The cause: the valve body or feed tails set at the wrong depth or position. What we do: set out to the finished tile face, so everything sits flush and true at second fix.
The leak behind the tiles
The tell-tale sign: a damp patch on a ceiling below, weeks after the job. The cause: a joint that was never pressure-tested before the wall was closed. What we do: pressure-test the whole first fix and hold it before boarding — every time, no exceptions.
None of this is exotic — it’s just the discipline of doing the invisible work as carefully as the visible work. That’s what “finished properly” means to us, and it’s why we’re happy to put our name on the parts of a bathroom you’ll never actually see.
First-fix plumbing FAQs
How long does first-fix plumbing take?
On a typical full bathroom it’s a day or two, usually days one to two of the job, running alongside the electrical first fix. It takes longer if the layout is changing and pipework has to be re-routed — moving the WC and its soil connection is the biggest single addition. We build the time into your schedule at survey so there are no surprises.
Can you reuse the existing pipework?
Sometimes, if it’s sound, correctly sized and in the right place — but we never assume. We check it at strip-out, and in many older Plymouth homes the feeds are tired or the waste has been added to over the years, so it’s better renewed while the walls are open. Reusing bad pipework to save a little now is a false economy that shows up as poor flow or a slow drain later.
Does first fix affect how much a bathroom costs?
Yes — it’s one of the biggest levers. Keeping outlets in roughly the same positions means the first fix is straightforward and quick. Moving the WC, bath or basin adds new feeds, waste runs and often a soil re-route, which takes longer and costs more. For the full picture, see our guide to the cost of a bathroom in Plymouth.
Why do you pressure-test before closing the walls?
Because a weeping joint found now, with the wall open, is a two-minute fix — and the same joint found after tiling means lifting the tiles you’ve just paid for. We fill the first-fix pipework, bring it up to pressure and hold it to confirm every joint is sound before anything is boarded or tanked. It’s a small step that removes the single worst risk in a bathroom: a slow leak hidden inside a finished wall.
Proud of every bathroom we fit
Get the hidden work done right
The pipework you never see is the part that makes a bathroom a pleasure — or a nuisance — for years. We check your supply, set every feed and waste properly, and pressure-test before we close a wall. Tell us about your room and we’ll give you one clear, fixed written quote.