When you’re renovating or building a bathroom, there’s a stage in the process that feels a bit like the backstage setup before a big show—you can’t see the magic yet, but every pipe, fitting, and connection you install now will decide how smooth (or stressful) the future will be. That’s first fix plumbing.
Think of it as the “skeleton” of your bathroom. You’re not installing taps, showers, or toilets yet—you’re installing the hidden framework that makes them work. And trust me, if you get this stage wrong, your dream bathroom can quickly turn into a money-sucking leak disaster.
1. Understanding What “First Fix Plumbing” Means
First fix plumbing is the stage where all water supply pipes, waste pipes, and hidden connections are installed before any plastering, tiling, or fixtures go in.
In simple terms:
- First fix = hidden work (pipes, drains, water feeds)
- Second fix = visible work (taps, basins, toilets, showers)
A plumber will chase walls, run pipework through floors and ceilings, and connect everything to the main supply and waste systems. At this point, you might see a tangle of copper pipes, plastic push-fits, and maybe even manifold systems—but no pretty sinks yet.
Why it matters: Around 80% of bathroom plumbing problems (leaks, low pressure, blockages) can be traced back to poor first fix planning. Once tiles are up, fixing them means smashing walls—never fun.
2. When First Fix Plumbing Happens in a Bathroom Project
This stage happens after the structural work is complete but before plastering or tiling.
Typical order in a bathroom renovation:
- Demolition & stripping out – removing old fixtures and finishes.
- Structural work – floor repairs, wall changes, stud wall installation.
- First fix plumbing – installing all hidden water and waste pipes.
- First fix electrics – adding wiring for lights, fans, heated mirrors.
- Plastering & wall prep.
- Tiling.
- Second fix plumbing – installing taps, toilets, showers.
- Finishing touches.
A good rule: First fix always comes before anything decorative.
3. What’s Included in First Fix Plumbing for a Bathroom
Here’s exactly what gets done during this stage:
- Hot and cold water supply pipework to all fixtures (sink, bath, shower, toilet).
- Waste (drain) pipework for baths, basins, toilets, and showers.
- Shower valve and concealed mixer installation if using built-in showers.
- Pipework for towel rails or underfloor heating if needed.
- Connections to mains water and heating system.
- Pressure testing to check for leaks before covering pipes.
Everything must be positioned to millimeter accuracy. A few cm off and your taps won’t align with basins, your toilet may sit awkwardly, or your shower head could be too high or low.
4. The Importance of Planning First Fix Plumbing
Here’s a harsh truth: Plumbing is not a “figure it out later” trade.
If you haven’t planned exactly where your bath, sink, and toilet will be before first fix, you’re gambling with your budget.
Why planning is vital:
- Avoids having to rip out tiles to fix mistakes.
- Ensures optimal water pressure to all outlets.
- Saves money—moving pipework later can cost 3x more.
Personal tip: When I did my first bathroom renovation, I didn’t double-check the measurements for my wall-hung toilet frame. The result? The flush plate ended up half hidden under a shelf. I had to spend another $400 redoing it. Learn from my pain—plan everything before pipes go in.
5. Materials Commonly Used in First Fix Plumbing
Different plumbers prefer different materials, but here are the most common:
- Copper pipes – Durable, excellent for both hot and cold supply. Can withstand high pressure and temperature.
- PEX plastic pipes – Flexible, easy to install, and resistant to scale build-up. Popular in modern builds.
- Push-fit fittings – Quicker to install, great for DIY-friendly connections.
- Solvent weld waste pipes – Strong, permanent connections for waste.
- Compression fittings – Reliable, removable, but bulkier.
Pro stat: In the UK, 60% of plumbers now use PEX piping for first fix due to speed and reliability.
6. Bathroom Layout and Pipework Routes
A good plumber will try to keep pipe runs as short and direct as possible.
Why? Shorter runs =
- Better water pressure.
- Less heat loss in hot pipes.
- Fewer joints (and fewer potential leak points).
Bathrooms often require careful coordination with carpenters—especially for wall-hung fixtures. Pipework may need boxing in or routing through stud walls.
7. Hot and Cold Water Supply Setup
For each bathroom fixture, you’ll need a dedicated hot and cold feed (except for toilets, which only need cold).
Typical pipe sizes:
- Basin & bath taps: 15mm.
- Shower: 15mm or 22mm depending on flow.
- Toilet: 15mm cold feed.
Flow rate matters:
If you’re installing a rainfall shower, you’ll need a minimum of 1.5–2 bar pressure and wide enough pipe bore to handle it.
8. Waste Pipe Installation
Waste pipes carry used water away to the main soil stack or external drain.
Key rules:
- Basins: 32mm waste pipe.
- Baths & showers: 40mm.
- Toilets: 110mm soil pipe.
All waste pipes must have a minimum fall (slope) of 18–22mm per metre to prevent standing water and bad smells.
9. Concealed Shower Valve and Mixer Installation
If you’re going for a built-in shower system, the first fix stage is when the valve gets mounted inside the wall.
Why this is tricky:
- Needs to be at the correct height for all users.
- Must be level to avoid crooked handles.
- Needs perfect sealing to prevent hidden leaks.
Once tiled, accessing a faulty valve means demolition—so triple-check connections before covering up.
10. Towel Rails and Underfloor Heating Pipework
If your bathroom includes heated towel rails or wet underfloor heating, the water pipework for these also gets installed now.
For underfloor heating:
- Manifold and loops are laid before screeding.
- Typically uses 16mm PEX pipe.
11. Common Mistakes to Avoid During First Fix
Mistakes at this stage can haunt you for years. Common ones include:
- Pipes not secured properly (causing banging noises).
- Wrong pipe size leading to weak water pressure.
- Waste pipes with no access points for cleaning.
- No isolation valves for easy maintenance.
- Mixing incompatible metals (can cause corrosion).
12. Testing the Pipework
Before walls are sealed, plumbers pressure test all pipework.
Typical method:
- Cap all outlets.
- Pressurize the system with water or air.
- Leave under pressure for at least 1 hour.
- Check for pressure drop or visible leaks.
Skipping this is asking for disaster.
13. Costs of First Fix Plumbing in a Bathroom
Costs vary depending on location and complexity, but on average:
- UK: £800–£2,000 for a standard bathroom.
- US: $1,000–$3,500.
- Australia: AUD $1,500–$4,000.
Adding underfloor heating or complex showers can push costs higher.
14. Timeframe for First Fix Plumbing
A straightforward bathroom first fix takes 1–3 days.
A complex layout or multiple bathrooms may take up to a week.
Delays often happen when decisions aren’t made early—like changing the position of a bath after pipes are laid.
15. Coordinating with Other Trades
First fix plumbing overlaps with:
- Electricians (for lighting, fans, heated mirrors).
- Carpenters (for boxing in pipes, wall-hung supports).
- Tilers (need pipe positions marked).
Good communication keeps the project on schedule.
16. Regulations and Building Codes
Plumbing must comply with local building regulations for safety and hygiene.
Examples:
- Backflow prevention devices for certain taps.
- Proper venting for waste systems.
- Pipe insulation for frost protection.
Non-compliance can mean fines or being forced to redo work.
17. Tools and Equipment Used
Plumbers typically use:
- Pipe cutters.
- Press-fit or push-fit tools.
- Blowtorches for soldering.
- Spirit levels for alignment.
- Test gauges for pressure testing.
18. DIY vs Hiring a Professional
While some homeowners can tackle first fix plumbing, mistakes can be costly.
DIY pros:
- Save labour costs.
- Learn a new skill.
DIY cons:
- Risk of leaks and water damage.
- No warranty.
- May not meet regulations.
19. Signs of a Good First Fix Job
You can spot quality work if:
- Pipes are neatly clipped and straight.
- Isolation valves are accessible.
- No sharp bends or awkward joints.
- Everything lines up with planned fixtures.
20. Final Checklist Before Moving to Second Fix
- All pipes installed and secured.
- Waste pipes tested for flow.
- Water supply tested under pressure.
- Valve heights checked.
- Positions marked for tilers.
Conclusion
First fix plumbing in a bathroom is the invisible but essential stage that sets the foundation for a fully functional, leak-free space. It’s all about precision, planning, and making sure every hidden pipe is in the right place before you close up the walls. Get it right, and your second fix will be smooth sailing. Get it wrong, and you could be redoing expensive work.
If there’s one takeaway here, it’s this: measure twice, plan thoroughly, and never rush your first fix.
FAQs on First Fix Plumbing in a Bathroom
1. What’s the difference between first and second fix plumbing?
First fix covers hidden pipework before walls are finished; second fix is installing visible fixtures afterward.
2. How long does first fix plumbing take in a bathroom?
Usually 1–3 days for a standard bathroom, longer for complex setups.
3. Can I do first fix plumbing myself?
Yes, if you have experience and understand local regulations, but mistakes can be costly.
4. Is first fix plumbing more expensive than second fix?
Yes, typically, because it involves more labour and materials.
5. Do I need a building permit for first fix plumbing?
In many regions, yes—check local regulations to avoid fines or failed inspections.