Rough-in plumbing is the stage where every pipe is installed before walls are closed up, floors are laid and fixtures are fitted. Get it right and the finished bathroom or kitchen works for decades. Get it wrong and you’re cutting into fresh plasterboard to fix a leak.
What Is Rough-In Plumbing?
Rough-in (first fix) plumbing is the installation of all the hidden pipework in a home: hot and cold water supply pipes, waste and drainage pipes, and vent pipes. Visible parts such as taps, basins, toilets, showers and radiators are added later during the second fix.
It usually happens:
- When building a new home or extension
- During a full bathroom or kitchen renovation
- When adding a new bathroom, utility room or en-suite
- When re-piping an older property
Before You Start: Rules, Permissions and Safety
Plumbing isn’t a free-for-all, even for confident DIYers.
- Water regulations: In the UK, all water fittings must comply with the Water Supply (Water Fittings) Regulations. Some work, such as new bathrooms or unvented systems, requires notifying your water supplier.
- Building Regulations: Drainage and sanitation fall under Part H and Part G of the Building Regulations. New drainage usually needs Building Control approval.
- Gas and heating: Any work on gas pipework must be done by a Gas Safe registered engineer. Unvented hot water cylinders need a suitably qualified installer.
- Electrical safety: Isolate power before drilling near cables, and use a cable and pipe detector.
If your project involves new drainage, gas or an unvented system, hire a qualified professional. Rough-in is often the stage where DIYers do the prep and a plumber does the connections.
Tools and Materials You’ll Need
Tools: tape measure, spirit level, pipe cutter, deburring tool, pipe benders or push-fit tools, hole saw and drill, pipe clips, marker pen, stud and cable detector, pressure-test kit.
Materials:
- Water supply pipe: copper or plastic (PEX or barrier pipe) in 15mm and 22mm sizes
- Waste pipe: typically 32mm, 40mm and 50mm
- Soil pipe: 110mm for toilets
- Fittings, isolation valves, pipe clips and insulation
- Fire-stop and acoustic sleeves where pipes pass through walls and floors
Step-by-Step: How to Do Rough-In Home Plumbing
Plan Your Layout
Start with a simple scale drawing of each room. Mark where every fixture will go: toilet, basin, bath, shower, sink, washing machine and dishwasher. Then decide how supply and waste pipes will run to each one.
Keep these principles in mind:
- Group wet rooms together, or stack them above each other, to shorten pipe runs
- Keep waste pipes as short and straight as possible
- Plan access points for valves and any joints you may need to reach later
Mark Out Pipe Routes and Fixture Positions
Using your plan, mark the exact positions of fixtures and pipe routes on the floor, walls and joists. Check the manufacturer’s specifications for each fixture, because toilet waste outlets, shower trays and baths all have different positions and connection heights.
Use a detector to check for existing cables and pipes before you drill or notch anything.
Drill and Notch Safely
Joists and studs can be drilled or notched, but only within limits. As a general rule for timber joists:
- Drill holes near the centre of the joist depth, not near the top or bottom edge
- Keep holes small relative to the joist depth
- Place notches only in the top edge, near the supports
- Never notch or drill structural beams without advice from a structural engineer
Poorly placed notches weaken floors, so when in doubt, ask a professional.
Install the Water Supply Pipes
Start from the main stopcock or manifold and work outward.
- Run cold supply pipes to every fixture, plus the hot water cylinder or boiler.
- Run hot supply pipes from the heater to each outlet.
- Fit isolation valves so individual fixtures can be shut off for repairs.
- Clip pipes securely at regular intervals to prevent sagging and noise.
- Leave capped ends at the correct heights for second fix connections.
Use 15mm pipe for most branches and 22mm for main runs or high-demand fixtures such as a bath. Keep hot and cold pipes apart where possible to prevent heat transfer.
Install the Waste and Soil Pipes
Waste pipes rely on gravity, so the gradient matters more than anything else at this stage.
- Slope waste pipes consistently downhill, typically around 1:40 for most domestic runs, and always follow the manufacturer’s guidance
- Use long-radius bends rather than sharp 90-degree elbows to avoid blockages
- Connect everything to the main stack or drain
- Fit access points or rodding eyes where required
Waste pipe diameters usually increase with demand: smaller for basins, larger for sinks, baths and showers, and 110mm for toilets.
Add Ventilation (Venting)
Every drainage system needs air. Without it, draining water creates suction that pulls water out of the traps and lets sewer smells into your home. Your system may use an extended soil stack, an air admittance valve (where permitted) or individual vent pipes. Check local requirements before choosing one.
Insulate and Protect the Pipes
- Insulate pipes in lofts, garages, crawl spaces and external walls to prevent freezing
- Insulate hot pipes to reduce heat loss
- Use sleeves where pipes pass through masonry and fire-rated walls
- Wrap pipes near joists to reduce noise
Pressure Test Before You Close Up
This is the most important step. Never cover up pipework untested.
- Cap all open ends.
- Fill the system with water and bleed out the air.
- Pressurise it to the level recommended for your pipe type.
- Hold the pressure for the recommended period and check the gauge for drops.
- Inspect every joint visually for weeping or drips.
Waste pipes can be checked by running water through them and watching every joint. If building control or an inspector needs to sign off, leave the pipework exposed until they’ve seen it.
Common Rough-In Plumbing Mistakes to Avoid
- Skipping the plan. Moving pipes later is costly.
- Wrong waste gradients. Too flat causes blockages, too steep can leave solids behind.
- Unsupported pipes. Sagging pipes create noise and weak points.
- No isolation valves. Every fixture should be isolatable.
- Covering pipes before testing. The most expensive mistake on this list.
- Overlooking expansion. Hot pipes expand, so leave room for movement.
- Ignoring local regulations. Failing inspection means ripping work out.
How Much Does Rough-In Plumbing Cost?
Costs vary widely by region, property size and pipe layout. As a rough guide, labour is the largest part of the bill, and a first fix for a single bathroom is typically a day or two of work for a plumber. Get at least three written quotes and ask exactly what’s included, such as pipework, drainage, testing and making good. Check current local prices before budgeting.
DIY or Hire a Professional?
Reasonable for confident DIYers: planning, marking out, drilling, running water supply pipes with push-fit or compression fittings, and insulating.
Better left to a professional: new drainage connections, soil stacks, gas pipework, unvented cylinders, and anything needing certification.
A good compromise is doing the prep and labour yourself and having a qualified plumber check and connect the final system.
Frequently Asked Questions
What does rough-in mean in plumbing?
It’s the stage where pipes for water supply, waste and venting are installed inside walls, floors and ceilings before fixtures are fitted and surfaces are finished.
Is rough-in the same as first fix?
Yes. “First fix” is the UK term for the same stage. Second fix is when taps, toilets, basins and appliances are connected.
How long does rough-in plumbing take?
A single bathroom can take one to two days. A whole house can take a week or more, depending on size and complexity.
Can I do rough-in plumbing myself?
You can do much of the water supply work yourself, but drainage, gas and certified systems should be handled by qualified professionals.
What comes after rough-in plumbing?
Inspection and pressure testing, then closing up walls, then tiling and decorating, and finally the second fix, where fixtures and appliances are installed.
Final Thoughts
Learning how to do rough-in home plumbing comes down to careful planning, correct gradients, secure supports and thorough testing before anything is hidden. Whether you tackle it yourself or manage a plumber, understanding each stage helps you spot mistakes early and protect your home from costly leaks.
