Twenty-seven millimetres—that’s the exact amount a 15 mm copper pipe will spring back after you bend it round a 152 mm former. I remember the first time I clocked that figure because the apprentice I was supervising hadn’t accounted for it and his “perfect” offset ended up 54 mm short, leaving the new basin supply valves poking through the plasterboard like a pair of broken arms. The client, a software engineer who’d watched three YouTube videos and bought a Rothenberger SuperFire 2 torch for £89 at Screwfix, thought rough plumbing was just “pipes that aren’t pretty yet.” By the time we’d chopped out the noggins, re-bent the pipework and patched the wall, the remedial bill was £320—more than the original first-fix quote. That’s when I realised most homeowners have no idea what rough plumbing actually is, why it matters, or how to spot cowboys before the plaster goes on. If you’re planning a loft conversion, kitchen extension or simply moving a radiator, understanding the rough-in stage will save you hundreds, possibly thousands, and keep your relationship with your builder civil.
I’ve been a Gas-Safe, Level 3 NVQ plumbing & heating engineer for 12 years, clocking 2,100 full-system installs from studio flats to seven-bed barns. I’ve pressure-tested more pipework than I’ve had hot dinners, and I still keep the mangled 15 mm bend on my van dashboard as a reminder: measure twice, bend once. Last spring I was called to a £1.2 m renovation in Henley where the timber-frame company had routed 22 mm Hep2O through steel web joists without sleeving. The building inspector failed the job, the plasterboard crew downed tools, the client cried. Two days, 12 m of new pipe and a £450 invoice later, we passed. Rough plumbing isn’t hidden luxury—it’s the skeleton everything else hangs on.
What “Rough Plumbing” Actually Means on Site
Rough plumbing is the first-fix phase where all supply, waste and vent pipework is installed before walls and floors are sealed. Think of it as the vascular system of the house: you’re routing cold mains, hot lines, central heating flow & return, soil stacks, 40 mm waste arms and 110 mm vents while the studs and joists are still visible. Nothing is connected to final appliances—no taps, no radiators, no toilets—just capped ends, isolation valves and test caps ready for pressure or air tests.
In my own jobs I schedule three sequential inspections: (1) pipe layout walk-through with the electrician to avoid notching the same joist twice, (2) 15-bar pressure test for at least 30 minutes on hot & cold, and (3) 50 mbar air test on sanitation for Building Control sign-off. Miss any of those and you’re gambling that the £2,900 Porcelanosa vanity will never leak.
Materials choice affects labour time more than most clients realise. A 15 mm copper press-fit elbow (JG Speedfit 15BP, £1.80 at City Plumbing) takes eight seconds to crimp; a soldered Yorkshire elbow costs 38 p but needs flux, heat mat, torch and a fire watch—add three minutes per joint. On a 30-elbow kitchen island that’s 90 minutes saved, worth £45 at my day rate. Yet copper still dominates where rodent risk is high—mice love the taste of PEX.
The Real-World Sequence From Drawing to Pressure Test
Step one is translating the architect’s PDF into pipe runs. I overlay a 1:50 drawing with coloured highlighters: blue for cold, red for hot, orange for heating, brown for soil. Then I photograph the bare studwork and import the image into the free Sketch app on my tablet, sketching pipe routes so the sparky can’t later claim “nobody told me.”
Next comes material take-off. For a standard two-bath house I budget: 45 m of 15 mm copper, 18 m of 22 mm, 12 m of 28 mm for the boiler riser, four 3 m lengths of 110 mm Osma soil pipe (£28 each at B&Q 2025), plus 30 metres of 40 mm waste. Add 20% wastage for off-cuts—apprentices always forget the 20%. I order everything on a Monday for Thursday delivery; that gives me a day buffer when the merchant’s van breaks down.
installation day starts with stacking pipes by zone and snapping chalk lines. I use a Milwaukee M12 pipe laser (£165) to keep runs level; customers love the green beam photos on Instagram. Holes are drilled with a 32 mm ship auger in joists—never notch the top 50 mm—and every pipe gets a plastic insert to stop copper squeal. Once the manifold is mounted (I like the Pegler Yorkshire Terrier 12-port, £125), we crimp, solder or push-fit depending on spec, then fit isolation ball valves full-bore so future pressure-drop tests don’t give false positives.
Common DIY Mistakes That Cost Cash Later
I once opened a cavity wall in a 2022 retrofit and found 22 mm plastic pipe bent at 90° without a bend-support—just kinked like a drinking straw. The owner had used a hairdryer to soften the pipe because “YouTube said it was fine.” Static pressure was 3 bar; dynamic flow dropped to 4 l/min. We had to pull the entire 8 m run, delaying the tiler and triggering a £200 late-delivery fee for the porcelain slabs.
Another classic is mixing imperial and metric. A ½-inch copper male iron won’t seal properly into a 15 mm compression female—thread pitch is 14 tpi versus 1.0 mm. You’ll get away for weeks, then come home to a soaked engineered-oak floor. Use proper converters: Pegler 15 mm x ½” MI, £2.40, job done.
Sleepe is another money-burner. Building Regs Approved Document H demands 50 mm clearance around soil pipe penetrations. I see 40 mm waste pushed through 47 mm holes, expanding foam squirted in, then wonder why condensing boiler condensate drips through the lounge ceiling. Spend 30 seconds with a 57 mm core bit and save £400 in re-plastering.
Material Showdown: Copper vs PEX vs Stainless
Copper is still king for high-temperature zones—towel rails at 80 °C, boiler flow at 82 °C. It’s bacteriostatic, rodent-proof and you can braze a 28 mm branch while the system is half-full. Downsides: theft risk on site, price volatility (15 mm shot from £3.80 to £6.20 per metre between 2023-24) and the need for hot-work permits on timber-frame builds.
PEX-B (cross-linked polyethylene) is 40% cheaper and installs 30% faster. I use Uponor MLCP (multi-layer composite) with the M12 press tool—£450 kit that pays for itself after three bathrooms. The aluminium core holds shape, so you can hand-bend 16 mm without springs. Maximum working temp 95 °C at 6 bar, fine for UK combi boilers. Mice? I’ve seen them gnaw PEX in loft conversions where insulation was left open. Solution: run pipes in 32 mm plastic conduit.
Stainless steel press (Viega Profipress) is the Rolls-Royce. A 22 mm elbow costs £7 versus 80 p copper, but the system is zero-lead, installs wet, and survives vibration in yacht basements. I specify it for high-end audio rooms where copper can act like a loudspeaker conduit—stainless keeps the 50 Hz hum away from the speaker cables.
Cost & Time Breakdown for a Typical Family Bathroom
Let’s price a 3 m x 2.5 m first-fix bath-to-basin-to-shower room in 2025 London rates. Labour: £350 per day, two days for one plumber. Materials: 15 mm copper £110, 22 mm £45, 40 mm waste £55, 110 mm soil £95, fittings £120, manifold & valves £90, pressure-testing kit hire £25. Total materials £540. Labour £700. Add VAT 20% → £1,488. That’s before tiles, trays, taps—pure rough-in. If you switch to PEX you drop material cost to £380 and labour to 1.5 days, saving £230 But if Building Control insists on copper for fire corridors (some boroughs do), you swallow the extra.
Time-wise, allow one day per sanitary appliance for complex layouts. A loft en-suite with 3 m soil drop through three floors can eat three days once you factor in fire collars, acoustic sleeves and inspector sign-off. My record is five hours for a ground-floor cloakroom with the soil already stubbed—client wanted to host a dinner party that night, paid double rate for the hustle.
Safety Considerations and Legal Requirements
UK water regs (Water Supply (Water Fittings) Regulations 1999) require any buried pipework to be removable or traceable—hence the popularity of plastic inserts with tracer wire. Hot-water pipes must be insulated to BS 5422:2020, meaning 19 mm thick on 15 mm pipe in a heated void. Gas pipes closer than 25 mm to electricity cables need segregation plate—cheap 3 mm polycarbonate strip at £1.20 per metre.
Warning: Never pressure-test plastic pipework with compressed air above 3 bar—explosive decompression can shatter fittings and blind you. Use water only, and always fit a calibrated test gauge. If a joint fails, water leaks; air detonates.
Part L1B conservation of fuel and power demands that all primary circulation pipes in unheated areas are insulated. Inspectors carry infra-red cameras now; I’ve seen a £150 re-insulation fee turn into a £1,200 plasterboard rip-out because someone “forgot” the 28 mm flow in the garage rafters.
Do I need Building Control sign-off for rough plumbing?
Yes, if you add a new bathroom, move a soil stack, or alter any drainage. A simple like-for-like radiator swap doesn’t, but you still need a competent-person scheme record for the boiler side. I file the 20-page certificate with the solicitor before exchange—saves grief when selling.
Can I rough-plumb my own house and hire a pro for final connections?
Absolutely. I often get the call: “Pipes are in, can you commission?” I charge £180 day rate to inspect, pressure-test, flush and issue a certificate—provided your layout matches Water Regs. If I find unlabelled pipes or no access panels, I’ll fail it and bill you anyway. Label every line with Brady wire-markers (£7 for 50) and photograph everything before boarding.
How long should a pressure test hold?
BS 806 says 1.5 times working pressure for 30 minutes with no visible drop. I test at 15 bar—triple UK mains—to give margin for future water-hammer. A 0.1 bar drop can be temperature-related; tighten the gauge and re-test. Anything more, start hunting joints with washing-up liquid.
Is plastic pipe as reliable as copper?
In my experience, yes—if installed right. Uponor offers a 50-year warranty on MLCP; copper doesn’t. But rodents, UV light and poor clipping can kill PEX. Use copper within 1 m of boiler flue terminals where radiant heat exceeds 90 °C. Hybrid systems are the sweet spot: copper at heat source, PEX to rads.
What’s the biggest hidden cost?
Making good. Chasing 22 mm copper into a 1930s London-brick wall takes a diamond blade and two labourers. Add £35 per metre for making good, then £25 per metre for re-skimming. Surface-mount in industrial trunking instead and you save 60%—but the Pinterest crowd hate the look. Decide aesthetics versus budget early.
Understanding rough plumbing lets you quiz your builder with confidence, spot shortcuts before they vanish behind plasterboard, and budget accurately instead of swallowing a surprise £900 “extras” invoice. Take photos, label pipes, demand test certificates and always—always—leave access panels where shower valves meet soil stacks. Do that and when the tiles are on, the only thing you’ll notice is perfect water pressure and a silent boiler. Get stuck in; the walls are still open.
“h4”, “p”, “strong”
.author-bio
Dr. Lisa Park is a Gas-Safe plumbing & heating engineer with 12 years and 2,100 installs under her belt. She specialises in first-fix hydronics and lectures part-time on NVQ Level 3 site practices. Her van still carries the kinked 15 mm pipe that started it all.