Bathroom renovation plumbing checklist, phase by phase

Most bathroom renovations that run over budget do so because of something behind the tiles: a waste pipe that couldn't reach the new shower position, water pressure that couldn't run the shower that was bought, a leak found after the tiling was finished. In almost every case the fix is order. Decisions made in the right sequence cost less than the same decisions made late.

This checklist follows the job from first look to final sign-off. It concentrates on the plumbing, with notes on the trades and rules that sit alongside it. Work through each phase before starting the next.

Phase 1: Survey what you've got

Before choosing a single tile, find out what the room can support.

• Locate the soil stack (the large vertical pipe the toilet drains into) and note where it runs: inside the room, inside a wall, or down the outside of the building.
• Find the stopcock and check it turns. A renovation is a good time to replace a seized one.
• Identify your hot water system: combi boiler, unvented cylinder, or a cylinder fed from a cold water tank in the loft. It decides which showers will work.
• Look at the existing pipework. Note any lead or old galvanised steel supply pipe; these are usually replaced during a refit.
• Find out how the floor is built. Timber joists let waste pipes run between them in one direction only. A concrete floor needs either chasing or a raised platform.
• Note which walls are solid and which are stud partitions, because that affects where concealed valves and pipes can go.
• Look for signs of past leaks: stained ceilings below, soft floorboards, loose tiles around the bath.

If you live in a flat, read your lease or building rules before planning anything that touches shared pipes, floors or the soil stack.

Phase 2: Plan the layout around the drains

Supply pipes are small and can go almost anywhere. Waste pipes can't. They need a continuous fall towards the stack, and the further an appliance sits from it, the more height that fall uses up.

• Place the toilet first, as close to the soil stack as practical. Its large-diameter outlet is the least flexible connection in the room.
• Plan each waste run (basin, bath, shower) with a steady downhill fall all the way to the stack. Local building rules limit length and gradient, and they differ between countries, so ask your installer to confirm them.
• Avoid long horizontal waste runs and tight bends. Every bend is somewhere a blockage can start.
• Plan access points on waste pipes where they change direction.
• Treat pumped waste (macerator toilets, shower waste pumps) as a last resort for awkward layouts, not a first choice. They work, but they're noisier and need more maintenance.

Shower tray or wet room: a quick decision guide

• A tray is simpler. It sits on a riser or on the floor with the waste beneath it, and only the area around it needs full waterproofing. Low-profile trays still need enough depth underneath for the trap and a waste run with fall.
• A wet room puts the drain in the floor, which means the floor itself has to be built to fall towards the drain and the whole room has to be waterproofed. On timber joists this usually involves a pre-formed former and some structural thought. On concrete it can mean breaking out the screed.
• Choose a tray if the floor is timber, the budget is tight, or access below is limited.
• Consider a wet room if you need step-free access, the floor construction allows the fall, and you're prepared for the extra waterproofing work.

Either way, plan the tanking. Tanking is a waterproof membrane or liquid coating applied to walls and floors before tiling, because tiles and grout aren't waterproof on their own. Allow for it in every wet area, not only inside the shower.

Phase 3: Match the shower to the water supply

The shower you want and the shower your system can run aren't always the same thing.

• Measure flow at the bath or kitchen cold tap: time how long it takes to fill a bucket of known size and work out litres per minute. Do it at a busy time of day as well as a quiet one.
• Check the shower's stated minimum pressure and flow. Large rainfall heads and body jets usually need far more than a standard head.
• With a combi boiler, hot water flow is limited by the boiler's output. A big head may run lukewarm.
• With a tank-fed cylinder, pressure depends on the height of the cold tank above the shower. A pump may be needed, and the right type depends on the system.
• An electric shower takes mains cold and heats it in the unit. It needs a dedicated circuit installed by a qualified electrician.
• Many mixer valves expect hot and cold at similar pressures. Check the valve suits your system before you buy it.

If pressure is already poor at other taps, find out why before buying anything. Low water pressure can come from a part-closed stopcock, a narrow or scaled supply pipe, or a shared main, and some of those are cheap to put right.

Phase 4: Budget and choose installers

• Set a contingency on top of the quoted total. A common rule of thumb is 10 to 20 per cent; lean towards the higher end in an older property, where surprises behind the walls are more likely.
• Decide what you're buying and what the installer is supplying. Get it in writing.
• Get at least three written quotes for the same specification, so you're comparing like with like.

Questions worth asking each installer:

• Who does each part: plumbing, electrics, tiling, plastering? Are they employed or subcontracted?
• Where will the waste pipes run, and how have you allowed for the fall?
• Will every outlet have its own isolation valve?
• Where will the access panels go?
• When and how will you pressure test the pipework?
• What waterproofing system will you use, and how will it be tested?
• What paperwork will I receive at the end?
• How long is the workmanship guaranteed, and what does that cover?

Clear, specific answers are a good sign. Vague ones aren't. The guide to choosing a plumber covers what else to check before signing anything.

Phase 5: First fix (pipes in, walls open)

First fix is everything that goes in before walls are boarded and floors are laid.

• Agree pipe materials. Copper is rigid, durable and tolerant of heat. Plastic systems (PEX and similar, usually with push-fit joints) are flexible and quick to install, and need pipe inserts and proper support. Both are sound in the right hands; follow the manufacturer's instructions for whichever is used.
• Fit an isolation valve on the hot and cold supply to every outlet: basin, toilet cistern, bath and shower. A future washer change then doesn't mean shutting off the whole house.
• Keep joints out of places nobody can reach. A pipe buried in a wall or floor should ideally be one continuous length, with joints in accessible positions.
• Clip pipes properly so they don't rattle or sag.
• Plan access panels for concealed cisterns, shower valves, bath traps and any pump. A panel tiled flush into the wall beats chiselling tiles out later.
• Photograph every wall and floor with a tape measure in shot before it's closed up. Those photos stop someone drilling into a pipe in five years' time.
• Keep pipes clear of where wall-hung fittings, grab rails and towel rails will be fixed.

Phase 6: Electrics and ventilation

• Bathrooms are divided into zones around the bath and shower, with rules on which electrical equipment can go where and what protection it needs. Those rules differ between countries. Use a qualified electrician for this part, and expect the work to be certified where local rules require it.
• Plan the extractor fan position: high up, close to the shower, and away from the door so air is drawn across the room.
• Duct the fan to outside, not into a loft or ceiling void. Keep the duct run short and as straight as possible.
• A fan with a run-on timer or humidity sensor keeps clearing steam after you've left the room.
• Decide on heating: a towel rail on the heating circuit, an electric one, or a dual-fuel rail. Electric elements again mean an electrician.

Phase 7: Test before anything is closed up

This is the phase that gets skipped, and the one that matters most.

• Pressure test all new supply pipework before boarding. The installer pressurises the system, isolates it, and checks that the gauge holds steady over a set period. Any drop means a leak somewhere.
• Fill and drain every waste. Run water through each waste connection and check every joint with dry kitchen roll.
• Flood test the waterproofing. For wet rooms and shower areas, the tanking can be tested with standing water before tiling, following the product's guidance on curing time.
• Leave it overnight if you can, then check again.

Finding a leak with the walls open costs a cut and a fitting. Finding it under new tiles costs the tiles. The guides in the leak detection section explain the methods used and what a hidden leak looks like once a room is finished, which is useful to know long after the builders have gone.

Phase 8: Second fix and sign-off

• Fit sanitaryware, taps, the shower valve and fittings.
• Seal around the bath, tray and basin with sanitary silicone. Fill the bath with water before sealing it, so it settles to its loaded position and the bead isn't torn the first time someone gets in.
• Run every tap and the shower at full flow, hot and cold, and check under and behind each one.
• Flush the toilet several times and check the cistern refills and stops cleanly.
• Turn every isolation valve and open every access panel once, to be sure they work.
• Collect the paperwork: electrical certificates, any building control sign-off required where you live, product manuals and guarantees, and the pipe-route photos.
• Write down where the stopcock and isolation valves are, for whoever lives there next.

Then check again after a week of normal use. Slow drips from a fitting often appear only after a few days of heating up and cooling down.