A bathroom installation that lasts comes down to getting the hidden work right before anything pretty goes on the walls. The order matters: first fix the pipework and waste connections, prove they hold, waterproof the wet areas, then tile and fit the visible items. Skipping or rushing the early stages is what causes most failures a year or two later.
This guide explains the plumbing decisions in plain terms so you can follow what a fitter is doing and ask the right questions. It stays focused on the bathroom itself — boiler sizing, external drainage runs and tracing existing leaks are separate subjects covered elsewhere.
What a bathroom installation actually involves
A full installation breaks into two phases. The first fix is everything that happens before plastering and tiling: running hot and cold supply pipes, setting waste and soil connections, and roughing in the positions for the basin, WC (water closet, the toilet), bath and shower. The second fix is fitting the items themselves once surfaces are finished — taps, shower valves, the WC pan and cistern, the basin, and sealing everything up.
Between those two phases sit two jobs that are easy to underestimate: pressure testing the pipework, and waterproofing the wet zones. Both belong before tiles go on, because correcting them afterwards means pulling finished work apart.
A sensible sequence usually runs: strip out, alter pipework and waste, test, board and tank the walls, tile, then second fix. Electrical work — extractor fans, lighting, shaver sockets — runs alongside and must follow the wiring rules for bathrooms, which restrict what can go where based on distance from water.
Getting water supply and pressure right
A bathroom installation that lasts comes down to getting the hidden work right before anything pretty goes on the walls.
The performance of every tap and shower depends on the supply behind it, so this is worth settling early. Two things matter: how much water arrives (flow rate) and how hard it is pushed (pressure).
Homes fed by a combi boiler or unvented cylinder usually have mains pressure, which suits most modern fittings. Homes with a cold water tank in the loft and a hot water cylinder often run at low, gravity-fed pressure — fine for filling a bath, but weak for a shower unless a pump is added. A fitter should check the existing setup before specifying a shower, because a high-flow shower head on a low-pressure system simply dribbles.
Shower valves are where this becomes obvious. A thermostatic shower valve holds the temperature steady when someone else turns on a tap elsewhere, and most are rated for a minimum pressure to work properly. It is worth confirming the valve matches the system rather than assuming. Other points worth raising:
- Whether existing pipe sizes can deliver the flow the new fittings need.
- Whether isolation valves are fitted so individual items can be serviced without draining the whole bathroom.
- Whether a balanced supply (equal hot and cold pressure) is needed for a mixer valve to behave.
Soil and waste connections that won't leak
Waste pipework carries used water and waste away, and most long-term problems trace back to it. Each fitting has its own requirements. A basin and shower waste need a trap (the U-bend that holds water to block drain smells) and a fall — a slight downward slope so water drains rather than sits.
The WC is the demanding one. It connects to the soil pipe, the large pipe that takes waste from the toilet to the drain. The join is usually made with a flexible or rigid pan connector, and it must line up cleanly with no strain on it. A connector forced at an angle, or one that has been stretched to reach, is a slow leak waiting to happen — and a soil connection leak is unpleasant and hard to reach once boxed in.
Where a WC or basin is being moved, the waste run has to be replanned. There are limits on how far waste can travel and how steep or shallow the fall can be before it stops draining reliably. Moving a toilet across a room is rarely as simple as it looks, because the soil connection often dictates what is possible. It is reasonable to ask a fitter to explain how a relocated waste will reach the existing stack.
All joints should be accessible or properly tested before they are concealed. Once a leak is hidden behind tiles or under a floor, finding it later is far more disruptive than the original job.
Waterproofing and tanking before tiling
Tiles and grout are not waterproof. They shed most water, but moisture passes through grout lines over time, especially inside a shower or wet room. The barrier that actually keeps water out is what sits behind the tiles.
Tanking means applying a waterproof layer to walls and floors in wet areas before tiling — typically a liquid membrane painted on, or a sheet membrane, with sealing tape at corners and joints. It is most important in shower enclosures and essential in a wet room, where the whole floor takes water.
The boards behind tiles matter too. Standard plasterboard is poor in wet zones; moisture-resistant or cement-based backer boards cope far better and give tanking something stable to bond to. Getting this layer right is the single biggest factor in whether a shower area stays sound for years or starts to soften behind the tiles.
Because tanking is hidden once tiling begins, it is worth seeing it done or confirming what system was used. There is no way to inspect it afterwards without removing tiles.
What shapes the budget for a new bathroom
Costs vary widely, and the visible fittings are often a smaller share than people expect. The bigger swings come from the work behind the walls.
- Moving services. Keeping the basin, WC and bath roughly where they are is far cheaper than relocating them, because moving fittings means new supply and waste runs.
- Pressure and supply changes. Adding a pump, upgrading pipework or changing the hot water system adds cost but may be unavoidable for the shower to work.
- Waterproofing and substrate. Proper boarding and tanking cost more than skipping them — and far less than fixing water damage later.
- Tiling. Labour scales with area, pattern complexity and tile size.
- Fittings and brassware. Taps, shower valves and the WC suite range enormously; spend tends to follow the valve and tap quality more than the ceramics.
- Hidden surprises. Old pipework, rotten floors or non-compliant wiring revealed during strip-out can change the scope.
When comparing quotes, it helps to check they cover the same scope — particularly testing, tanking and waste alterations — rather than just the items on show. A cheaper quote that omits the hidden work is not really cheaper; it just defers the cost.