Most of the plumbing behind a new bathroom is hidden: the work that decides whether the suite drains properly, stays leak-free and passes building rules happens inside walls, under floors and behind the bath. A typical installation involves three connected systems — hot and cold supply pipes feeding the taps and shower, waste pipework carrying water away from each fitting, and the soil stack that removes waste from the toilet. Getting the layout and falls right at first-fix stage matters far more than the finish you see at the end.
What plumbing does a new bathroom involve?
A bathroom installation breaks into two stages. First-fix is the pipework run before walls and floors are closed up: supply pipes for each tap and the shower, waste pipes from the basin, bath and shower tray, and the connection from the toilet to the soil stack. Second-fix is fitting the visible items — taps, valves, the WC and the trap connections — once tiling and surfaces are done.
Supply pipes carry water at mains or stored pressure, so they only need to be the right diameter and well secured. Waste pipes are different: they rely on gravity, so each one must fall at a steady gradient towards the stack. A trap — the U-bend under each fitting — holds a plug of water that stops drain smells coming back into the room. If a trap is sited too far from the stack, or the pipe runs too flat, that water seal can be siphoned out and the room will smell.
Common pipe sizes follow the fitting. A basin usually drains through 32mm waste pipe, a bath or shower through 40mm, and a toilet through a 110mm soil pipe. Mixing these up, or trying to push a bath waste through an undersized pipe, leads to slow draining and gurgling.
Getting waste falls and the soil stack right
Getting the layout and falls right at first-fix stage matters far more than the finish you see at the end.
The single most important hidden detail is the fall — the downward slope of each waste pipe. Too little and water sits in the pipe; too much and water races away leaving solids behind, which eventually blocks the run. Building guidance gives working ranges rather than a single figure, and the right fall depends on pipe diameter, so this is something a competent installer will calculate rather than guess.
The soil stack is the vertical pipe, usually 110mm, that takes waste from the toilet and carries it down to the underground drain. It also needs to breathe. Open to the air at the top — traditionally a vent through the roof, or via an air admittance valve inside the loft — the stack lets air in so that flushing the toilet does not suck the water seals out of nearby traps. Moving a toilet during a refit often means the connection to the stack has to be reworked, and that is one of the costlier parts of any bathroom job.
Where a new bathroom sits a long way from the existing stack, or below the drain level, a normal gravity run may not be possible. In those cases a small pumped unit — a macerator for a toilet, or a waste pump for a basin and shower — can move waste through narrow pipework instead. These work, but they add a mechanical part that can fail and they are noisier, so most installers treat a gravity connection as the first choice and a pump as the fallback.
It is worth knowing that certain drainage and stack work is notifiable under the building regulations. Anyone planning to relocate a soil stack or alter underground drains should check whether the work needs to be signed off, as this affects sign-off and any future sale of the property.
Choosing and plumbing a shower valve
A shower valve is the unit that mixes hot and cold and controls the flow. The hidden part — the valve body — is usually buried in the wall during first-fix, with only the control and handle visible afterwards. Because it is built in, the type of valve has to be decided before tiling, not after.
The main choice is between a manual mixer, which blends hot and cold by hand, and a thermostatic valve, which holds a set temperature and shuts off if the cold supply fails. Thermostatic valves are the usual choice for safety, especially where children or older people use the bathroom, because they prevent a sudden scald if someone elsewhere in the house runs a cold tap.
The valve also has to suit the system pressure. A home fed by a gravity-fed hot water cylinder in the loft often has low pressure, and some valves will barely trickle on it without a pump. A combination boiler or an unvented cylinder gives higher pressure and suits a wider range of valves. Mismatching the valve to the pressure is a common cause of a disappointing shower, so the pressure should be established before buying. The position of the valve body in the wall matters too: it needs to sit at the right depth for the chosen finishing plate, which is why the valve is normally bought before the wall is closed up.
Tanking and waterproofing a wet room
Tanking is the waterproof layer applied behind tiles to stop water reaching the structure of the building. In a standard bathroom with an enclosed shower, it is sensible around wet zones. In a wet room — where the shower drains straight into a sloped section of the floor with no tray or enclosure — full tanking is essential, because water meets the floor and lower walls directly.
The process is usually a flexible waterproof membrane or a liquid coating, run across the floor and up the walls to a sensible height, with extra attention at corners, the floor-to-wall joint and around the drain. The floor itself needs a built-in slope, or "fall to gully", so that water runs to the drain rather than pooling or escaping into the rest of the room. On a timber floor this often means reinforcing the structure first, since a wet-room floor takes more weight and movement than a standard one.
Because tanking sits underneath the tiles, mistakes here are invisible until water appears in the ceiling below or in an adjoining room. That makes it one of the parts of a bathroom installation where careful preparation matters most, and where it is reasonable to ask an installer exactly what waterproofing system they intend to use and how the drain and floor falls will be formed.