Wet rooms and tanking: how a level shower floor stays watertight

A wet room is a bathroom, or part of one, where the shower floor is level with the rest of the floor and the water runs away through a drain in the floor itself. There is no tray with a raised edge, and often no shower door. The whole floor, and the walls around the shower, are treated as a waterproof area.

Wet rooms look clean and are easier to use for anyone who struggles to step into a tray. They are also less forgiving than a standard bathroom. This guide explains the main parts, what to ask about, and where things go wrong. It does not give a step-by-step build method; the waterproofing system maker's instructions decide the details.

Wet room or walk-in shower?

People use the terms loosely:

• Walk-in shower: a low-profile tray with a fixed glass screen. The tray is a manufactured product with its own fall and waste. Our shower trays and wastes guide covers these.
• Level access shower: the floor of the shower area is flush with the room, formed with a wet room former or tray that is set into the floor and then tiled or covered.
• Full wet room: the whole room is waterproofed and the floor falls to a drain, so water can splash anywhere without harm.

The more of the room that gets wet, the more of it needs proper waterproofing.

What tanking means

Tiles and grout are not waterproof. Water gets through grout lines and hairline cracks, and into the boards, plaster and timber behind.

Tanking is the waterproof layer applied to the walls and floor before tiling. It is usually a system from one maker made up of:

• a primer suited to the backing board or screed;
• a liquid membrane brushed or rolled on, or a sheet membrane bonded down;
• reinforcing tape at the joints between boards, in corners and where floor meets wall;
• sealing collars or gaskets around the drain, pipes and valve outlets that pass through it.

The weak points are always the joins: corners, board joints, the drain and anything that penetrates the membrane. A system is only as good as how carefully those details are done. Most makers set out curing times before tiling, and some recommend a flood test before the floor is covered. Follow the system's instructions, not a general rule.

In a bathroom with a standard tray, the shower enclosure walls are usually tanked too. Our guide to bathroom refit order explains where tanking sits in the sequence of trades.

Falls to the drain

A wet room floor must slope, or fall, towards the drain so water does not sit on the floor or run towards the door. The fall is usually built in by:

• a pre-formed wet room former or board, with the slope already shaped in; or
• a screed laid to falls on a solid floor.

On a timber floor, the joists and boards must be stiff enough to stop the floor flexing, because movement cracks tiles and grout and strains the membrane. The former may need the joists cut down or packed so it sits at the right level. That is structural work; get the floor checked before you commit.

Getting the fall right is one of the hardest parts of a wet room. Too little and puddles sit on the floor. Too much and it feels steep underfoot and tiles are hard to lay. Large tiles also struggle to follow a fall in more than one direction, which is why many wet rooms use small mosaics or a linear drain along one edge.

The drain and trap

The drain is a fitting set into the former or screed with a trap below it. The trap holds water to stop drain smells coming back. Wet room drains are often shallow to fit within the floor depth, and some have lower water seals than a standard trap.

Things to plan:

• Where the waste can run. The drain must connect to the soil or waste pipe with enough fall. On an upper floor this means going through or between joists, which may limit where the shower can go.
• Access for cleaning. Hair builds up in shower drains quickly. Choose a drain with a removable grate and hair catcher, and a trap you can lift out to clean. Our blocked shower drain guide covers routine clearing.
• Flow. The drain has to cope with the shower's flow, especially from a pumped or high-flow shower. Check the drain maker's figures against the shower.

The water supply side

The shower valve and pipework in a wet room are plumbing like any other shower. Water Regs UK's page on bathrooms names wet rooms among the places where water fittings must be supplied via an appropriate form of backflow protection for the risk. A handheld shower that can reach a drain or a bath needs particular care. Ask your plumber what applies.

If the shower is electric, the electrical supply is work for a qualified electrician. Our guide to electric and mixer showers explains the difference.

Who does what

A wet room usually involves several trades:

• Plumber: moves the waste and supply pipes, sets the drain and fits the valve at first fix.
• Builder or joiner: strengthens or adjusts the floor and fits the former.
• Tiler or wet room installer: boards, tanks and tiles. Some wet room systems are installed by trained fitters only.
• Electrician: lighting, extractor fan and any electric shower or underfloor heating.

Agree who is responsible for the waterproofing in writing. When a wet room leaks, the first argument is often whose part failed. Building Regulations may apply to parts of the work, such as drainage, electrics or ventilation. Ask your installer what needs approval or notification, or check with your local building control. Our guide to hiring a plumber covers what to ask.

Why failures are messy

A shower tray leak is usually at the waste or the seal and can sometimes be reached. A wet room leak is often under or behind tiles:

• water gets through a failed corner, joint or drain collar;
• it spreads under tiles into boards, screed or timber;
• it shows up as a stain on the ceiling below, loose tiles, a musty smell or a soft floor, often months later.

Fixing it can mean lifting the floor or wall tiles, replacing boards and redoing the tanking. That is why the waterproofing should be done carefully, inspected before it is covered, and photographed.

Keep sealant at the junctions in good order; see sealant and silicone. Silicone is a finish, not the waterproofing.

Is a wet room the same as a walk-in shower?

Not quite. A walk-in shower normally uses a low-profile tray and a screen. A wet room has a level, waterproofed floor that falls to a drain, with no separate tray edge.

Can I tank a shower just with tile adhesive and grout?

No. Tile adhesive and grout are not a waterproof layer. Tanking is a separate membrane system, applied and detailed as its maker says, before tiling.

Can a wet room go on an upstairs timber floor?

Often, yes, but the floor must be stiff enough, the drain needs a route to the waste with enough fall, and the former may need the joists adjusting. Get a builder or plumber to check the floor before you commit.