Wet Room Guide

Wet rooms are space-efficient, accessible and increasingly common in London homes. They are also the one bathroom type where the expensive part is invisible: done properly a wet room lasts decades, done badly it wets the structure for years before anyone notices. Here is what the standards require and what it costs to meet them.

What is a wet room?

A wet room is a fully waterproofed bathroom with an open, level-access shower area draining through a floor gully, with no tray and no upstand. The floor is laid to a fall towards the drain and the whole room is tanked behind the tile, so water reaching the substrate has nowhere to go but the gully.

The build-up depends on what you are standing on. Over a suspended timber floor a pre-formed former 22 to 30 mm deep is set into an 18 mm plywood deck; over a ground-floor slab the falls are screeded. That difference drives most of the cost and most of the problems.

Which regulations apply?

No Approved Document says "tank your wet room" in those words, which is why the British Standards matter here. Four separate requirements apply.

• Drainage, Approved Document H. A 50 mm waste runs a maximum of 4 m unventilated at 18 to 44 mm of fall per metre, into a trapped gully with at least a 50 mm water seal.
• Ventilation, Approved Document F. 15 litres per second intermittent, or 8 litres per second continuous. A wet room needs it more than a standard bathroom, because the whole floor is wet after every shower.
• Electrics, Part P and BS 7671 Section 701. With no tray, Zone 1 is a 1,200 mm radius from the outlet, floor to 2,250 mm. Zone 0 takes only 12 V SELV at IPX7, Zones 1 and 2 need IPX4, and every circuit needs 30 mA RCD protection. The work is notifiable.
• Waterproofing, BS 5385-4 and BS 8000-11. These set the tanking and tiling standards for wet environments. Regulation 7 then requires workmanlike execution using adequate materials, which is the hook Building Control and an insurer use if it fails.

There is a structural limit too. Dropping a gully and trap through a timber floor means notching or drilling a joist, and Approved Document A allows notches no deeper than 0.125 of the joist depth, between 0.07 and 0.25 of the span from a support, with holes no larger than 0.25 of the depth on the neutral axis. A trap body does not fit those limits on a shallow joist, and that is the constraint that quietly rules a wet room out upstairs. See our building regulations guide.

Why tanking is non-negotiable

Tile, adhesive and grout are not waterproof. They resist water; the membrane behind them excludes it. That is why a wet room that looks immaculate can be wetting joists for five years.

• Coverage: full height in the shower zone, at least 150 mm up every other wall above finished floor level.
• Junctions: reinforcing tape bonded in at every internal corner and floor-to-wall junction, collars at every pipe penetration, and the drain flange bonded into the system rather than sealed against it.
• Substrate: tile backer board in the shower zone, not plasterboard, primed first.
• Application: two coats of liquid membrane at the stated wet film thickness, or a bonded sheet system. One thin coat is the commonest failure we are called to.
• Proof: a 24 hour flood test before a single tile goes down. It is the only evidence you will have, and it costs nothing.

Where it has already failed, the symptoms read as rising damp on the floor below rather than as a leak. Our guide to damp causes and fixes covers telling them apart.

What fall does the floor need?

BS 5385-4 sets a minimum of 1:80, or 12.5 mm per metre, and 1:60 is the figure to build to. Anything flatter pools, and pooled water finds the weakest joint eventually.

A 1,200 mm shower zone therefore drops about 20 mm at 1:60. Forming falls by building up adhesive is not a method, and large-format tiles cannot follow a four-way fall without cracking, which is what pushes most wet rooms towards a linear drain.

Linear or point drain?

Choose a linear channel if you want large-format tiles, and a point drain if the shower zone is square and the tile is small. The drain decides the tiling, not the other way round.

Match the gully to the shower, not to the drawing. A standard 50 mm gully passes around 25 litres per minute and high-flow versions reach 36 to 48, while a normal shower runs 8 to 12 litres per minute and a rain head 15 to 20. A rain head with body jets outruns a low-flow gully, and the result is standing water in a room designed not to have any.

How a wet room is built

1. Survey. Establish joist depth and direction, or slab construction, and confirm the trap can drop within the notching limits.
2. Strip out to the deck, checking for damp or rot.
3. Form the falls: a pre-formed former set into an 18 mm ply deck on timber, or a screed to 1:60 on a slab.
4. Set the gully and run the 50 mm waste to the stack, within 4 m at 18 to 44 mm per metre.
5. Board the shower zone in tile backer board.
6. Tank: prime, then two coats of liquid membrane or a bonded sheet, taping every junction and collaring every penetration.
7. Flood test for 24 hours. Only then does tiling start.
8. Tile on a solid bed, grout, and silicone every change of plane. See bathroom tile ideas.

Where a wet room works, and where it does not

Wet rooms suit small rooms, second bathrooms and level access. They suit a family's only bathroom much less well.

• Good candidates: en-suites, bathrooms under about 4 m², ground floors on a slab, and households staying put who want step-free access.
• Poor candidates: the only bathroom in a family house, where losing the bath costs on resale; upper floors with shallow joists or a fixed floor level; and unheated rooms, where the floor never dries.
• Leasehold flats: the lease usually requires the freeholder's written consent to alter waste runs, and a wet room above another flat is a conversation with the insurer too.

Where the room stays warm, the floor dries, which is why underfloor heating is close to standard here rather than a luxury. A fixed screen at 900 to 1,000 mm keeps towels and paper out of the splash zone without reintroducing a threshold. More in small bathroom ideas.

Pros and cons

• For: level access, an open feel in a small room, easy to clean, no enclosure to replace, genuine future-proofing.
• Against: dearer than a standard bathroom, the whole room gets wet so storage needs placing, the build-up may not be available upstairs, and everything depends on the installation.

What does a wet room cost in London?

Nationally the average is £5,500, with most projects between £4,000 and £7,000. In London a wet room detailed to the standards above runs £10,000 to £18,000, more than a standard bathroom of the same size because of the tanking, the falls, the recessed drain and usually underfloor heating.

If the adaptation is for someone with a chronic illness or disability, VAT relief at 0 or 5% may apply to the installation and the specialist products, worth confirming before work starts. See bathroom renovation London, or contact us on 07472 424 226.