A wet-room with a level-threshold shower, full-height tile, and a linear channel drain reads as a clean architectural space. Built correctly it holds up for twenty years. Built ordinary it leaks through the ceiling of the room below by the third winter. The difference between good and ordinary wet-room construction is nearly always in the substrate, the fall, and the movement joints, not the tile.

The substrate

A tiled wet-room floor sits on a waterproofing membrane that sits on a substrate. The substrate has to be rigid enough that the tile does not flex, flat enough to hold the fall, and waterproof enough that an imperfect membrane does not lead to catastrophic failure.

We build most wet-room floors on a concrete nib or poured topping over the sub-floor, fall applied during the pour. The concrete is waterproofed with a two-pack sheet or liquid applied system. The membrane is bonded to the substrate, sealed at all penetrations, and turned up the walls at least one hundred and fifty millimetres above the finished floor level at the dry side and two hundred millimetres on the shower side.

On a timber sub-floor, we use a proprietary tile backer board with a factory fall built in, fixed to the joists with a vapour gap below. The falls have to be set out carefully on the joist layout.

Fall and drainage

A level-threshold shower requires a fall of not less than one in fifty across the shower zone toward the drain, and a fall of not less than one in eighty across the wider wet-room floor toward the shower zone. Most Auckland wet-rooms fail to hit the second of these two, which is why water puddles outside the shower in older builds.

The linear channel drain at the shower wall is our default over a central point drain. The reasons are that the channel accepts water flowing in from two directions, the fall can be set in one plane, and the tile layout does not need to be cut around a central grate. The channel has to be sized for the expected flow rate. A low-flow rain shower over three bodies per day can be served by a smaller channel. A high-flow dual shower with body jets needs a wider channel and a larger outlet.

The waterproofing system

The two-pack liquid applied membrane or sheet membrane must be specified as a system with the tile adhesive and grout. Mixing systems invalidates the warranty and sometimes compromises the bond between membrane and tile. We use Ardex, Mapei, or Sika systems on wet-rooms. Each has its own preferred tile adhesive and grout.

The membrane is dressed into the channel drain, around penetrations for the mixer and shower head, and up the wall to the specified height. Every corner, every internal angle, every external angle gets an additional reinforcing strip over the membrane. These strips are the difference between a membrane that holds for twenty years and one that opens a split at a corner after five.

Movement joints

A tile floor over a concrete substrate will move. The concrete expands and contracts with temperature. The tiles are rigid. Without movement joints, the stress is taken out on the weakest grout line, which cracks, lets water through, and starts a slow failure.

Movement joints are specified at every internal corner where the floor meets a wall, at every external corner where one material meets another, and across the floor at maximum intervals of four metres. The joint is a silicone filler in a colour matched to the grout. It is the detail most often skipped on mid-range bathrooms and the one we will not skip on our own work.

Tile selection and layout

For slip resistance, the shower zone tile should have an R10 or R11 slip rating. For visual continuity, the dry-zone tile can be the same format. A large-format tile of six hundred by six hundred or larger reads cleanly but is harder to set into the falls. Small-format mosaic works well in a shower zone because the many grout lines accommodate the fall naturally.

The layout is set out by the tile-layer with the architect before the cuts are made. The visible cut lines at doors, skirting, and the shower threshold should be considered rather than accepted as they fall out of a grid drawn from a random starting corner.

Heating and ventilation

Underfloor heating in a wet-room pushes the drying out of the room between uses. A wet-room without heat or ventilation in the south face of an Auckland house in winter will take three days to dry between uses. The consequence is visible mould within twelve months.

We specify underfloor heating on a separate thermostat and a ceiling extractor vented to the outside, not into a roof void. Those two items resolve ninety percent of the mould problems that turn up on the defect walk twelve months in.