How should drainage be designed for a wet room?

A wet room has no tray or threshold, so the whole floor is the shower area. Drainage design therefore starts with flow capacity, then the fall, then the drain position — and the waterproofing must cover the entire floor, not just the shower zone.

Design sequence

Step	Decision
1. Flow	Total output of all outlets; specify drain capacity above it
2. Drain position	Wall position for single-plane fall; consider travel distance
3. Slope	Single-plane fall, as shallow as accessibility allows but enough to clear water
4. Waterproofing	Full floor plus walls to the required height, continuous over the drain flange
5. Threshold	How water is prevented from leaving the room if no physical barrier exists
6. Access	Hair trap serviceable without dismantling the floor

The threshold problem

With no tray and no step, nothing physically contains the water except the floor slope and the drain capacity. This is why flow rate matters more in a wet room than in a conventional shower:

A conventional shower has a tray edge containing water while the drain catches up
A wet room has no such buffer — the drain must keep pace with the shower
If the drain is at capacity, water spreads across the floor and reaches the door
A linear drain at the wall, with the floor falling toward it, is the usual solution

Waterproofing in a wet room

The entire floor is treated as a wet area, not just the shower corner
Walls are waterproofed to the height required by the system and the application
The membrane is continuous over the drain flange
Junctions between wall and floor, and around the drain, are the critical details
Flood test before screed and tile

Common design errors

Drain sized on the shower head only, ignoring other outlets
Slope reduced below what the tile format requires
Waterproofing treated as a shower-area detail only
No thought given to how water is stopped at the doorway
Hair trap inaccessible without lifting tile

See our wall-position drain solutions and flow rate guidance.