Wet room drainage: linear drains, gullies and floor falls
Drainage decides how the floor is tiled, how it feels underfoot, and whether water leaves the room or sits on it.
How does wet room drainage work?
Wet room drainage works by falling the whole shower zone toward a drain set into the floor, at a gradient of roughly 1:80 to 1:40. Water passes through the grate into a trapped body and out to the existing soil stack through pipework laid at a continuous fall.
Linear drain or centre gully
A linear drain is a long channel, usually set against a wall or at the entry to the shower zone. Because water only has to travel in one direction, the floor can be tiled in large-format tiles with a single-plane fall. It is more comfortable underfoot and easier to keep clean.
A centre gully is a square or round point drain in the middle of the shower zone. The floor has to fall toward it from four directions, which means either a prefabricated four-way former or small-format tiles that can follow the compound slope.
How much fall a wet room needs
A fall of 1:80 to 1:40 across the shower zone is the working range — around 12mm to 25mm of drop per metre. Too little and water pools; too much and the floor feels noticeably sloped and a shower seat sits unevenly.
The fall is built into a prefabricated former in most installations. On a solid concrete floor it can also be screeded in, which allows a bespoke shape but relies entirely on the accuracy of the person laying it.
Flow rate and trap depth
The drain has to remove water at least as fast as the shower delivers it. A standard mixer shower delivers around 9 to 12 litres per minute; a large rainfall head with a body jet system can exceed 25. The drain is chosen against the actual shower specification, not by default.
Trap depth matters in Glasgow's older properties where the connection to the stack is often shallow. Shallow-profile traps exist for exactly this situation, and they trade a little flow rate for the depth you do not have.
Drainage in suspended timber floors
In a timber floor, the drain body and waste pipe sit within the joist void. The pipe run needs a continuous fall to the stack, and it cannot cut through joists wherever it is convenient — notching and drilling limits protect the structure.
This is why the drain position in a tenement flat is often dictated by the pipe run rather than the ideal shower layout. A good survey establishes this before the design is agreed, not after the floor is up.
Maintenance
Every wet room drain has a removable hair trap. Lifting and rinsing it monthly prevents the slow drainage that leads people to believe the falls are wrong. An annual check of the silicone at the drain flange takes two minutes and protects the membrane junction underneath.