Wet Room Drainage Calculator — Fall, Screed & Drain Sizing

1:80, which is 12.5mm per metre, is the minimum fall for a screeded wet-room floor, and this calculator will not go flatter. Read the ratios the right way round first, because it is the usual source of confusion: a larger second number is a SHALLOWER fall, so 1:100 is gentler than 1:80, not steeper. The working band for a tiled wet area is 1:80 to 1:35. Flatter than 1:80 and water will not drain off the tiles; steeper than 1:35 and the floor becomes awkward and unsafe to stand on. Many installers work at 1:40 to 1:60 in the showering area so water reliably reaches the drain across a tiled, less-than-perfect surface, and pre-formed gradient formers build a fall in for you. One source words it differently and it is worth knowing about: LABC Warranty say falls to the wet area must prevent ponding and should fall between 1:80 and 1:100 to the drainage point, which would allow up to 1:100. We apply the stricter 1:80, in line with BS 5385-4 as it is commonly quoted for tiled wet areas and with our own shower tray fall calculator. The fall should be consistent and directed towards the drain from all points.

Linear (channel) drains are the preferred choice for modern wet rooms. They sit along one wall, meaning the floor only needs to fall in one direction — making screeding much simpler. Channels are typically rated around 40–42 L/min, comfortably above any domestic shower. Point (centre) drains require the floor to fall in four directions, creating a more complex screed profile, and their capacity is set by the 90mm waste (standard ~32 L/min, high-flow 43–50 L/min). Linear drains cost more (£150–400) but are easier to install correctly.

Wet room screed depth depends on the fall distance and the base depth. Minimum screed depth at the drain point should be 25mm, with the depth increasing towards the walls based on the 1:80 fall. For a typical 1.5m × 1.2m wet room with a wall-mounted linear drain, you need approximately 0.1-0.15 m³ of screed. Use a rapid-set or polymer-modified screed suitable for wet areas.

Your drain must handle the maximum shower output: 9 L/min for electric showers, 12 L/min for standard mixers, 15 L/min for thermostatic mixers, and up to 20 L/min for rain/drencher heads. A drain's capacity is governed by its outlet and trap, not by a per-metre channel rule. As a guide, Impey Aqua-Dec linear channels are rated at 40 L/min for a 600mm channel and around 42 L/min for an 800mm channel — comfortably above any domestic shower. Point drains depend on the 90mm waste: the BS EN 274 minimum is 24 L/min (0.4 l/s), standard fast-flow wastes manage around 32 L/min and high-flow units 43–50 L/min. Always check the manufacturer's stated flow rate for the specific drain and waste you fit.

A pre-formed gradient former (like those from Impey, Kudos or CCL Wetrooms) makes installation much easier and more reliable. The former creates the correct 1:80 fall and integrates with the drain. Sizes are fixed, and they come from the manufacturer's range rather than being made to your room: Impey's Aqua-Dec Linear channel formers run from 900 x 900mm to 2000 x 1200mm, and their square EasyFit formers, which take a central point drain, run from 800 x 800mm to 1850 x 900mm (Impey Wetroom Design and Specification Brochure, Edition 5.8, April 2025). Pick the next size up from your showering area, because the trimmable ones only cut down, and the Aqua-Dec Linear 3 only within 75mm each side of the drain edge. If your showering area is longer than the largest former, no single former covers it and the fall has to be formed in screed instead, which is what this calculator will tell you rather than naming a size that nobody makes. While you can create the fall with screed alone, a former kit provides a guaranteed gradient and is strongly recommended for DIY and professional installations.