What fall do I need for a wet room floor?

The standard fall for a wet room floor is 1:80 (12.5mm per metre) to the drain. This is the minimum gradient recommended by BS 8204-1 for screeded floors with waterproof finishes. For larger wet rooms, you may need to increase the fall to 1:60 to ensure water reaches the drain quickly. The fall should be consistent and directed towards the drain from all points.

Should I use a linear or point drain?

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. They also have a higher flow capacity than point drains. Point (centre) drains require the floor to fall in four directions, creating a more complex screed profile. Linear drains cost more (£150-400) but are easier to install correctly.

How much screed do I need for a wet room?

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.

What flow rate does my wet room drain need to handle?

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. Most linear drains handle 24-48 L/min depending on length. A 600mm linear drain typically handles 24 L/min — adequate for most showers. Always check the manufacturer's stated flow rate.

Do I need a wet room former kit?

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. They come in standard sizes from 800mm to 1800mm. 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.

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Wet Room Drainage Calculator — Fall, Screed & Drain Sizing

Calculate the correct floor fall, screed volume, drain length and flow capacity for a wet room installation to BS 8204 and Building Regulations.

Room Length

Internal length of the wet room

Room Width

Internal width of the wet room

Drain TypeLinear drains are easier to install and higher capacity

Drain PositionWall position simplifies the fall to one direction

Shower OutputMaximum flow rate of your shower

Base Screed Depth

Minimum depth at the drain point (25mm min)

Former Kit Price (optional)

Pre-formed gradient kit

Drain Price (optional)

Linear or point drain unit

Screed Price/Bag (optional)

Per 25kg bag of screed

How We Calculate This

This calculator determines the correct floor fall gradient, screed volume, drain sizing and flow capacity for a wet room based on your room dimensions and shower output.

The formula

Fall distance = Max distance to drain ÷ Fall ratio

Screed volume = Floor area × (Base depth + Fall/2)

For a 1500mm × 1200mm wet room with wall-mounted linear drain at 1:80 fall: fall = 1200 ÷ 80 = 15mm. Average screed depth = 50 + 7.5 = 57.5mm. Volume = 1.8 m² × 0.0575 = 0.104 m³ (approximately 208 kg or 9 bags).