What is the correct fall for a wet room floor?

The minimum recommended fall for a wet room floor is 1:80, which equals 12.5mm per metre. This means for every metre of floor between the edge and the drain, the floor drops by 12.5mm. Some installers prefer 1:60 (about 17mm per metre) for faster drainage. BS 5385-4 covers tiling in wet areas and references these gradients.

Where should the drain be positioned in a wet room?

Centre drains are the most common and create a four-way fall which is easiest to form in screed. Linear (channel) drains along one wall create a simple one-way fall and are popular in modern designs — they are also easier to waterproof. Corner drains work but create a longer fall distance and can be trickier to screed evenly.

What is the minimum screed depth at the drain point?

The minimum screed depth at the drain (thinnest point) should be 25mm. This ensures structural integrity and provides enough depth for the screed to bond properly to the substrate. The total screed depth at the edges depends on the fall distance — our calculator works this out for you.

Do I need Building Regulations approval for a wet room?

Wet rooms in England and Wales must comply with Building Regulations Part C (moisture), Part E (sound in flats), and Part L (energy). While you do not typically need formal approval for a like-for-like bathroom renovation, a new wet room may require a building notice if it involves structural changes. Waterproofing (tanking) must meet BS 5385-4 — see our wet room tanking calculator (/tiling/wet-room-tanking-calculator/) for material quantities. Ventilation must comply with Part F — use our bathroom extraction calculator (/tiling/bathroom-extraction-calculator/) for fan sizing.

Can I create the fall with tile adhesive instead of screed?

No — you should not rely on tile adhesive to create the fall. The gradient must be formed in the screed layer beneath the tiles. Tile adhesive should be applied at a consistent thickness (typically 3-6mm). Using excessive adhesive to create falls can lead to cracking, poor adhesion and waterproofing failure. Always form the fall in a sand-cement screed or proprietary wet room former.

Shower Tray Fall Calculator — Wet Room Gradient & Screed Depth UK

How We Calculate This

Our shower tray fall calculator determines the height difference needed between the drain and the furthest edge of your shower area, based on the UK standard minimum fall of 1:80 (12.5mm per metre). This gradient is recommended by BS 5385-4 for tiled wet areas.

The formula

Fall depth = Distance to drain × 12.5mm per metre

Total screed at edge = Minimum screed at drain (25mm) + Fall depth

How drain position affects fall

Centre drain: Water falls from all four edges to the centre. Maximum distance is half the longest dimension. Creates a four-way fall (inverted pyramid).
Corner drain: Fall runs diagonally from the opposite corner. Maximum distance is the diagonal of the shower area. Requires more screed depth at the far corner.
Linear drain: Simple one-way fall across the width. Maximum distance is the full width from the far edge to the channel drain. Easiest to form and waterproof.

UK Building Regulations

Wet room floors must comply with Building Regulations Approved Document C (Site preparation and resistance to contaminants and moisture). The floor must be waterproofed (tanked) using a system compliant with BS 5385-4. The minimum 1:80 fall ensures water drains efficiently and does not pool on the tiled surface.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Shower Tray Fall Calculator — Wet Room Gradient Guide