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Shower Tray Fall Calculator — Wet Room Gradient Guide
Calculate the correct fall gradient for a wet room or walk-in shower. Uses the UK standard 1:80 minimum fall set by BS 5385-4 (tiling) and BS 8204-1 (screeds).
Length of the shower/wet room floor
Width of the shower/wet room floor
Approximate screed volume using the average depth across the floor — add 5–10% for wastage
Fall gradient in mm per metre. BS 5385-4 range: 12.5 (1:80, minimum) to 28.6 (1:35, maximum). Default 12.5.
Minimum screed depth at drain point in mm (default 25)
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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). BS 5385-4 (tiling in wet areas) sets the gradient range at 1:80 to 1:35, and BS 8204-1 sets the same 1:80 minimum for the screed that forms the fall.
The formula
Fall depth = Distance to drain × 12.5mm per metre
Total screed at edge = Minimum screed at drain (25mm) + Fall depth
The optional screed volume is an approximate estimate using the average screed depth across the floor. It is a guide only — order a little extra (typically 5–10%) for wastage and to account for how the fall is formed around a point or corner drain.
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
The minimum fall for a wet room floor is 1:80, which equals 12.5mm per metre — for every metre between the edge and the drain, the floor drops 12.5mm. BS 5385-4 (tiling in wet areas) sets the gradient range at 1:80 (shallowest) to 1:35 (steepest, about 28.6mm per metre); gradients steeper than 1:35 can be inconvenient and unsafe to walk on. BS 8204-1 sets the same 1:80 minimum for screeded wet-area floors. Many installers use 1:60 (about 16.7mm per metre) for faster drainage, which sits comfortably within the range. Do not go shallower than 1:80 or water can pond on the tiles.
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.
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.
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 for material quantities. Ventilation must comply with Part F — use our Bathroom Extraction Calculator for fan sizing.
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.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.