Shower Tray Fall Calculator — Wet Room Gradient Guide

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.