Floors and substrates
Wet room and shower floor fall
- where
- fall over the run (mm)
- run from high point to drain (m)
- gradient expressed as 1 in N
- gradient (%)
- angle of fall (°)
Explain this equation
Works out how much a wet room or shower floor must drop from its high point to the drain. Use it when setting out a former or screeding falls.
Lr is the longest path water travels to the drain, usually from the far corner. N is the gradient as “1 in N”: 1 in 80 means the floor drops 1 mm for every 80 mm across. The percentage and the angle are the same slope said differently.
A gradient is just height over length, so the fall is the run divided by N. The 1000 turns the run from metres into millimetres. The angle comes from the arctangent of that same rise-over-run.
Sense check: 1.2 m at 1 in 80 needs 15 mm of fall; at 1 in 40 it needs 30 mm. Halving N doubles the fall.
Watch out: shower areas are normally laid between 1 in 80 and 1 in 35, and whole wet room floors between 1 in 80 and 1 in 100 to the drain. Build the fall into the base, not the adhesive bed.
Works out how much the floor must drop from its high point to the drain for a chosen gradient.
Enter your values to see the result.
Good to know
Published ranges: falls within the shower area between 1:80 and 1:35; wet room floors falling 1:80 to 1:100 to the drain (warranty provider guidance). Measure the run along the longest path water travels, usually from the far corner to the gully. Remember the waste, the tile and the adhesive bed: the fall must be built into the base or former, not the adhesive. Check the former or tray maker’s stated fall and the drain’s required tile height.
Common mistake
Measuring to the nearest edge of the drain rather than from the furthest corner, so the far corner ends up flat and ponds.
Limits
Straight single-plane fall. Envelope (four-way) falls to a centre gully need each face set out separately, with cuts along the hips. Large-format tiles cannot follow multi-plane falls without lipping.
Sources
- BS 5385-4:2015 Wall and floor tiling – Design and installation of ceramic and mosaic tiling in specific conditions – Code of practice
- Building CPD 14 2015: Integrated solutions for wetrooms with tile and stone coverings (falls 1:80 to 1:35 in showers)
- Premier Guarantee technical update: Showers and wet room construction (falls 1:80 to 1:100 to the drainage point)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.