Summary

Floor falls are the part of a wet room that nobody sees once it's tiled, and the part that generates the most callbacks when it's wrong — standing water, black mould at the low points, and, worst case, water finding its way past the tanking at a badly detailed upstand because the substrate was never properly formed to fall in the first place. Unlike a shower tray, which arrives with the fall built in, a true wet room floor (tiled, no tray, wheel-in or level threshold) has to have its gradient created on site, either in a formed screed, a preformed foam former, or a proprietary tray-former system bonded to the substrate.

The confusion in the trade usually isn't about direction of fall — everyone knows water needs to run to the drain — it's about how steep, how far, and whether one gradient serves the whole floor or whether you need a compound fall: gentle across the main area, steeper right at the drain. Get the ratio wrong in either direction and you get a problem: too shallow and water sits, ponds, and is slow to clear (a particular issue for barefoot use and for keeping the floor genuinely dry between uses); too steep and the floor is uncomfortable, unsafe for anyone with balance or mobility issues, and can visually read as "sloping" rather than flat.

This article works through the numbers: what fall ratio to use where, how to calculate the actual rise across a given room, the difference between point-drain (conical) and linear-drain falls, and how substrate choice changes the buildable depth you have to work with. For the wider construction sequence — tanking membrane selection, substrate types, and waterproofing detailing — see waterproofing and bathroom waterproofing tanking.

Key Facts

Quick Reference Table — Fall Ratios and Rise per Metre

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Fall ratio Rise per metre Typical use Notes
1:80 12.5mm/m Standard minimum fall for a wet room floor Widely used minimum per BS 5385-4 / BS 8204-1; the default minimum on most manufacturer wet-room systems
1:60 16.7mm/m Preferred/comfortable fall, especially accessible wet rooms BS 8300-2 comfort zone; drains faster than 1:80 while remaining safe and comfortable to stand or wheel on
1:50 20mm/m Steeper option on standard (non-accessible) wet room floors Some manufacturer systems specify this as their default for faster drainage; not the minimum, an alternative to it
1:40 25mm/m Drain collar / transition zone Common "second fall" in a compound-fall design
1:35 28.5mm/m Tightest collar immediately around gully Fastest drainage; not suitable across a full walking area, and not the floor's minimum fall
1:25 40mm/m Not recommended for domestic wet rooms Too steep for comfortable/safe standing use

Detailed Guidance

Working out the fall you actually need

Start from the two constraints, not the ratio:

  1. How far is the furthest point from the drain? — this determines total rise, and therefore total floor build-up, at a given ratio.
  2. Is this an accessible/level-access wet room? — if yes, the main area gradient is capped around 1:60 for comfort and safety, and you push the extra fall you need into a tight collar around the drain rather than steepening the whole floor.

Worked example — point drain, standard wet room: A 2.4m × 1.8m shower room, drain positioned centrally against the back wall. The furthest point (opposite corner) is roughly 2.2m away.

Worked example — linear drain, accessible wet room: A 3.0m-wide room with a linear drain running along the far wall from the entry threshold, main area falling 1:60, with the last 400mm before the drain falling at 1:35.

Always calculate the total rise before committing to floor buildup, especially where the wet room sits over a timber floor with limited depth to the joists, or where a level (kerb-free) threshold to an adjoining room is required — the fall has to be absorbed entirely within the wet room's own floor construction if the adjoining room's finished floor level can't move.

Point drain vs linear drain — why the choice changes the fall design

Point drain (gully): needs fall converging from every direction, which means every wall-to-drain distance must independently achieve the minimum gradient. In a square or rectangular room this produces a "hipped" floor shape, like a shallow pyramid inverted toward the drain, which is harder to form accurately by hand-screeding and needs the deepest overall buildup in the corner furthest from the drain.

Linear drain: needs fall in one direction only, perpendicular to the drain channel. This is significantly easier to form (a single-direction float or a preformed former panel), gives a shallower maximum buildup for the same fall ratio, and is the preferred choice where floor depth is constrained — over timber joists, in a loft conversion, or where the finished floor height has to match an adjoining room without a step. Linear drains also tend to clear larger volumes of water faster because the full width of the channel is doing the work, not a single point.

Substrate and floor buildup options

Method Typical total buildup Pros Cons
Hand-floated sand:cement screed 40–100mm depending on fall and room size Any shape/fall achievable, cheap materials Skilled labour, slow cure (days before tiling), risk of uneven fall if not checked with a laser level
Preformed foam tray-former (Wedi, Impey, Schlüter) 30–65mm depending on model Consistent factory-made fall, fast install, integral waterproofing on some ranges Fixed sizes/gradients — may need cutting/adapting for non-standard rooms; higher material cost
Self-levelling compound over pre-formed fall Thin topping layer, 3–10mm Flat, fast-curing finish surface over a formed substrate Not a fall-forming method on its own — needs the fall already built into the base
Timber floor with firring pieces + ply/backer board 25–50mm Works over suspended timber floors without deep screed Requires structural check for point loads, deflection limits, and careful detailing at the drain penetration

Whichever method is used, the fall must be checked with a laser level or long straight-edge before the waterproofing membrane goes on — a membrane will faithfully follow every hump and dip in the substrate beneath it, so correcting a fall after tiling means stripping back to substrate.

Compound falls for accessible wet rooms

A wet room built to Approved Document M / BS 8300-2 principles for level-access, wheelchair-accessible use has a genuine conflict to solve: comfort and safety want a shallow gradient, drainage wants a steeper one. The standard solution — and the reason the range "1:80 to 1:35" appears together in the same design — is a two-zone fall:

This detail needs to be agreed and drawn before the substrate work starts — it's a screed-forming and tray-former specification decision, not something that can be corrected once the floor is built. If working to a Disabled Facilities Grant specification or an occupational therapist's brief, confirm the exact accessible gradient limit specified for that project — some briefs specify 1:60 as an absolute maximum across the entire floor with no exceptions, which rules out a steep collar zone entirely and pushes the whole solution toward a linear drain with a very shallow, very slightly graded floor instead.

Common fall-related failures on site

Symptom Likely cause Fix
Standing water away from the drain Low spot / dip in the screed, not checked with a laser level before tiling Full or partial strip-out and re-form the fall; cannot be patched from above
Water pooling at the threshold Fall running the wrong way, or a level threshold with insufficient fall built in before it Re-form the last section of floor; check threshold detail and door bar/upstand
Slow-draining floor despite correct fall Undersized or partially blocked drain/gully for the shower flow rate Check drain flow rate against BS EN 274 waste fitting capacity, clear/upsize the outlet
Cracked tiles along the fall line Screed not cured before tiling, or inadequate movement joint provision on a large falling floor Allow full screed cure time before tiling; add movement joints per BS 5385 spacing rules
Membrane pinholing / early leak at the drain collar Fall too tight around a point drain, causing the membrane to be stretched or poorly bedded at a sharp change of gradient Widen the transition radius around the drain; use a manufacturer's pre-formed drain collar piece rather than hand-forming a sharp change

Frequently Asked Questions

Is 1:80 too shallow for a wet room floor?

No — 1:80 (12.5mm per metre) is the widely recognised minimum drainage fall for a wet room floor, drawn from BS 5385-4 / BS 8204-1 site practice and used as the default minimum by most manufacturer wet-room systems. It clears water reliably at a gradient that's still comfortable and safe to stand or wheel on. Some manufacturer systems specify a steeper default (commonly 1:50) for faster drainage — check the specific tray-former or tanking system's technical data sheet, since minimum falls do vary by product — but 1:80 is the floor below which you shouldn't go.

Can I use the same fall ratio for the whole floor in an accessible wet room?

You can, but a single shallow ratio (say 1:60 right to the drain) may drain more slowly than a compound fall, and a single steep ratio (1:35 right across the room) will usually fail accessibility comfort requirements. A compound fall — shallow main area, steeper collar at the drain — is the standard way to satisfy both requirements at once; confirm against the specific accessibility brief for the project.

Does the fall need to be checked before or after the tanking membrane goes on?

Before. The membrane, whether liquid-applied or sheet, follows the shape of whatever is beneath it. Any error in the fall — a low spot, an uneven gradient, a flat patch — will show up as standing water once tiled, and the only fix is to strip back to the substrate and re-form it. Always check with a laser level across multiple points before applying the membrane.

How does floor buildup affect a suspended timber floor wet room?

Timber floors have limited spare depth before the finished floor level clashes with door thresholds, adjoining rooms, or ceiling heights below. A linear drain with a shallow single-direction fall over firring pieces and a rigid backer board typically gives the most buildup-efficient solution. Always get the joists checked for point loading and deflection at design stage — a wet room floor loaded with screed and tile is considerably heavier than a standard timber floor finish, and deflection under a tiled floor causes grout and tile cracking even if the waterproofing holds.

What's the minimum trap seal depth for a wet room gully?

Building Regulations guidance (Approved Document H) generally expects a minimum 50mm trap seal depth to prevent foul air and odours being drawn back through the gully, though this can vary with drain layout and venting — check the specific gully manufacturer's installation instructions and the project drainage design.

Regulations & Standards