Walk-In Shower Design: Falls, Drainage, Waterproofing and Glass

Quick Answer: A walk-in (level-access or low-threshold) shower must drain reliably and stay watertight, so the floor needs a positive fall to the gully — typically 1:48 to 1:80 (around 12–20 mm per metre) for a point drain, or a slightly gentler linear-drain fall — and the whole wet zone must be tanked (waterproofed) to BS 5385-4 before tiling. Building Regulations Part G caps bath hot-water fill at 48°C for scald prevention; Part G does not set a 48°C cap on shower outlets, but a thermostatic shower mixer is strongly recommended best practice (commonly set around 41–43°C). Glass screens must be toughened safety glass to BS EN 14428 / Part K (and Part N in older terminology), and a level-access threshold should follow Part M accessibility principles. Get the fall, the tanking and the drain capacity right or the floor pools, leaks downward, and you get a callback.

Summary

The "walk-in shower" covers everything from a low-profile shower tray with a glass screen up to a fully tiled level-access wet floor with a linear drain. What they share is the removal of the high bath-style step, which is exactly what makes water management harder: there is no upstand holding water in, so the fall, the drain capacity and the tanking carry the whole burden of keeping water where it belongs. A bath surround forgives a sloppy bead of silicone; a level-access wet floor does not.

This matters to plumbers, tilers, bathroom fitters and wet-room specialists, and increasingly to anyone doing accessible or "ageing-in-place" adaptations where the brief is no step and a chair-accessible floor. The single biggest cause of walk-in shower failure is water escaping the wet zone — either because the fall is wrong (water runs the wrong way, or sits flat and creeps), the drain can't shift the flow fast enough, or the tanking membrane was skipped, lapped wrong, or punctured at the drain flange. Every one of these is invisible once the tiles are on and ruinously expensive to put right.

The common misconceptions are: that a tray is always easier than a tiled floor (a quality tray is often the safer waterproofing choice); that you only need to tank the shower corner (you tank the whole wet zone and turn the membrane up the walls); that any waste will do (drain flow rate must match the shower's flow); and that fall is optional on a linear drain (it isn't — it's just more forgiving). Decide tray-vs-tiled early, because it changes the build-up, the drainage and the price.

Key Facts

• Governing waterproofing standard — BS 5385-4: Code of practice for the design and installation of ceramic and natural stone wall and floor tiling in specific conditions (wet areas / wet rooms).
• Floor fall to point gully — typically 1:48 to 1:80 (about 12–20 mm/m). Steeper end (1:48) drains faster and suits smaller-format tiles; too steep and large tiles "lippage" and rock.
• Linear drain fall — single-plane fall to a linear channel can be gentler (often around 1:80) because water only has to travel to a line, not a point. Falls to one wall, or a central channel with falls from both sides.
• Hot water temperature — Building Regulations Part G3 limits bath fill to a maximum 48°C (scald prevention). Part G sets no 48°C cap on shower outlets, but a thermostatic shower mixer is strongly recommended (commonly set ~41–43°C); TMV2 for domestic, TMV3 for healthcare/vulnerable users.
• Thermostatic mixer — thermostatic shower valve required to hold temperature and fail safe if cold supply is lost.
• Level-access threshold — Part M principles: flush or near-flush entry, suitable for wheelchair/walking-frame use; no upstand the user must step over.
• Glass safety standard — shower screens/enclosures to BS EN 14428 (shower enclosures), glass toughened/safety glass per Building Regulations Part K (critical locations) — formerly the subject of Part N in older Approved Documents.
• Glass thickness — frameless walk-in panels typically 8–10 mm toughened; framed/sliding enclosures often 5–6 mm.
• Drain flow rate — choose a waste rated for the shower's flow; high-flow rainfall heads can exceed a standard 50 mm trap's capacity, causing pooling. Linear drains and 50 mm fast-flow traps suit high flow.
• Waste size — shower wastes are commonly 50 mm (vs 40 mm for basins/baths) for higher flow; trap seal depth must satisfy BS EN 12056-2.
• Trap seal — minimum 50 mm water seal to prevent drain odours (BS EN 12056-2).
• Tanking up the walls — turn the waterproof membrane up the walls to at least 100 mm above the finished floor, and to full height in the shower spray zone.
• Substrate — tiling-grade backer board (cement board / foam tile-backer / suitably prepared and primed concrete or screed). Standard plasterboard is not a wet-zone substrate.
• Slip resistance — floor tiles should have adequate wet slip resistance; aim for a tested anti-slip rating (PTV / R-rating). HSE references the Pendulum Test (PTV ≥ 36 for low slip risk wet).
• Falls construction — pre-formed sloped tray/former, or screed laid to falls, or a proprietary level-access tray; the drain sits at the low point.
• Electrical zones — shower zones follow BS 7671 bathroom zoning (Zones 0/1/2); IP ratings and RCD protection apply.

Quick Reference Table

Detailed Guidance

Tray vs tiled level-access floor — decide first

A quality level-access shower tray is, paradoxically, often the more reliable waterproofing solution: the tray is the waterproof layer and the fall is moulded in by the manufacturer. A tiled wet floor looks seamless and is unavoidable for true wheelchair access or unusual layouts, but it puts the entire waterproofing burden on the tanking and on getting the falls right on site. For most domestic walk-ins, a low-profile or flush-fit tray with a frameless glass panel gives the look with far lower leak risk. Choose tiled-to-falls when the floor area is large, the geometry is awkward, or full level access is the brief — and then tank it properly.

Falls and setting out the drain

Water has to be led to the drain; it won't find a flat gully. For a single point gully, form a fall of roughly 1:48 to 1:80 from all directions into the drain (a four-way "pyramid" fall, or a former that does this for you). For a linear drain, you only need a single-plane fall toward the channel, which is gentler to build and looks better with large-format tiles (no diagonal cuts, no four-way ridges). Linear drains can sit against the wall (fall away from the room) or be set in the floor (falls from both sides). Set the drain at the true low point, and check the fall with a level and a straightedge before tiling — once tiles are on, a reverse fall means ripping it out.

Waterproofing (tanking) to BS 5385-4

Tanking is the make-or-break stage. The whole wet zone — floor and the lower part of the walls, full height in the spray area — is sealed with a continuous waterproof system: liquid membrane, sheet membrane, or a proprietary tanking kit, with reinforcing tape at every internal corner and movement joint, and a bonded collar at the drain flange. Lap the floor membrane up the walls (at least ~100 mm, more in the spray zone) so any water on the floor can't get behind the wall membrane. The most common failure point is the drain flange — the membrane must bond to the manufacturer's bonding flange so water on the membrane drains into the gully, not around it. Tile only onto a fully cured, continuous tank.

Drainage capacity — match the waste to the head

A standard shower trap is fine for a normal head, but big rainfall and "deluge" heads can deliver more litres per minute than a basic 50 mm trap can clear, so water backs up and pools. Choose a high-flow 50 mm waste or a linear drain rated above the showerhead's flow rate, and keep the waste run short with adequate fall to the stack (per BS EN 12056-2). Maintain a minimum 50 mm trap water seal for odour control, and make sure the trap is accessible for clearing (removable grate / accessible inspection).

Glass screens and safety glass

Walk-in screens and enclosures should be made to BS EN 14428 and use toughened safety glass because shower glass is a "critical location" under Building Regulations Part K (impact safety — historically also Part N). Frameless walk-in panels are typically 8–10 mm toughened glass on a channel or with a bracing bar; framed and sliding enclosures commonly use 5–6 mm. Fix into solid, properly waterproofed substrate (not just tile adhesive over board), seal the glass-to-tile junction with a low-modulus sanitary silicone, and make sure any fixed panel can't be walked into without being seen — manifestation/visibility matters for large clear panels.

Temperature control and Part G

Building Regulations Part G3 requires bath hot-water fill to be limited to a maximum of 48°C for scald prevention. Part G does not impose a 48°C limit on shower outlets — but a thermostatic shower valve is strongly recommended best practice (commonly set around 41–43°C), holding the set temperature and shutting down if the cold supply is lost. For vulnerable users (elderly, disabled, children) specify a TMV3 valve and consider a lower set maximum. Don't rely on a manual mixer "set carefully" — fit a thermostatic device.

Slip resistance and accessibility

A wet floor that drains well can still be dangerous if it's slippery. Choose floor tiles with a tested wet slip resistance (Pendulum Test Value / R-rating); HSE treats PTV ≥ 36 as low slip risk in the wet. For accessible installs, follow Part M: flush threshold, level approach, space for a chair, grab rails fixed to solid pattressing (not just board), and a fold-down or fixed seat where specified. Small-format mosaic on the wet floor both improves grip and lets the tiles follow a point-drain fall without lippage.

Frequently Asked Questions

What fall do I need for a walk-in shower floor?

For a point gully, form a fall of roughly 1:48 to 1:80 (about 12–20 mm per metre) from all directions into the drain. A linear drain can use a gentler single-plane fall (around 1:80) because water only travels to a line. Always check the tray/drain manufacturer's stated fall — it governs.

Do I have to tank the whole floor, or just the shower area?

Tank the whole wet zone — the floor and the lower walls, full height in the spray area — as a continuous waterproof system to BS 5385-4, and turn the floor membrane up the walls. On a level-access wet floor there's no upstand to contain water, so partial tanking is asking for a leak. A tray install still needs the surrounding walls tanked.

Why does my walk-in shower pool water with a rainfall head?

The waste can't clear the flow fast enough. Big rainfall/deluge heads can exceed a standard trap's capacity. Fit a high-flow 50 mm waste or a linear drain rated above the head's litres-per-minute, and check the fall actually leads water to the gully.

Can I use normal plasterboard behind the tiles?

No. The wet-zone substrate must be a tiling-grade backer board (cement board or foam tile-backer) or properly prepared and tanked masonry/screed. Standard plasterboard degrades when wet and isn't a wet-area substrate.

What temperature is the legal maximum for a shower?

There is no Part G legal maximum specifically for shower outlets — the 48°C Part G3 limit applies to bath fill (scald prevention). For showers it's strong best practice to fit a thermostatic mixing valve set to a comfortable, safe temperature (commonly ~41–43°C). For vulnerable users specify a TMV3 valve and consider setting it lower.

Regulations & Standards

• Building Regulations Part G (Sanitation, hot water safety and water efficiency) — 48°C bath-fill scald-prevention limit (G3), water efficiency.

• Building Regulations Part M (Access to and use of buildings) — level-access and accessible shower provision.

• Building Regulations Part K (Protection from falling, collision and impact) — safety glazing in critical locations (historically Part N in older guidance).

• Building Regulations Part P / BS 7671 — electrical safety and bathroom zones (0/1/2), RCD protection, IP ratings.

• BS 5385-4 — Wall and floor tiling: design and installation of ceramic/stone tiling in wet conditions (tanking).

• BS EN 14428 — Shower enclosures: functional requirements and test methods.

• BS EN 12056-2 — Gravity drainage systems inside buildings (waste falls, trap seals).

• Water Supply (Water Fittings) Regulations 1999 — backflow and water fittings.

• HSE slips guidance / Pendulum Test (PTV) — wet floor slip resistance.

• Approved Document G — hot water safety, 48°C bath-fill limit

• Approved Document M — accessible bathrooms

• Approved Document K — safety glazing

• BSI — BS 5385-4 wet area tiling — tanking and tiling standard

• HSE — assessing slip resistance (Pendulum/PTV) — wet floor slip risk

• The Tile Association — wet room guidance — UK tiling trade body

• wet room installation guide — full wet-room build from substrate to tiling

• walk in shower installation — step-by-step install of a walk-in shower

• bathroom waterproofing tanking — tanking systems and BS 5385-4 detailing

• part g sanitation — hot water temperature and scald limits