Summary
A shower tray looks like the simplest fit in the bathroom and is one of the most frequently botched. The tray itself rarely fails — what fails is the bed under it and the waste below it. An acrylic or stone-resin tray that is not fully supported across its whole underside will flex when someone stands on it, crack, and weep water into the floor for months before anyone notices the joist rot.
This article covers bedding methods for the common tray types (acrylic/ABS, stone resin, slate-effect composite, and tiled-in formers), waste and trap selection, and the falls and access you need to get right before the tray ever goes down. The recurring theme is support and fall: the tray must be solid underfoot, and the water must run to the waste under gravity. If you are building a level-access wet room rather than fitting a tray, the same principles apply but the fall is created in the screed/former — see wetroom construction.
The order of work matters. First-fix the waste, prove the fall to the soil/waste stack, tank and prepare the substrate, then bed the tray. Trying to retrofit a fall or fix waste access after the tray is bedded and tiled is how a one-day job becomes a floor-up rebuild.
Key Facts
- Full bed, no voids — acrylic and ABS trays must be bedded on their entire underside (sand/cement mortar bed, or a tray-maker's proprietary support kit), not on perimeter dabs. Voids = flex = cracks.
- Stone resin trays are rigid and can sit on a flat, solid floor, but a mortar bed still eliminates rock and spreads point loads. Always follow the tray maker's bedding instruction.
- Tray is the fall — the fall to the waste is moulded into the tray. Bed the tray dead level; do not tilt it to create a fall. Tilting moves the waste out of position and ponds water at the high edge.
- Waste size — shower waste runs in 40mm waste pipe (1½"). Trap must give a minimum 50mm water seal per Approved Document H / BS EN 274.
- Flow rate — high-flow showers (rain heads, body jets) need a high-flow waste (often 60mm tray hole / high-flow trap) or the tray floods; match the waste to the shower's litres-per-minute.
- Waste pipe fall — the waste run to the stack needs a fall of roughly 18–90mm per metre (1:55 to 1:11) for 40mm pipe per Approved Document H; aim for a steady, well-supported fall with no bellies. See waste pipes.
- Access to the trap — provide a removable panel or upstand access to the trap for clearing blockages; a fully tiled-in inaccessible trap is a future grief.
- Tank before tiling — the substrate (walls and any tiled-in former) must be tanked/waterproofed before tile; the tray seals onto tile, not bare board. See waterproofing.
- Seal on the wet side — silicone the tray-to-wall and tray-to-tile junctions on the inside (shower) face with low-modulus neutral-cure sanitary silicone; leave the outside able to weep.
- Upstands — many trays need a tiling upstand or the tile must lap over the tray's upstand lip so water runs onto the tray, never behind it.
- Wet-room fall — a level-access wet-room floor needs a minimum fall to the gully of around 1:80 (12.5mm/m); some specs use 1:40–1:60 closer to a linear drain.
- Joist trimming — recessed (flush) trays in timber floors need joists trimmed and noggined to take the tray and waste; this is a structural alteration — keep within safe notching/trimming limits.
Quick Reference Table
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Try squote free →| Tray type | Bedding method | Rigidity | Notes |
|---|---|---|---|
| Acrylic / ABS capped | Full sand:cement mortar bed OR maker's support kit | Low — flexes if unsupported | Never bed on dabs; full contact essential |
| Stone resin | Flat solid floor or full mortar bed | High | Heavy; mortar bed kills any rock |
| Slate-effect composite | Full mortar bed per maker | Medium–high | Follow maker's bed depth exactly |
| Tileable former (wet-room) | Bonded to floor, then screed/tile over | N/A (becomes floor) | Fall built into former; see wet-room article |
| Low-profile / flush-fit | Recessed into trimmed joists, full bed | Varies | Structural floor alteration; access to waste essential |
Detailed Guidance
Bedding the tray — the make-or-break step
The bed exists to do one thing: support the tray's whole underside so it cannot flex. For an acrylic or ABS tray with a contoured underside, perimeter mortar alone leaves the centre unsupported — exactly where someone stands. Two correct approaches:
- Full mortar bed. Lay a stiff sand:cement bed (typically 3:1 or 4:1), set the tray into it, and press/tap down with a level until it is dead level and fully in contact. Excess squeezes out; clean it off. Let it cure before loading or tiling.
- Manufacturer's support kit. Some trays ship with adjustable feet plus a structural panel or require a specific tile-backer support board. Follow the kit — these are designed to give full contact and a defined finished height.
Check level both ways with a long spirit level, and confirm the tray does not rock before the mortar goes off. Once it is set, stand on it at every corner and the centre — no movement, no creak.
WRONG (dab bedding) RIGHT (full bed)
▔▔▔▔▔▔▔▔▔▔▔▔▔▔ ▔▔▔▔▔▔▔▔▔▔▔▔▔▔
tray flex point ↓ tray fully supported
░░ ░░ ░░ ░░░░░░░░░░░░░░░░░
▒▒ void ▒▒ void ▒▒ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ mortar
───────────────────── ───────────────────── floor
Waste, trap and getting the fall right
The waste must be fitted and tested before the tray is bedded, because once the tray is down and tiled you cannot reach it.
- Use a 40mm shower trap with at least a 50mm water seal. Shallow/low-profile traps exist for flush trays but verify the seal depth — anti-vac or HepvO-type valves can substitute where seal depth is tight.
- Match the trap's flow rate to the shower. A 12 l/min rain head over a standard trap will pool around your ankles. High-flow trays use a 90mm/60mm tray hole and a high-flow trap.
- Connect to 40mm waste pipe run to fall back to the soil stack or gully. Support the pipe at the correct fall with no dips (bellies hold water, smell, and block).
- Pressure/flood test the waste before bedding: fill the trap, run water, check every joint underneath.
- Build in access — a removable bath/tray panel, or an access hatch in the ceiling below — so the trap can be cleared.
Falls: tray vs wet-room former
A standard tray has the fall moulded in — your job is to bed it level so that built-in fall does its work. Tilting the tray to "help" the water actually defeats the moulded fall and lifts the waste out of position.
A level-access wet room has no tray; the fall is formed in the floor. Use a pre-formed tileable former (fall built in) or screed a fall by hand. Target a minimum fall to the gully/linear drain of around 1:80 (12.5mm per metre) for a point gully; linear drains can run shallower close to the channel. Always confirm against the former and membrane manufacturer's stated minimum — getting this wrong leaves standing water that the waterproofing membrane will not fix.
Substrate, tanking and the tile lap
The tray seals onto tiled, tanked walls — not bare plasterboard or backer board. Sequence:
- Backer board / tile substrate fixed to walls.
- Tank/waterproof the wall substrate and any tiled former (see waterproofing).
- Tile the walls down to (or lapping over) the tray's upstand so water runs onto the tray, never behind it.
- Seal the tile-to-tray junction on the wet side.
If the tile finishes above the tray with a gap, water runs behind the tray — the defining detail to avoid. Either the tray has an upstand the tile laps over, or the tile drops behind/onto the tray lip.
Sealing the tray
- Low-modulus neutral-cure sanitary silicone on the inside (wet) face of the tray-to-wall and tray-to-tile junctions.
- Tool a clean concave bead; mask both sides for a crisp line.
- Leave the outside junctions able to weep — do not seal both faces and trap water.
- Cure 24 hours minimum before use.
Flush / recessed trays in timber floors
Low-profile and flush-fit trays drop the finished floor to the tray top — that means trimming joists and forming a recess. This is a structural alteration: keep notches and holes within safe limits (notches in the top, near supports; holes on the neutral axis), add trimmers and noggins to carry the tray and waste, and make sure the waste still has fall to the stack within the reduced depth. If the fall cannot be achieved within the floor zone, the flush tray is the wrong choice for that floor.
Frequently Asked Questions
Do I need to tilt the shower tray to make the water drain?
No. The fall is moulded into the tray — bed it dead level and the built-in fall does the work. Tilting the tray moves the waste off the true low point, ponds water at the raised edge, and stresses the tray. Level in both directions is the target.
Can I bed an acrylic tray on mortar dabs to save time?
No. Acrylic and ABS trays must be bedded on their whole underside. Dab bedding leaves voids under the centre, the tray flexes when stood on, and it cracks — then leaks into the floor. Use a full mortar bed or the maker's support kit.
What size waste pipe does a shower use?
40mm (1½") waste pipe with a trap giving at least a 50mm water seal (Approved Document H). For high-flow rain/body-jet showers use a high-flow trap and the larger tray waste hole, or the tray will flood faster than it drains.
What fall do I need for a wet-room floor?
Around 1:80 (12.5mm per metre) minimum to a point gully is the common good-practice figure; linear drains can be shallower near the channel. Always confirm against your tileable former or waterproof membrane manufacturer's stated minimum. Too shallow and water stands; too steep and large-format tiles "lippage" at the fall.
Do I need access to the shower trap after it's tiled in?
Yes — provide a removable panel or a hatch in the ceiling below. Traps block with hair and soap; a fully sealed, inaccessible trap means lifting the tray or the ceiling to clear a blockage.
Regulations & Standards
Approved Document H (Drainage and waste disposal) — trap seal depths (min 50mm), waste pipe sizing and falls.
BS EN 274 — waste fittings for sanitary appliances (traps and outlets).
BS 5385 (wall and floor tiling series) — substrate, falls and tiling good practice for showers and wet areas.
BS 8000 workmanship series — general workmanship standards including bedding and sealing.
BS 8204 / floor screeds guidance — for hand-screeded wet-room falls.
Building Regulations Part P / BS 7671 — for electric showers and any electrical work involved.
Approved Document H: Drainage and waste disposal (gov.uk) — trap seals and waste falls
BS 5385 tiling standards (BSI) — wet-area substrate and falls
Bathroom Manufacturers Association (BMA) — tray bedding and product guidance
TTA – Tile Association technical advice — wet-room falls and substrate preparation
wetroom construction — level-access former, falls and tanking when there is no tray
shower screen installation — the screen is set out and sealed on top of a correctly bedded tray
waterproofing — tanking the substrate the tray and tile seal to
waste pipes — 40mm waste sizing, falls and trap seals feeding the tray waste