Waterproofing a Bathroom with a Recessed Shower and Timber Subfloor
You are tiling a 2.4 m x 1.8 m first-floor bathroom over 19 mm particleboard sheeting fixed to timber joists at 450 mm centres. The client wants a 900 x 900 mm tiled shower recess with a central waste, and large-format 600 x 300 mm porcelain tiles on the floor. The existing floor deflects noticeably when walked on near the doorway. The builder says the membrane can go straight onto the particleboard because 'the tiles are heavy and will hold it down'. You have a 2-day window before the vanity and frameless glass arrive. The shower area must fall to the waste at a minimum gradient, and the floor outside the shower must be level with the adjoining hallway threshold, which leaves only 25 mm of build-up.
1. Identify the substrate and structural problems you must resolve before any tiling or waterproofing proceeds, and explain how you would verify and address them.
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Two separate problems exist: structural movement and unsuitable substrate.
Deflection. Noticeable bounce near the doorway indicates the joists or sheeting are under-sized or poorly fixed. Tiling over a flexible floor will crack tiles and rupture the waterproof membrane at movement joints. Before proceeding, check joist size, span, spacing and any bearers against the manufacturer's span tables, and confirm the floor meets the required stiffness for the tile type. If it does not, the fix is structural - additional bearers, blocking or a thicker structural sheet - not extra adhesive. Document the finding and refer it to the builder in writing.
Particleboard. Particleboard is moisture-sensitive and generally not an acceptable direct substrate for a tiled wet area. It swells and delaminates if water reaches it. The correct approach is to remove it in the wet area or overlay with a suitable tile underlay fixed per manufacturer instructions, with joints offset and fixed through to the joists. The builder's 'tiles will hold it down' claim is wrong: tiles add load, not stiffness, and a bonded rigid surface over a moving substrate fails at the bond line.
Only after deflection and substrate are corrected can you set out falls, build the shower screed, and apply the membrane.
2. Explain how you would set out the shower fall and floor levels to meet the 25 mm build-up limit while keeping the hallway threshold flush.
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Work backwards from the finished floor level (FFL) at the threshold. With only 25 mm total:
- Tile thickness (e.g. 10 mm porcelain) + adhesive bed (approx. 6 mm) = about 16 mm.
- That leaves roughly 9 mm for membrane and screed at the threshold - very tight.
Because a 900 x 900 shower with a central waste needs fall on four planes, the perimeter of the shower will be higher than the waste. A typical minimum fall of 1:80 to 1:100 over 450 mm from waste to edge gives about 5 mm drop, but the shower perimeter must still sit below or level with the surrounding floor. Options:
- Recess the shower floor by lowering the joists or sheeting in the shower zone so the screed sits within the structural depth rather than on top.
- Use a preformed shower base with built-in fall, which is thinner than a site-built screed.
- Raise the whole bathroom floor and adjust the threshold - but this breaches the 25 mm limit, so it must be negotiated with the client and builder.
Set out the waste position first, establish the perimeter height, then screed to the waste with a straight edge and check with a spirit level. The membrane must continue up the walls and over the hob/step. If the 25 mm cannot be met with correct falls, stop and escalate - do not reduce the fall below the minimum to make the level work.