Large-Format Porcelain Over a Plywood Joist Floor

The first check is deflection. Porcelain tile generally requires a floor system deflection of not more than L/360 under live load; the 12 ft span must be verified against a span table for the actual joist species, grade, and condition. A 2 × 10 SPF at 16 in. on centre and 12 ft may be close to the limit, so I would not accept the builder's assumption. If the deflection is inadequate, the joists must be strengthened or the span reduced before any tile work. The second check is the substrate: 18 mm plywood is a single layer, and large-format tile over wood framing typically requires a second layer or an uncoupling membrane system approved for the assembly. The 4 mm dip over 1.8 m exceeds the flatness tolerance for large-format tile, generally 3 mm in 3 m, so it must be flattened. The adhesive residue must be removed because it can prevent proper bonding. The builder's proposal to screw down the plywood and tile directly does not address deflection, flatness, or the single-layer plywood, and it risks cracked tile and debonding.

An uncoupling membrane separates the tile from the substrate so that differential movement in the plywood does not transfer directly into the tile. It does not correct joist deflection or excessive substrate variation, so the floor must still meet the deflection and flatness requirements before the membrane is installed. For large-format porcelain, I would require a modified thin-set that meets ANSI A118.4 and is approved by the membrane manufacturer, mixed to the specified consistency. The membrane must be embedded in thin-set with a trowel notch sized to the manufacturer's instructions, and the tile must be back-buttered to achieve at least 95% mortar coverage; large-format tile is not reliably bonded by trowel-notching alone. I would check coverage by lifting a tile after the first few are set. The tile should be laid with a medium-bed or large-and-heavy-tile mortar if the membrane manufacturer allows it, and joints should be at least 2 mm to allow movement. Waterproofing at the tub and shower must be integrated with the membrane system.

I would first remove the adhesive residue and screw down or replace any loose plywood, then add a second layer of plywood or a membrane-compatible underlayment to reach the required assembly. The dip can be corrected with a patching compound or self-leveling underlayment rated for plywood, but the cause of the dip should be checked; if a joist is high or low, flattening compound alone may not be enough. For the transition, I would calculate the finished height: plywood, membrane, thin-set, tile, and any uncoupling mat. If the tile is higher than the 15 mm hardwood, a tapered transition strip or a reducer is needed rather than a flush butt joint. At the tub and shower, I would install a waterproofing membrane that turns up the wall and overlaps the tub flange, with a sealant at the change of plane. The uncoupling membrane is not automatically waterproof, so a separate waterproofing system or a waterproof membrane must be used. The transition and waterproofing details should be drawn and approved before installation.