Bathroom Underfloor Prep Calculator — Floor Build-Up & Threshold Check

On a concrete subfloor with tanking and porcelain tiles: tanking (1mm) + adhesive (3mm) + tile (10mm) = approximately 14mm total. On a timber subfloor: 15mm ply overlay + backer board (6mm) + tanking (1mm) + adhesive (3mm) + tile (10mm) = approximately 35mm. Adding underfloor heating adds another 13mm (insulation 6mm + mat 4mm + SLC 3mm).

Yes. BS 5385-3 and BAL guidance recommend overlaying timber floorboards with a minimum 15mm (typically 15-18mm) flooring-grade WBP/marine plywood to create a stable, rigid base. Screw the ply down (do not nail) at 200-300mm centres in a grid, laid in a brick-bond/staggered pattern so joints do not coincide with the floorboard joints, with a 2-3mm expansion gap around the perimeter. Note that in wet areas (showers, wet rooms) plywood alone is not recommended because it swells — use a cement backer board with a tanking system over the ply. Chipboard and OSB subflooring also need a ply overlay as they swell when exposed to moisture.

Backer board (6mm cement-based boards such as HardieBacker, sold in 1200×800mm = 0.96 m² sheets) is recommended over timber subfloors for moisture resistance and rigidity, and is the correct base for tanking in wet areas. On concrete it is optional but still beneficial for creating a flat surface. Backer board also provides a better key for tanking membrane and tile adhesive than ply alone.

If the total build-up exceeds the available threshold height, you have several options: use thinner tiles, reduce adhesive bed depth, omit backer board on concrete floors, or trim the bottom of the door (the ply overlay should not go below 15mm on timber). Note the 15mm figure people quote is from Approved Document M and is the maximum raised threshold height for EXTERNAL accessible thresholds (any upstand over 5mm chamfered or rounded), not a rule for internal doorways. An internal bathroom threshold is not regulated to a fixed step, but it should be kept to a minimum and any level change finished with a tapered trim or ramp to avoid a trip hazard.

Yes, provided the concrete is sound, level, clean and has been primed. Any cracks should be treated with a flexible repair compound. If the concrete is uneven (more than 3mm over 2m), apply self-levelling compound first. Tanking membrane should be applied before tiling in wet areas. Concrete must be fully cured (minimum 6 weeks for new concrete) and tested for moisture content.