What is the typical floor build-up for a tiled bathroom?

On a concrete subfloor with tanking and porcelain tiles: tanking (1mm) + adhesive (3mm) + tile (10mm) = approximately 14mm total. On a timber subfloor: ply overlay (12mm) + backer board (6mm) + tanking (1mm) + adhesive (3mm) + tile (10mm) = approximately 32mm. Adding underfloor heating adds another 13mm (insulation 6mm + mat 4mm + SLC 3mm).

Do I need plywood over timber floorboards before tiling?

Yes. You should overlay timber floorboards with 12mm WBP (weather and boil proof) plywood to create a stable, rigid base. The ply must be screwed (not nailed) at 150mm centres into the joists, with joints staggered from the floorboard joints. Chipboard and OSB subflooring also need a ply overlay as they swell when exposed to moisture.

Do I need tile backer board in a bathroom?

Backer board (6mm cement-based boards like HardieBacker or No More Ply) is recommended over timber subfloors for moisture resistance and rigidity. 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.

What if the floor build-up is too high for the door threshold?

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, use a thinner ply, or trim the bottom of the door. Building Regulations allow a maximum 15mm threshold step in accessible dwellings. A sloping threshold strip can accommodate up to 15mm level change.

Can I tile directly onto concrete in a bathroom?

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.

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

Calculate the complete floor build-up for a bathroom including ply, backer board, tanking, adhesive and tiles. Check against door threshold height to avoid step-up issues.

Room Length

Internal room length

Room Width

Internal room width

Subfloor Type

Determines whether ply/backer board is needed

Tile Type

Affects tile thickness in build-up

Available Threshold Height

Height from subfloor to bottom of door (0 to skip check)

Underfloor heating (electric mat)

Adds ~13mm to build-up

Tanking membrane (wet area)

Required for showers, wet rooms and around baths

Ply Price (optional)

Per 1200×2400mm sheet

Backer Board Price (optional)

Per 600×1200mm board

Adhesive Price (optional)

Per 20kg bag

How We Calculate This

This calculator builds up each layer of a bathroom floor from subfloor to finished tile, calculating material quantities and checking the total height against your door threshold to ensure there are no step-up issues.

The formula

Total build-up = Sum of all layer thicknesses

Threshold clearance = Door threshold height − Total build-up

For a timber subfloor with UFH and porcelain tiles: 12 + 6 + 6 + 4 + 3 + 1 + 3 + 10 = 45mm. If door threshold is 40mm, you are 5mm over and need to adjust.

Typical layer thicknesses

Plywood overlay: 12mm WBP ply (timber subfloors only)
Tile backer board: 6mm (HardieBacker, No More Ply)
UFH insulation: 6mm foil-faced board
Heating mat + SLC: 4mm + 3mm = 7mm
Tanking membrane: ~1mm (liquid, 2 coats)
Tile adhesive: 3-6mm depending on tile type
Tiles: 8-20mm depending on material

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.