Bathroom Floor Preparation: Levelling, Tanking, Tile Backer Boards & Falls

Summary

The floor is the most critical substrate in any bathroom project. Unlike walls, bathroom floors have to carry live loads, resist water ingress from above, and — in wet rooms — provide a controlled gradient to ensure water runs to the waste. Poor floor preparation is the number one cause of tile failures, cracked grout, and water damage in bathroom renovations. It pays to spend significant time on substrate preparation before a single tile is laid.

The substrate type dictates the preparation approach. Concrete ground floors need levelling with a latex-modified floor levelling compound, and tanking if the bathroom is below ground or at risk of rising damp. Timber suspended floors need to be stiffened (screwing at 150mm centres, noggins between joists if accessible), overlaid with a rigid backer board, and then tanked if in a shower area. Screed floors need to be primed and levelled if not flat to specification.

Wet rooms are the most demanding application: the floor must be tanked across its full surface area (not just in the shower zone), and a consistent fall of 1:80 minimum (1:60 is more comfortable) must be maintained to the waste. The waste must be positioned correctly before the floor build-up is decided, because the fall needs to drain to a specific point and the total build-up thickness affects the fall gradient achievable.

Key Facts

• BS 5385-4 — British Standard for wall and floor tiling in internal environments; defines substrate flatness tolerances (3mm under 2m straightedge) and adhesive requirements
• 3mm under 2m rule — maximum deviation allowed for a tiled floor; greater deviation requires levelling before tiling
• Timber floor deflection — BS 5385 limits deflection to L/360 of span for tiled floors; most standard domestic joisted floors (at 400mm centres, adequate depth) meet this with adequate boarding
• Tile backer board — cement-based rigid boards (Hardiebacker 12mm, Wedi, Aquapanel Outdoor) give a stable, dimensionally consistent substrate; preferred over plywood under tiles
• Plywood overlay — 12mm exterior-grade plywood, screwed at 150mm centres, is acceptable but provides less flatness tolerance and expands/contracts more than backer boards
• Tanking — waterproofing the floor and wall junctions in wet areas; liquid polyurethane or cementitious systems, or sheet membranes; applied before tiles
• Wedi or Schlüter Kerdi — proprietary wet room board and membrane systems; waterproof from the board surface; good for complex wet room designs
• Wet room fall — minimum 1:80 gradient (12mm drop per 1000mm run); 1:60 preferred; all areas of the wet room floor must fall to the drain
• Linear drain vs centre drain — linear drain allows a single-direction fall (simpler to tile); centre drain requires a four-directional fall (more complex cutting)
• Floor levelling compound — latex-modified self-levelling compound (Mapei Ultraplan, Ardex K360, BAL Level Max); used over primed concrete or plywood; maximum depth per pour depends on product
• DPM on concrete ground floors — 1200-gauge polythene DPM under screed/levelling compound where rising damp risk exists; or liquid DPM applied over the surface
• Movement joints — in larger bathrooms, movement joints at internal corners and over structural breaks prevent tile cracking from building movement
• Underfloor heating — if UFH mats are being installed under tiles, confirm adhesive compatibility (flexible S1/S2 adhesive required per BS EN 12004)

Quick Reference Table

Detailed Guidance

Assessing the Existing Floor

Before any preparation, assess:

1. Deflection test — walk the floor, jump lightly; any springiness indicates inadequate rigidity. Acceptable deflection for tiling is L/360 of span (approximately 3mm for a 1.1m span at 400mm joist centres) — most solid floors pass, but older joisted floors with narrow joists at 450mm+ centres may need investigation.
2. Flatness — lay a 2m straightedge (or a long spirit level) across the floor in multiple directions; measure any gaps. Record the maximum deviation.
3. Moisture — check for damp, particularly on ground floors and in properties with no DPC or a failed DPC. A concrete floor showing dampness may need a surface-applied DPM before any preparation begins.
4. Existing coverings — vinyl, carpet, or old tiles should generally be removed. Old ceramic tiles in good condition can be tiled over if the total build-up height is acceptable, the tiles are firmly adhered, and the floor is flat.

Levelling a Concrete Floor

1. Remove all loose material, grease, and contamination.
2. Check and repair any cracks (inject with epoxy or use a flexible repair mortar for moving cracks).
3. Prime with a diluted PVA or purpose-made primer (e.g. Mapei ECO Prim Grip or Ardex P4); allow to dry.
4. Mix the self-levelling compound per manufacturer's instructions; pour and spread with a spiked roller to release air.
5. Allow to cure (typically 3–24 hours depending on depth and product); do not tile until fully cured.
6. Sand down any high spots or ridges at joints after curing.

Maximum pour depths vary: most compounds are 3–20mm; deep pour products allow 50mm+. For larger deviations, use a float and set screed before the levelling compound.

Stiffening and Overlaying a Timber Floor

1. Check joist condition — if joists are accessible from below (crawl space), inspect for rot, woodworm, or damage.
2. Add noggins — if joists are at 450mm or greater centres, or if the floor is springy, add 47x75mm timber noggins between joists at mid-span to reduce deflection.
3. Screw existing boarding — screw the existing floorboards or chipboard deck at 150mm centres to reduce movement at joints. Use 50–65mm countersunk screws.
4. Overlay with backer board — cut tile backer board (12mm Hardiebacker or equivalent) to fit; fix with 25mm screws and proprietary bonding adhesive at 150mm centres; stagger joints from the boarding below; tape joints with alkali-resistant scrim tape and flex adhesive.
5. Check flatness with 2m straightedge; spot-fill any low areas with tile adhesive and allow to cure.

Tanking a Shower Area or Wet Room

Full tanking is required in all shower areas and wet rooms. In a standard shower tray installation, tanking is typically applied to the wall area within the shower enclosure. In a wet room with a level-access shower, the entire floor (and walls up to splash height) must be tanked.

Liquid tanking (cementitious):

1. Apply the tanking membrane (Mapei Mapelastic, BAL Tank-It, Weber.tec 824) with a brush or trowel; allow first coat to cure.
2. Embed fleece tape in the first coat at all wall/floor junctions.
3. Apply a second coat.
4. Allow to fully cure before tiling (typically 24 hours; check product guidance).

Sheet membrane (Schlüter Kerdi, Laticrete HydroBar):

1. Apply Schlüter Kerdi or similar thin polyethylene membrane in the adhesive bed.
2. Embed securely; overlap seams per manufacturer instructions.
3. Apply second adhesive coat over the membrane before tiling.

Creating Falls in a Wet Room

A 1:80 fall means the floor drops 12.5mm over 1000mm of run. In practice, for a typical 1500mm shower area draining to one corner, the far corner is raised approximately 19mm above the drain.

Method with fixed waste position:

1. Establish the waste position and drain height (top of drain grate = finished floor level).
2. Calculate required floor level at each wall: corner furthest from drain should be (maximum distance / 80) mm higher.
3. Use a float and set screed to create the fall — this can be done in one or two coats depending on depth.
4. Allow to cure; apply tanking over the screed.
5. Tile to a consistent joint width; use wedge spacers to maintain fall during tiling.

Pre-formed wet room formers (Wedi, Schlüter Kerdi-Shower, Marmox) provide a pre-formed sloped base with integrated waste — much easier than screeding a fall from scratch and highly recommended for new installations.

Frequently Asked Questions

Do I need a structural engineer if the bathroom floor is bouncy?

Not necessarily — most springy bathroom floors just need the joists investigating and possibly some noggins added or the floorboarding upgraded. Only if there is actual structural damage (rotten joists, joists cut for services) would an engineer be needed. A competent builder can assess this.

Can I tile directly on existing vinyl flooring?

Generally no — vinyl is dimensionally unstable and will cause tile debonding. Strip the vinyl, check what's beneath, and follow the appropriate substrate preparation. If the vinyl is fully adhered, perfectly flat, and you can confirm the substrate beneath is solid, plywood overlaying may be acceptable in non-wet areas — but check the adhesive manufacturer's guidance.

How thick does tanking need to be?

Liquid tanking membranes are typically applied in two coats to achieve a dry film thickness of 0.5–1.0mm (per the product technical data sheet). Thicker applications don't add proportionally more protection. The critical areas are the wall/floor junctions and corners — make sure the fleece tape is fully embedded and the second coat bridges all joints.

Regulations & Standards

• BS 5385-4:2015 — Wall and floor tiling: design and installation of ceramic and porcelain tile and mosaic systems in internal and external environments

• Approved Document C — Site preparation and resistance to contaminants and moisture: DPM requirements for ground floors

• BS EN 12004 — Adhesive requirements: S1/S2 flexibility classification for tile adhesives over UFH

• BS 8102:2022 — Protection of below ground structures against water from the ground: relevant for tanking in basements

• BAL Technical Advice — Bathroom Floor Prep — Substrate assessment and preparation guides

• Schlüter Systems — Wet Room Installation — Kerdi and Ditra system installation manuals

• British Ceramic Tile — BS 5385 Guidance — Interpretation of tiling standards

• Hardiebacker Installation Guide — Fixing specification and joint treatment

• wetroom construction — Wet room design, drain positioning, tanking systems

• waterproofing — Bathroom waterproofing and tanking membrane comparison

• floor insulation — Floor insulation including under bathroom floors

• bath installation — Bath installation sequence and waste positioning