Hardie™ Ceramic Tile Underlay
The Stable Base That Stops Tiles Cracking
Tiles crack when the surface they’re bonded to moves. It’s that simple. Timber, particle board and plywood all expand and contract with temperature and humidity changes, and when they do, the adhesive bond between the substrate and the tile either accommodates the movement or fails. In practice, it usually fails. The tiles crack or lift, the grout opens up, and the floor needs retiling long before its time.
Hardie™ Ceramic Tile Underlay is the decoupling layer that solves this problem. A 6mm fibre cement sheet laid over an existing timber, particle board or plywood subfloor, it provides a stable, dimensionally consistent tile bed that doesn’t move with humidity and temperature the way organic substrates do. Pre-marked with a nailing pattern for fast gun-nail installation, pre-sealed on the face to improve adhesive working time and moisture resistance, and suitable for both wet and dry area floor applications.
Technical Specifications
Material
Fibre cement – stable, moisture-resistant, dimensionally consistent
Thicknesses
6mm
Sheet sizes
900mm × 900mm · 1200mm × 900mm · 1800mm × 1200mm
Mass
Approx. 17.5 kg/m² (1200mm × 900mm sheet: approx. 17.5 kg per sheet)
Surface
Pre-sealed face – increases cement-based adhesive working time by 50%
Nailing pattern
Pre-marked on the sheet face – gun-nail directly, no measuring required
Fixing
25mm × 2.5mm annular threaded galvanised nails (Hardie underlay gun nails)
Suitable substrates
Over existing timber · particle board · plywood subfloors
Suitable finishes
Ceramic tiles · porcelain tiles · natural stone · slate · marble · vinyl tiles · cork
Wet/dry suitability
Both wet and dry area floors – residential applications
Fire performance
Non-combustible – does not rot, swell or degrade with moisture
Warranty
Australian-made · 25-year product warranty
The Movement Problem - and How Ceramic Tile Underlay Solves It
Most tiled floors that fail prematurely fail for the same reason: the subfloor is moving and the tile system can’t accommodate the movement. Understanding why Hardie™ Ceramic Tile Underlay works requires understanding the specific movements it’s designed to interrupt.
Moisture expansion and contraction
Timber, particle board and plywood absorb and release moisture with changes in ambient humidity, expanding when wet and contracting when dry. This repeated dimensional cycling stresses the adhesive bond between the tile and the substrate. Fibre cement does not absorb moisture in the same way Hardie™ Ceramic Tile Underlay remains dimensionally stable regardless of humidity variations.
Thermal movement
All floor materials expand with heat and contract with cold. The rate and magnitude of that movement differ between materials, and where a timber subfloor and a ceramic tile move at different rates, the adhesive bond is the failure point. Fibre cement’s thermal movement coefficient is closer to ceramic tile than timber is, reducing the differential movement that stresses the bond.
50% longer adhesive working time
The pre-sealed face of Hardie™ Ceramic Tile Underlay reduces the rate at which the substrate draws moisture from the tile adhesive, increasing the open time of cement-based adhesives by approximately 50%. This gives tilers more time to achieve perfect tile alignment and full adhesive coverage before the adhesive sets, reducing lippage and incomplete bonds.
Pre-marked nail pattern - faster installation
The nailing pattern is pre-marked on the face of every Hardie™ Ceramic Tile Underlay sheet, eliminating the need for the tiler or labourer to measure and mark fixing positions. Gun-nail directly to the marks. On large bathroom and kitchen floor projects, the time saving from pre-marking is consistent across every sheet.
Movement Joints - Important
Movement joints must be provided where floor dimensions exceed 5 metres in one or both directions or where existing structural joints are located. Position joints symmetrically about the centre of the floor. Tiles bridging movement joints will crack; the joint must be maintained through the full floor system from substrate to tile surface.
Featured Projects with Hardie™ Ceramic Tile Underlay
Frequently Asked Questions
Can Hardie™ Ceramic Tile Underlay be used directly on concrete slabs?
Hardie™ Ceramic Tile Underlay is designed for use over existing timber, particle board and plywood subfloors, not directly on concrete slabs. On concrete slabs, tiles can generally be laid directly over the concrete with an appropriate tile adhesive without an underlay product. If the concrete slab has surface movement, cracking or joint issues, contact our team or a structural engineer for advice on the appropriate treatment before tiling.
What nails should I use to fix Hardie™ Ceramic Tile Underlay?
The specified fixing fastener is a 25mm × 2.5mm annular threaded galvanised nail; these are often called ‘Hardie underlay gun nails’ and are available specifically for use with an underlay gun. Do not use standard bright nails for fibre cement underlay; the annular thread provides the mechanical grip that prevents nails from withdrawing under the dynamic loads that tile floors experience.
Is Hardie™ Ceramic Tile Underlay suitable for slate and natural stone tiles?
Yes, Hardie™ Ceramic Tile Underlay is suitable for ceramic tiles, porcelain, natural stone, slate and marble. Slate should not be less than 9 mm thick when used over Hardie™ Ceramic Tile Underlay. For heavy natural stone installations, confirm the adhesive system with the adhesive manufacturer and ensure the existing subfloor structure can carry the combined load of the underlay, adhesive and stone.
How does the pre-sealed face increase adhesive working time?
Standard fibre cement and particle board substrates are porous; they draw moisture from cement-based tile adhesives rapidly, reducing the open time and making it difficult for tilers to achieve full contact and perfect tile placement before the adhesive starts to set. The pre-sealed face of Hardie™ Ceramic Tile Underlay reduces this moisture absorption by approximately 50%, giving the tiler significantly more working time before the adhesive sets. This improves adhesive coverage and reduces the risk of lippage.