Hollow spots — those dull, drum-like sounds when you tap a tile — are one of the most common problems with large format floor tiles, and they're almost always preventable. The root cause is insufficient or uneven adhesive contact between the tile and the substrate. With the right preparation, products and technique, you can achieve the solid, full-coverage bond that large format tiles demand.
Why Large Format Tiles Are More Demanding
Tiles larger than 600 × 600 mm (sometimes called large format tiles or LFTs) are increasingly popular in NZ homes, but they're far less forgiving than smaller mosaic or 300 mm tiles. Their size means:
- Any slight bow or warp in the tile itself creates a gap in the middle or at the edges.
- Substrate imperfections are magnified across a wider span.
- Standard notched-trowel technique alone rarely achieves adequate coverage.
The general industry benchmark — and a requirement under good trade practice — is 95% adhesive contact for floor tiles in wet areas, and at least 80% in dry areas, with no voids at corners or edges. Always check the tile and adhesive manufacturer's specifications, as requirements can vary.
Step 1 — Prepare a Flat, Solid Substrate
No amount of adhesive skill will fix a bad base. Your substrate must be:
- Flat to within 3 mm over a 3-metre straightedge (some manufacturers specify tighter tolerances for very large tiles — check the datasheet).
- Structurally sound, clean, dry and free of dust, grease or curing compounds.
- Deflection-resistant — timber floors in particular must have adequate joist depth and noggin spacing. Excessive flex causes tiles and grout to crack over time.
For timber subfloors, a fibre-cement sheet underlayment (such as a James Hardie product) or a suitable decoupling membrane is strongly recommended. For concrete, grind down high spots and fill low spots with a floor-levelling compound. Products from Ardex and Mapei are widely used by NZ tilers for this purpose.
Step 2 — Choose the Right Adhesive
Not all tile adhesives are created equal. For large format tiles, use a polymer-modified, non-slump adhesive rated for the tile size you're installing. Check the product's maximum tile size on the technical datasheet.
- Rapid-set adhesives are useful when you need to grout quickly, but they require confident, fast working.
- In wet areas (bathrooms, laundries), use a waterproof or water-resistant adhesive appropriate for the area classification under NZS 3604 and the NZ Building Code clause E3 (Internal Moisture).
- Always mix powder adhesives to the manufacturer's ratio — too much water weakens the bond.
Sika, Ardex and Mapei all offer large-format-rated adhesives available through NZ tile and building suppliers.
Step 3 — Use the Correct Trowel and Technique
This is where most DIYers go wrong. For large format tiles:
- Use a large-format notched trowel — typically a 12 mm or 15 mm square-notch trowel, though always follow the adhesive manufacturer's recommendation.
- Comb the adhesive in one direction on the substrate. This allows air to escape as the tile is pressed down.
- Back-butter the tile as well — apply a thin, smooth skim coat of adhesive to the back of the tile before laying. This is essential for achieving full coverage with large tiles.
- Press the tile firmly and slide it slightly (5–10 mm) back and forth perpendicular to the combed ridges to collapse the notches and fill any voids.
- Periodically lift a tile to check coverage — you should see full, even contact with no bare patches, especially at corners.
Step 4 — Use a Tile Levelling System
A tile levelling system (TLS) is almost essential for large format tiles. These clip-and-wedge or screw-based systems:
- Hold adjacent tiles at the same height while the adhesive cures, preventing lippage (uneven tile edges).
- Apply even downward pressure across the tile face, helping to bed the tile fully into the adhesive.
Levelling systems are available at Bunnings and Mitre 10, as well as specialist tile suppliers.
Step 5 — Work in Manageable Sections
Adhesive has an open time (the window during which it remains workable). In NZ's warmer months or in direct sun, this can shorten significantly. Spread only as much adhesive as you can tile within the open time — typically 20–40 minutes depending on the product and conditions. Tiling onto skinned adhesive is a leading cause of hollow spots.
Substrate and Adhesive Compatibility at a Glance
| Substrate | Recommended Approach |
|---|---|
| Concrete slab | Prime if dusty; use floor leveller for flatness |
| Timber subfloor | Fibre-cement sheet or decoupling membrane first |
| Existing tiles | Check adhesion; use appropriate primer and adhesive |
| Wet areas | Waterproof membrane before tiling (Building Code E3) |
When to Call a Professional
Large format tile installation is a skilled job. If your floor has significant level variation, structural flex, or you're tiling a wet area like a shower or bathroom, it's worth engaging a qualified tiler. Wet area waterproofing in particular must meet the NZ Building Code, and mistakes can be very costly to fix. A building consent may be required for certain renovation work — always check with your local council, and visit MBIE Building Performance for guidance on what requires consent.
For electrical work associated with underfloor heating beneath tiles, this is restricted electrical work that must be carried out by a registered electrician — see the Electrical Workers Registration Board for details.
You can find qualified, local tilers on tradedirectory.co.nz to get the job done right the first time.
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