In many NZ homes, tiling directly over a concrete bathroom floor is perfectly achievable — but only if the slab meets some important conditions. Skip the prep work and you risk cracked tiles, failed adhesive, and costly repairs down the track. Here's what you need to know before you mix a drop of adhesive.
When can you tile directly on concrete?
Concrete makes a solid substrate for tiles, but it must satisfy three key criteria before you lay a single tile:
- Flatness: The floor must be flat to within 3 mm over a 3-metre straightedge (a commonly used industry tolerance). Dips or humps beyond this will cause hollow spots, cracked tiles, or lippage between tiles.
- Soundness: The concrete must be fully cured (typically at least 28 days old), free of cracks, delamination, or crumbling areas.
- Dryness: Moisture content must be within the tile adhesive manufacturer's limits. In NZ's often damp climate, rising damp through a slab without a proper damp-proof membrane is a real risk — especially in older homes.
If your concrete ticks all three boxes, you may be able to tile straight onto it with the right preparation. If not, a screed or levelling compound will be needed.
Checking and preparing the concrete surface
Test for flatness
Use a long straightedge or spirit level across the floor in multiple directions. Mark any high spots or hollows with chalk. High spots can be ground back with an angle grinder fitted with a diamond cup wheel (wear appropriate PPE — dust from concrete grinding is hazardous; check WorkSafe NZ guidelines on silica dust). Low spots of a few millimetres can often be filled with a floor-levelling compound such as those from Ardex or Mapei, avoiding the need for a full screed.
Test for moisture
Tape a piece of plastic sheeting (about 500 mm × 500 mm) to the concrete and leave it for 24–48 hours. If moisture appears underneath, you have a rising damp issue. In this case, apply a waterproof membrane or moisture barrier before tiling, or consult a tiler about the right system for your situation.
Prime the surface
Almost all tile adhesive manufacturers require a primer on concrete before applying adhesive or levelling compound. Products from Ardex, Mapei, and Sika all offer suitable primers — always follow the specific product's technical data sheet.
Clean thoroughly
Remove all paint, sealers, grease, dust, and curing compounds. These will prevent adhesive from bonding properly. A mechanical grind or scarify is often the most reliable method.
When is a screed actually necessary?
A sand-cement screed or self-levelling compound becomes necessary when:
- The floor is out of flat by more than 3–5 mm and grinding alone won't fix it
- You need to raise the floor level to meet a door threshold or drainage fall
- You're installing in-slab or underfloor heating (a screed encases the heating elements and provides an even surface)
- The concrete surface is badly damaged, contaminated, or has extensive cracking
A full sand-cement screed is typically laid at a minimum of 50–75 mm thick and must be allowed to cure before tiling. Self-levelling compounds can be applied much thinner (check manufacturer specs) and are often the faster, easier option for minor unevenness.
Waterproofing — a non-negotiable step in bathrooms
Regardless of whether you screed or tile directly, waterproofing is mandatory in NZ bathrooms under NZ Building Code clause E3 (Internal Moisture). A compliant membrane must be applied to the floor and up the walls to the required height before tiling. This is not optional — it protects the structure of your home from water damage.
Applying waterproofing membrane is work a competent homeowner can do, but it must be done correctly and in accordance with the membrane manufacturer's specifications. Products from Ardex, Mapei, and Sika are widely used in NZ. Check BRANZ for guidance on best-practice waterproofing systems.
Choosing the right tile adhesive
Use a polymer-modified, flexible tile adhesive rated for wet areas. Standard cement-based adhesives without flexibility can crack as the concrete moves slightly with temperature changes — a real consideration in NZ's variable climate. Always check the adhesive is compatible with your tile type (large-format tiles, for example, need full-bed coverage of at least 95% in wet areas).
A quick comparison
| Situation | Screed needed? | Action |
|---|---|---|
| Flat, sound, dry concrete | No | Prime, waterproof, tile |
| Minor dips (< 5 mm) | No | Level with compound, prime, waterproof, tile |
| Significant unevenness | Yes | Screed or self-levelling compound |
| Rising damp present | Depends | Moisture barrier or specialist advice |
| Underfloor heating install | Yes | Screed over heating system |
When to call a professional
Tiling is generally non-restricted work that a capable homeowner can undertake, but bathroom tiling involves waterproofing that directly affects your home's internal moisture management and structural integrity. A poor waterproofing job can lead to serious, expensive damage hidden behind walls and under floors.
If you're unsure about the condition of your slab, have persistent moisture issues, or are dealing with a complex layout, it's well worth engaging a qualified tiler. You can find experienced local tilers on tradedirectory.co.nz.
Note: Any work involving moving or adding floor waste drains is restricted plumbing work and must be carried out by a licensed drainlayer or plumber registered with the Plumbers, Gasfitters and Drainlayers Board.
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