Tiling over existing tiles is a tempting shortcut — no messy demolition, no skip bin, and a faster finish. In many situations it's perfectly viable, but get it wrong and you could end up with cracked tiles, failed adhesive, or a floor that's too high for your doors. Here's what you need to know before you reach for the tile adhesive.
When Is It Acceptable to Tile Over Existing Tiles?
The short answer: sometimes yes, sometimes no. The key factors are the condition of the existing tiles, the substrate beneath them, the added weight and height, and whether the space is a wet area.
Tiling over existing tiles is generally acceptable when:
- All existing tiles are firmly bonded — no hollow spots, cracks, or lifting edges
- The substrate (concrete slab or timber floor) is structurally sound
- The added height won't cause problems with door clearances, skirting boards, or adjacent floor levels
- You're not in a wet area that requires a waterproof membrane under the new tiles
How to Check If Your Existing Tiles Are Suitable
Before you buy a single tile, do a tap test. Use a small hammer or coin and tap across every tile. A solid "thud" means good adhesion; a hollow "click" means the tile has debonded from the substrate. Even a few hollow tiles can cause the whole new layer to fail over time.
Also check for:
- Cracks — hairline cracks in tiles or grout lines can indicate movement in the substrate
- Lippage — significant height differences between tiles will telegraph through your new layer
- Contamination — grease, soap scum, or sealers on the surface will prevent adhesion
- Existing layers — if tiles have already been tiled over once, a third layer is almost certainly too much
If more than a small number of tiles are hollow or cracked, removal is the safer option.
Weight and Height Considerations
Every layer of tiles adds weight and height. A standard ceramic tile with adhesive and grout can add several kilograms per square metre. On a timber-framed floor, you need to be confident the structure can handle the additional load — this is especially relevant in older NZ homes.
BRANZ provides guidance on floor loading and substrate requirements for tiling. For timber floors, the deflection (flex) must be within acceptable limits — excessive bounce will crack grout and tiles regardless of how well you prepare the surface.
Height is equally important. Adding 10–15 mm to a floor can mean:
- Doors won't clear the new surface
- A trip hazard at transitions to other rooms
- Plumbing fixtures (like toilets) sitting at the wrong height
Measure carefully before committing.
Wet Areas: Special Rules Apply
In bathrooms, showers, and laundries, the NZ Building Code (specifically clause E3 — Internal Moisture) requires that wet areas are properly waterproofed. If you tile over existing tiles without a waterproof membrane, you may be relying entirely on the original waterproofing — which could be degraded, incomplete, or non-compliant by today's standards.
For shower areas in particular, the safest approach is usually to remove old tiles and install a fresh membrane system. Products from Ardex or Mapei are widely used by NZ tilers for wet area waterproofing and are formulated to meet NZS 4858 (wet area membranes).
If you do tile over existing tiles in a wet area, use a flexible adhesive rated for wet areas and ensure all grout joints are sealed properly.
Surface Preparation: The Most Important Step
Good prep is everything. Even if your existing tiles pass the tap test, you need to:
- Clean thoroughly — remove all grease, soap residue, and sealers. A sugar soap wash followed by a light sand or grind of the tile surface improves adhesion significantly.
- Grind down grout lines — high grout lines create an uneven surface. Use an angle grinder with a diamond blade to flatten them, or apply a levelling compound over the top.
- Apply the right adhesive — use a polymer-modified tile adhesive suitable for tile-on-tile applications. Check the manufacturer's datasheet; not all adhesives are rated for this use.
- Back-butter large tiles — for tiles larger than 300×300 mm, apply adhesive to both the tile back and the substrate for full coverage.
You'll find suitable adhesives and prep products at Bunnings, Mitre 10, or PlaceMakers.
Tile-on-Tile: A Quick Comparison
| Factor | Tile Over Tiles | Remove & Re-tile |
|---|---|---|
| Cost | Lower upfront | Higher (demo + disposal) |
| Time | Faster | Slower |
| Risk of failure | Higher if prep is poor | Lower with fresh substrate |
| Wet area suitability | Often not recommended | Preferred |
| Height/weight impact | Adds to existing | Starts fresh |
| Long-term durability | Good if done correctly | Best outcome |
When to Call a Professional
While tiling is a DIY-friendly trade for many homeowners, there are situations where you should bring in a qualified tiler:
- Wet areas and showers — waterproofing in these areas is critical and errors are costly to fix
- Large or complex areas — getting consistent lippage and layout right takes experience
- Structural concerns — if you're unsure about floor loading or substrate condition, get a professional assessment
- Heated floors — in-slab or under-tile heating systems require careful installation
Note: if your tiling project involves moving or adjusting plumbing (e.g. repositioning a floor waste or shower drain), this is restricted work under the Plumbers, Gasfitters and Drainlayers Board and must be carried out by a licensed tradesperson. Similarly, any electrical work (such as installing heated tile systems or bathroom lighting) must be done by a registered electrician — see the Electrical Workers Registration Board.
You can find qualified, local tilers and related tradespeople on tradedirectory.co.nz.