Getting the trowel size right is one of the most important — and most overlooked — steps in laying floor tiles. Use a notch that's too small and your tiles won't bond properly; too large and you'll waste adhesive and struggle to keep tiles level. This guide walks you through how to choose the correct notched trowel for your floor tiling project.
Why Trowel Size Matters
A notched trowel combs ridges of tile adhesive onto your substrate. When the tile is pressed down, those ridges collapse and spread to create full, even coverage beneath the tile. The goal — and the industry standard — is to achieve at least 80% adhesive contact on the back of each tile in dry areas, and even higher coverage in wet areas like bathrooms and laundries. Getting this right is especially important in New Zealand homes where timber subfloors can flex, and where wet-area waterproofing requirements under the NZ Building Code (clause E3) depend on a solid, well-bonded tile installation.
Understanding Trowel Notch Shapes
Before choosing a size, you need to pick the right notch profile:
- Square notch – the most common for floor tiles; gives consistent, full coverage.
- V-notch – better suited to wall tiles and smaller mosaic tiles; less adhesive volume.
- U-notch (round bottom) – used for large-format tiles and natural stone where maximum coverage is needed.
For most floor tiling jobs, a square or U-notch trowel is the right choice.
Matching Trowel Size to Tile Size
As a general rule, the larger the tile, the larger the notch you need. Here's a practical guide:
| Tile Size (longest edge) | Recommended Notch Size |
|---|---|
| Up to 100 mm (mosaics on mesh) | 3 mm V-notch or 6 mm square |
| 100 mm – 300 mm | 6 mm square notch |
| 300 mm – 600 mm | 10 mm square or U-notch |
| 600 mm – 900 mm | 12 mm square or U-notch |
| 900 mm and larger (large format) | 15 mm U-notch or larger |
These are starting points — always check the adhesive manufacturer's recommendations, as products from suppliers like Ardex or Mapei often specify a minimum trowel size on the packaging.
Other Factors That Affect Your Choice
Tile Thickness and Back Profile
Thicker tiles (such as porcelain pavers or natural stone) and tiles with deep ribbed backs need more adhesive to fill the voids. Go up a notch size if your tile has a heavily profiled back.
Substrate Type
A perfectly flat concrete slab needs less adhesive build-up than a timber subfloor or a surface with minor undulations. On uneven substrates, back-buttering the tile (applying a thin skim of adhesive to the tile back as well as the floor) is strongly recommended alongside a larger notch trowel.
Adhesive Type
Standard cement-based adhesives, flexible adhesives, and epoxy adhesives all behave differently. Flexible adhesives — important on timber subfloors common in NZ homes — are often thicker and may require a slightly different notch to achieve the right coverage. Always read the product data sheet.
Wet Areas and Outdoor Use
For bathrooms, laundries, and outdoor areas, aim for 95% or greater coverage to prevent water ingress behind tiles. This usually means going up a notch size and back-buttering. BRANZ provides good guidance on wet-area construction best practice.
How to Check You're Getting Good Coverage
Don't just assume — test your coverage before committing to a full run of tiles:
- Comb adhesive onto a small area with your chosen trowel.
- Press a tile firmly into place, twisting slightly as you go.
- Lift the tile back up immediately and inspect the back.
- The adhesive should cover at least 80% of the surface evenly, with no large bare patches.
If coverage is patchy or low, switch to a larger notch or adjust your technique.
Tips for Better Results
- Hold the trowel at a consistent angle (roughly 45–60°) to get uniform ridge height.
- Work in manageable sections so the adhesive doesn't skin over before you lay the tile.
- Use a spirit level and tile spacers to keep large-format tiles flat — lippage (uneven tile edges) is a common problem with big tiles on NZ timber floors.
- For large-format tiles over 600 mm, consider hiring or buying a notched float rather than a standard trowel for better control.
When to Call a Professional
While laying floor tiles is a popular DIY project, there are situations where it pays to bring in a qualified tiler:
- Large-format tiles (600 mm+) on timber subfloors require experience to avoid cracking and lippage.
- Wet areas such as showers and bathrooms involve waterproofing membranes that, if done incorrectly, can lead to costly water damage and may not comply with the Building Code.
- Heated floor systems (underfloor heating) require specific adhesives and installation methods.
- Structural or substrate issues — if your subfloor is bouncy, uneven, or damaged, a professional can assess whether it needs reinforcing before tiling.
You can find qualified, vetted local tilers on tradedirectory.co.nz to get the job done right the first time.