The rule that makes sense of it
The finished bed is roughly half the depth of the notch. The ridges collapse and spread sideways when the tile is pressed in, so a 6 mm notch leaves you with about a 3 mm bed.
That one relationship connects your trowel to your material quantity. Go up a notch size and your bags per square metre go up with it.
A starting point by tile size
- Mosaic and small tiles — 3 to 4 mm notch, roughly a 2 mm bed
- Standard wall tiles — around 6 mm notch, roughly a 3 mm bed
- Standard floor tiles — 8 to 10 mm notch
- Large format — 12 mm notch and back-buttering
Treat these as a starting point, not a specification. The tile's own flatness and the substrate matter as much as the tile's size.
Too small is the common error
An undersized notch is the classic cause of hollow tiles. There is simply not enough material to fill the space behind the tile, so you get stripes of adhesive with bare channels between them.
It is a false economy. You save a little adhesive and buy yourself tiles that drum underfoot and lift within a couple of years.
Too big causes its own problems
An oversized notch pushes excess adhesive up into the joints, so you spend the day raking squeeze-out out of them before it sets. On thin or translucent tiles the ridge pattern can telegraph through and show as stripes in certain light. It also simply costs more in material.
The test that settles it
Comb an area, set one tile, then lift it straight back up and look at the back.
- Near-complete transfer — the trowel is right.
- Stripes with bare gaps — go up a notch size, or start back-buttering.
- Adhesive squelching out of every joint — go down a size.
Do this once at the start of each job. It takes two minutes and it is worth more than any chart, because it tests your actual tile against your actual substrate.
Comb straight, and keep the angle
Hold the trowel at a consistent angle — around 45 degrees — so the ridges are a consistent height. Comb in straight parallel lines rather than arcs: straight ridges collapse cleanly and let air escape, while swirls trap it.
Worn trowels round off at the notch and quietly under-deliver adhesive. If the teeth have visibly lost their edges, replace it. It is the cheapest insurance on the job.
Note: the figures here are typical values for Maira Tile Bond 3100 and are given as guidance. Real results vary with substrate, tile format, trowel size, temperature and site conditions. Confirm quantities and working times with Ali Traders before you order or start work.