Large-format tiles
Adhesive bed thickness from data-sheet consumption
- where
- mean bed from the trowelled ribs (mm)
- adhesive consumption for the trowel used (kg/m²)
- consumption per mm of bed (kg/m² per mm)
- total bed after embedding (mm)
- back-buttering thickness (mm)
Explain this equation
Use this to check the bed you will end up with under a large tile, from the figures on the adhesive bag, and see whether it falls within the product’s minimum and maximum bed thickness.
C is how much adhesive the data sheet says your trowel uses per m². cm is the weight of a 1 mm layer per m², also from the data sheet. tb is any back-buttering skim on the tile.
Once the tile is pressed in, the ribs collapse into a layer of even thickness. Its mass per m² is C, and each millimetre weighs cm, so the thickness is C divided by cm. Buttering just adds on.
Ceresit CM 14 at 3.3 kg/m² for a 10 mm notch and 1.15 kg/m² per mm gives about 2.9 mm: thin for a large floor tile. Double the consumption and the bed doubles.
Watch out: this is an average. Trowel angle, substrate flatness and how hard the tile is beaten in all change the real bed.
Works backwards from the adhesive used per m² to the bed thickness left under the tile, so you can check it sits inside the product’s allowed range.
Enter your values to see the result.
Good to know
Manufacturers quote consumption for each notch and a figure per mm of bed; dividing one by the other gives the mean bed thickness after the ribs collapse. Ceresit CM 14, for example, gives about 3.3 kg/m² for a 10 mm notch and 1.15 kg/m² per mm, a bed of about 2.9 mm. BAL Flex One is used at 2–8 mm (up to 15 mm in small areas). ANSI-tested thin-bed mortars should leave at least 3/32 in (2.4 mm) after embedding; LHT mortars up to 1/2 in (12.7 mm).
Common mistake
Choosing a trowel for a large-format floor by habit; a 10 mm notch may leave under 3 mm, too little to take up bow and substrate tolerances.
Limits
Average thickness only; actual thickness varies with substrate flatness, trowel angle and how hard the tile is pressed. Use your own product’s figures.
Sources
- Ceresit CM 14 Express technical data sheet (10 mm notch ≈ 3.3 kg/m²; thick bed ≈ 1.15 kg/m² per mm)
- BAL Flex One technical data sheet (bed 2–8 mm typical, up to 15 mm in isolated areas; 10 mm notch solid bed 0.26 m²/kg)
- CTEF, How to select the right trowel notch to obtain required mortar coverage (80% dry / 95% wet and exterior; 95% for stone; embedded thin-bed ≥3/32 in; LHT mortar ≤1/2 in)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.