Adhesive and grout
Tile adhesive from notched trowel coverage, with back-buttering
- where
- adhesive per m² (kg/m²)
- coverage for the notched trowel used (kg/m²)
- back-buttering thickness (mm)
- adhesive per mm of bed (kg/m² per mm)
- adhesive needed (kg)
- area to tile (m²)
- allowance for losses (%)
- bags to buy
- bag size (kg)
Explain this equation
Estimates powder adhesive for an area from the coverage figure the manufacturer gives for the notched trowel you are using, plus an allowance for back-buttering the tiles.
Cn is the adhesive used per square metre with that notch, from the data sheet. tb is the thickness of the thin layer buttered onto the back of each tile and Cm the adhesive used per square metre per millimetre of thickness. q is their total per square metre, M the total for area A with a loss allowance w, and B the bags of size Bs.
The combed bed and the buttered layer simply add together. A layer tb millimetres thick over a square metre uses tb × Cm kilograms, because Cm is the weight of powder per square metre for each millimetre of bed.
Sense check: a 10 mm notch on a floor uses about 3.85 kg/m² of a typical flexible adhesive; adding a 1 mm butter coat brings it to about 5 kg/m², so a 20 kg bag does about 4 m².
Watch out: coverage figures differ between brands and notch shapes. Use the figures for the product you are buying.
Powder adhesive for an area from the manufacturer’s coverage for your notched trowel, plus a thin back-buttered layer on each tile where solid bedding is needed.
Enter your values to see the result.
Good to know
Published examples: BAL Flex One 0.5 m²/kg (2.0 kg/m²) with a 6 mm notch on dry walls, 0.4 m²/kg (2.5 kg/m²) with 6 mm on wet walls and 0.26 m²/kg (3.85 kg/m²) with 10 mm on floors; Ceresit CM 14 about 2.1, 2.7 and 3.3 kg/m² for 6, 8 and 10 mm notches. Floors, wet areas, external work and tiles of 0.1 m² or more must be solidly bedded (BS 5385), usually by floating the bed and buttering the tile back.
Common mistake
Taking a coverage figure from one brand and using it for another. Coverage depends on the powder density and the notch shape, so use your product’s data sheet.
Limits
Coverage assumes a flat background and the ribs fully collapsed. Uneven floors, heavily ribbed tile backs and large-format tiles use more. The waste allowance is a judgement value.
Sources
- BAL Flex One technical data sheet (coverage per notched trowel, 2–8 mm bed, 3 mm minimum on floors)
- Ceresit CM 14 Express technical data sheet (consumption per notch size and per mm of bed)
- Mapei Keraflex Maxi S1 product page (consumption 1.2 kg/m² per mm of thickness)
- The Tile Association: Brief guidance on the use of adhesives (solid bed, floating and buttering)
- The Tile Association: Recent changes to BS 5385-1:2018 (solid bedding of tiles 0.1 m² and over)
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.