Adhesive and grout
Solid-bed adhesive from bed thickness (BS 5385 full coverage)
- where
- adhesive per m² (kg/m²)
- finished bed thickness (mm)
- 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 adhesive for a solid, fully bedded installation from the finished bed thickness under the tile, as required on floors, in wet areas, outside and for large tiles.
tb is the finished bed thickness once the tile is pressed in and tt the thickness of the butter coat on the tile back. Cm is the adhesive used per square metre per millimetre, from the data sheet. q is the adhesive per square metre, M the total for area A with loss allowance w, and B the bags of size Bs.
A solid bed is a uniform layer, so the adhesive is simply total thickness times the consumption per millimetre. Cm is effectively the weight of powder needed to make one litre of mixed adhesive, spread one millimetre thick over a square metre.
Sense check: a 4 mm bed with a 1 mm butter coat at 1.2 kg/m² per mm is 6 kg/m², so about 3.3 m² per 20 kg bag.
Watch out: the finished bed is thinner than the notch depth, because the ribs collapse. Do not take up out-of-level floors in the adhesive.
Adhesive for a solid (100% contact) bed from the finished bed thickness and the product’s consumption per millimetre. Use for floors, wet areas, external work and large tiles.
Enter your values to see the result.
Good to know
BS 5385 requires solid bedding on floors, in wet internal areas, externally and for tiles of 0.1 m² or more; the TTA describes this as floating a combed bed and buttering the tile back. Typical cementitious adhesives use about 1.15–1.2 kg of powder per m² per mm of finished bed. BAL quote 2–8 mm typical beds and a 3 mm minimum on floors; thin-bed adhesive on gypsum plaster is limited to 3 mm.
Common mistake
Using the notch depth as the bed thickness. A combed bed collapses to roughly half the notch depth once the tile is pressed in.
Limits
Assumes an even bed. Out-of-level floors should be levelled first rather than taken up in the adhesive. Check the product’s own maximum thickness.
Sources
- Mapei Keraflex Maxi S1 product page (consumption 1.2 kg/m² per mm of thickness)
- Ceresit CM 14 Express technical data sheet (consumption per notch size and per mm of bed)
- BAL Flex One technical data sheet (coverage per notched trowel, 2–8 mm bed, 3 mm minimum on floors)
- 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)
- The Tile Association Tiling Guide: thin bed (3 mm max) and 100% coverage on floors and wet areas
- NHBC Standards 9.3.5 Ceramic, concrete, terrazzo and similar tile finishes (solid bed, 3 mm joints, BS 5385-3/-4)
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.