Large-format tiles
Adhesive contact coverage from a lift test
- where
- tile back area (mm²)
- tile length and width (mm)
- adhesive contact coverage (%)
- total uncovered area seen on the tile back (cm²)
Explain this equation
Use this after a lift test: bed a tile, lift it, estimate the bare patches on the back, and get the contact coverage as a percentage to compare with the solid-bed requirement.
At is the whole back of the tile in mm². Av is the area with no adhesive transfer, measured in cm² because that is easier on site. C is the share of the back in full contact with adhesive.
Coverage is simply one minus the void fraction. The factor 100 converts cm² to mm² so both areas are in the same units before dividing.
On a 600 × 600 tile (3600 cm²) a void total of 150 cm² gives about 96%, enough for a wet or external area at 95%. On a 300 × 300 tile the same 180 cm² of voids is 20%, leaving only 80%.
Watch out: voids at corners and edges matter most, even if the percentage looks fine. Back-buttering improves bond but does not by itself fill ridges.
Turns what you see when you lift a freshly bedded tile into a coverage percentage, so you can check it against the solid-bed requirement.
Enter your values to see the result.
Good to know
Lift a tile soon after bedding, add up the bare patches (estimate each as length × width in cm) and enter the total. BS 5385 and The Tile Association ask for a solid bed on all floors, wet areas and external work, and at least 50% contact on dry internal walls; tiles of 0.1 m² and over should be solidly bedded. ANSI A108.5 puts numbers on it: 80% in dry interiors and 95% wet or external, with edges and corners supported.
Common mistake
Checking only the middle of the tile. Voids at corners and edges are what crack and chip under point loads.
Limits
A visual estimate from one lift. Check several tiles at the start and periodically. Coverage is not bond: back-buttering improves bond but on its own does not fill ridges.
Sources
- The Tile Association, Brief guidance on the use of adhesives (≥50% contact on internal dry walls; solid bed in wet areas, all external work and all floors)
- The Tile Association, Presentation on changes to BS 5385-1:2018 (tiles ≥0.1 m² solidly bedded; tiles >0.1 m² fixed above 3 m need mechanical fixings; gypsum plaster 20 kg/m²; large format >3600 cm²)
- Stone World / CTEF, Do you know the ANSI standards for tile coverage, offset and acceptable lippage? (A108.5 coverage 80% / 95%; A108.02 offset ≤33% for sides over 15 in)
- 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.