Tile adhesive and grout: a class-by-class buying guide

The five tile adhesive classes

The numeric class indicates the minimum bond strength after twenty-eight days. C1 is a cementitious adhesive with a minimum bond strength of 0.5 N per square millimetre, suitable for low-water-absorption ceramic tiles on cementitious substrates in dry interior areas. C2 is an improved cementitious adhesive with bond strength above 1.0 N per square millimetre, suitable for vitrified tiles with low water absorption. C2 with the F suffix is fast-setting, the T suffix indicates slip-resistance for wall application, the E suffix extends open time for hot-weather work, and the S1 or S2 suffix adds deformability for substrates that flex. T1 to T5 are not standard letter codes; the formal system is C1, C2 with optional suffixes, and reaction-resin R1 or R2 grades for chemical-resistance applications.

The grout classes

Grout is classified under EN 13888 as CG1 for normal cementitious, CG2 for improved cementitious, RG for reaction-resin epoxy and W for the wider joint variants. CG1 is the sand-and-cement-style grout used in residential interior tiling at modest cost. CG2 is improved with polymer additives for higher abrasion resistance and lower water absorption, suitable for kitchens and bathrooms. RG epoxy grout is non-absorbent, stain-resistant, chemically resistant and the only correct choice for kitchens, food-prep areas, swimming pools, hospitals and laboratories.

The colour selection is independent of the technical class and matters more than is generally appreciated. Light grout in a heavy-traffic area discolours within months and cannot be cleaned back to its original colour without bleaching the surrounding tile. Dark grout in a low-traffic decorative area is fine. Where colour matters more than performance, an epoxy grout in the chosen colour outperforms a cementitious grout of any colour and will not need re-grouting in five years.

Application errors that destroy bond

The most damaging error in residential tile work is dry-set application of a vitrified tile against an unprimed cementitious substrate without back-buttering. The vitrified tile body absorbs almost no water, the substrate absorbs the adhesive water before the cement can hydrate, and the bond strength on the central area of the tile collapses to less than half of the rim bond. The tile will sound hollow when tapped and will fail under impact within months. The fix is to back-butter the tile so that the adhesive ridges engage with a continuous adhesive bed on the tile rather than just the substrate.

The second common error is over-watering the adhesive mix to make it easier to spread, which dilutes the polymer and reduces bond strength by twenty to thirty percent. The product data sheet specifies the water-to-powder ratio, which should be measured rather than guessed. The third common error is laying tiles on adhesive that has skinned over, typically more than thirty minutes after notching in hot weather; the skinned adhesive will not bond to the tile and the joint will fail.

Coverage and quantities

Coverage on a 6-millimetre notched trowel is approximately 1.5 to 1.8 kilogrammes per square metre on a flat substrate; on a 10-millimetre trowel for large-format tile the figure is 2.5 to 3.0 kilogrammes per square metre. A 20-kilogramme bag of C2 adhesive covers approximately ten to twelve square metres on a 6-millimetre trowel. Grout consumption depends on tile size and joint width; a typical 600 by 600 millimetre vitrified tile with a 3-millimetre joint takes 0.4 to 0.5 kilogrammes of grout per square metre.

Related reading

The category page on adhesives and chemicals carries the brand roster. The article on waterproofing systems covers the substrate preparation that must precede tiling in wet areas. For floor finish selection see the category page on tiles and stone.

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