Tile Adhesive & Grout Explained

Walk into any builders’ merchant and you’ll find an entire aisle of tile adhesives and grouts. Standard, flexible, rapid-set, S1, S2, unsanded, epoxy, the terminology can be baffling, and the consequences of choosing the wrong product range from cracked grout lines to tiles delaminating off a wet room wall. This guide explains what the classifications mean, how to match product to application, and what you should expect to pay.

Tile Adhesive: The Basics

Tile adhesive (also called tile cement or fixing adhesive) bonds tiles to a substrate, typically plaster, plasterboard, cement board, screed, or existing tiles. Most tile adhesives are cement-based powders that are mixed with water; ready-mixed (dispersion) versions are available for light wall tiling, and reaction-resin (epoxy or polyurethane) adhesives for specialist use.

The European standard BS EN 12004-1 classifies tile adhesives by type (cementitious, dispersion or reaction resin) and by performance.¹

• Class C, cementitious (powder mixed with water)
• Class D, dispersion (ready-mixed, acrylic)
• Class R, reactive (epoxy or polyurethane, specialist use)

Within Class C, you’ll see numbers and letters indicating performance:²

• C1, standard adhesive
• C2, improved adhesive (higher bond strength than C1)
• T, slip-resistant (thixotropic, won’t slump on vertical surfaces)
• F, fast-set
• E, extended open time (useful for large-format tiles)
• S1 or S2, deformable, often sold as flexible (S1: a transverse deformation of 2.5–5 mm; S2: more than 5 mm)²

So a product labelled C2 T F S1 is an improved, reduced-slip, fast-setting, deformable adhesive, suitable for wall and floor tiles in most domestic situations, including heated floors and wet rooms.

Which Adhesive for Which Application?

As a rule of thumb: use at least an S1 adhesive wherever the background can move or heat up, such as plasterboard, timber, wet rooms and underfloor heating, and follow the adhesive maker’s data sheet for the substrate. The small cost premium over standard adhesive is insignificant against the cost of re-tiling a failed installation.

Costs

Selco lists Dunlop’s CX-24, the standard-setting C1T adhesive whose data sheet is quoted below, at £17.10 a 20 kg bag including VAT.³ Dunlop’s CF-24 flexible adhesive is from £25.06 a 20 kg bag at the same merchant.⁴ Toolstation sells Mapei’s Keraquick Easy S1, a rapid-setting flexible adhesive, at £17.99 a 20 kg bag including VAT.⁵

Coverage varies with tile size and bed thickness. Dunlop’s data sheet for its standard-setting C1T adhesive gives 2 kg/m² on dry walls and 3.5 kg/m² on floors and wet areas at a 3 mm bed, so a 20 kg bag covers about 10 m² of wall or 5.7 m² of floor.⁶ Use a notched trowel sized to the tile (larger tiles need a deeper notch and a thicker bed) and follow the adhesive maker’s guidance on notch size.

For large-format tiles (above 600 × 600 mm), back-buttering, applying a thin skim of adhesive to the back of the tile in addition to the wall bed, is recommended to achieve full contact coverage. Adhesive coverage is not set by the Building Regulations but by the tiling code of practice, BS 5385, which since its 2018 revision calls for heavier wall tiles to be solidly bedded whatever their size.⁷ Poor coverage, where tiles are not solid-bed fixed, is a common cause of cracked grout, so aim for a solid bed in wet areas and on floors.⁸

Grout: Types and Selection

Grout fills the joints between tiles, prevents water ingress, and is the element that most affects the visual finish. The main types are:

Cementitious Grout (Powder Grout)

The standard for most domestic tiling. Available in:

• Unsanded (fine): for joints up to 3 mm, used for wall tiles with tight joints
• Sanded: for joints 3–15 mm, better for floor tiles and wider joints
• Polymer-modified (often sold as flexible grout): contains added polymer for better flexibility and lower water absorption, useful on floors and wherever the background can move

Epoxy Grout

Two-part resin system mixed on site. Extremely hard, virtually impervious to staining, chemical resistant, and long-lived. Preferred by professionals for kitchen floors, restaurant kitchens, and any area exposed to oils, fats, or acids. It’s significantly harder to work with than cementitious grout: Mapei gives its Kerapoxy epoxy grout a pot life of 45 minutes once mixed, and residue has to be cleaned off before it hardens.⁹ It also costs several times as much per square metre.

Epoxy grout is worth specifying in: kitchen floors where cooking oil or beetroot will land, swimming pool copings, commercial food-prep areas.

Grout Joint Width

Joint width affects both appearance and structural performance:

Rectified tiles, whose edges are ground to a consistent size, look best with narrow joints, but for wall tiling BS 5385-1 still sets minimum widths: 2 mm for tiles under 0.1 m² with no side over 600 mm, and 3 mm for tiles of 0.1–1 m².⁷ Hairline joints under 2 mm fall below the code.

Grout Costs

Coverage depends heavily on tile size and joint width, larger tiles with wider joints consume much more grout per m².

Grout Colour

Grout colour is the most visible design decision in a tiled space. A contrasting grout (dark tiles, light grout or vice versa) emphasises the tile pattern but shows any inconsistency in joint width. A matching or near-matching grout creates a more monolithic, sophisticated look but can look dirty if it’s a light colour in a heavily used kitchen floor.

Whiter grouts stain more easily; darker grouts fade unevenly if bleach cleaning products are used regularly. Mid-tones (warm grey, beige, taupe) are the most forgiving in practice.

Tip: grout samples look different wet and dry, and dry they look different again on the wall versus in the bag. Always ask your tile merchant to show you a cured sample on the actual tile before committing.

Sealing Grout

Cementitious grout is porous and benefits from sealing after it has fully cured (typically 7 days). A penetrating grout sealer reduces staining and is especially worthwhile on floor grout in kitchens and bathrooms. Re-seal every 2–3 years depending on wear.

Epoxy grout does not need sealing.

Common Tiling Mistakes

• Tiling a shower without tanking, whatever the adhesive, the background must be protected from water; even standard adhesives rated for wet areas and power showers assume the background has been waterproofed first.⁶
• Grouting too early, wait until the adhesive has set (typically 24 hours for a standard-setting adhesive, longer in cold conditions, and a few hours for rapid-set products); grouting before the adhesive has cured is a common cause of grout problems.⁶⁸
• Not mixing grout to the right consistency, too wet and it slumps from vertical joints; too stiff and it pulls away before it levels. Aim for a thick peanut butter consistency.
• Forgetting to leave movement joints, British Standards call for movement joints at perimeters, over structural joints and at changes of background, and for internal floors to be divided into bays of no more than 10 m in each direction (3 m outside), filled with flexible sealant rather than grout, to stop tiles cracking or lifting as the floor and substrate move.¹⁰
• Using powder grout at the tile-to-bath or tile-to-tray junction, this joint must be silicone sealant, not grout; it will crack within weeks if grouted.