Tiling in London Renovations: Format, Adhesive, Grout, and What Goes Wrong

Tiling is one of the most skill-dependent trades in a renovation. A good tiler working with the correct adhesive and grout on a properly prepared substrate produces a floor or wall that will last 30 years without intervention. A poor tiler, or the correct tiler working with an incorrect specification, produces hollow tiles, cracked grout joints, and eventually tiles that lift — one of the most disruptive and expensive remediation exercises in a finished bathroom.

This guide covers the specification decisions and quality standards that determine whether a tiled surface performs for its intended lifespan.

Tile format and weight

The dominant trend in London renovation tiling over the last decade has been large-format tiles: 600×600mm, 800×800mm, and 1200×600mm formats are now standard in high-specification bathrooms and kitchens, replacing the 300×300mm and 200×200mm formats that were conventional in the 2000s.

Large format tiles have advantages: fewer grout joints (visually cleaner, easier to maintain), and a more contemporary appearance. They also have demanding installation requirements:

Subfloor flatness: a 600×600mm tile bridging a 5mm hollow in the subfloor will either rock or crack at the point of loading. British Standard BS 5385 specifies that the substrate variation under a tile must not exceed 3mm under a 2m straight edge (for tiles up to 600mm) or 5mm under a 3m straight edge (for larger formats). Any deviation beyond this must be corrected before tiling begins.

Adhesive coverage: large format tiles require back-buttering (adhesive applied to both the substrate and the back of the tile) to achieve full coverage. The minimum coverage requirement under BS 5385 is 80% in dry internal areas and 95% in wet areas. Under-coverage creates voids behind the tile — the tile sounds hollow when tapped, the adhesive bond is reduced, and in a wet environment moisture penetrates to the void and causes failure.

Tile weight: a 1200×600mm porcelain tile typically weighs 20–25kg. At ceiling or high-wall positions, this requires a high-performance wall tile adhesive and appropriate key in the substrate. Standard wall tile adhesives are not appropriate for large heavy tiles in wall positions.

Adhesive selection

Adhesive selection is determined by:

1. 1.Substrate type: plasterboard, cement board, existing tiles, concrete, or timber
2. 2.Location: floor, wall, or ceiling
3. 3.Wet or dry environment: bathroom, kitchen, or dry room
4. 4.Tile format and weight
5. 5.Underfloor heating: if present, a flexible adhesive is required

Standard areas (dry, wall tiles up to 600mm): S1 flexible adhesive (e.g. BAL Rapidflex, Mapei Keraflex). Standard C2 adhesive is acceptable in dry areas for smaller formats but S1 flexible provides better performance.

Wet areas (showers, wet rooms, bath surrounds): S1 flexible polymer-modified adhesive applied to a tanked substrate. The waterproofing system (tanking membrane) must be installed before tiling — tiles do not provide waterproofing, the membrane beneath them does.

Over underfloor heating: S1 or S2 flexible adhesive throughout. A rigid adhesive over a heated screed cracks as the screed expands and contracts. The UFH system must be commissioned and cycled before tiling to pre-condition the screed.

Over existing tiles (adhesive-on-adhesive): possible in some circumstances but requires the existing tiles to be fully bonded (no hollow tiles), the surface prepared with a primer, and an S1 flexible adhesive used. Better practice is to remove existing tiles entirely and tile on a prepared substrate.

Timber substrates: direct tiling onto timber subfloors requires an uncoupling membrane (e.g. Schlüter DITRA) to decouple the tiles from the movement of the timber. Tiling directly onto timber without decoupling will crack grout and eventually crack tiles as the timber moves seasonally.

Grout selection

Unsanded (fine) grout: for joints up to 3mm. Used for wall tiles and joints where a fine finish is required.

Sanded grout: for joints 3–12mm. Floor tiles and wider joints.

Epoxy grout: two-component grout with excellent stain, chemical, and moisture resistance. Appropriate for heavily used floors, commercial kitchens, and areas where hygiene is critical. More expensive and more difficult to apply than cementitious grout — requires an experienced tiler. Clean-up of excess epoxy must be done before it sets; once cured, it is extremely difficult to remove.

Grout joint width: the grout joint width affects the visual outcome significantly. Rectified tiles (mechanically cut to precise dimensions) allow joints as narrow as 1.5–2mm, which reads as seamless from a distance. Non-rectified tiles have dimensional variation that requires wider joints (3–5mm) to accommodate. Specifying 1.5mm joints with non-rectified tiles produces inconsistent-width joints — always confirm tile rectification status before specifying joint width.

Grout colour: grout colour is a design decision with lasting consequences. Light grout reads clean initially but stains over time in floor applications. Dark grout hides staining but shows efflorescence (white salt deposits) more visibly. Mid-tone grout matching the tile is the most forgiving specification for floor joints in wet areas.

Substrate requirements for wet areas

The most important principle in wet area tiling: tiles do not provide waterproofing. Water penetrates grout joints — particularly after grout shrinks and micro-cracks develop with age. The waterproofing must be in the substrate beneath the tiles.

Bathroom walls: tanking membrane applied to the substrate before tiling. In standard bathrooms (no direct water impingement on walls except in the shower zone), water-resistant plasterboard (pink or green board) with waterproof adhesive is the minimum specification. In shower zones, a full liquid-applied membrane (BAL Waterproofing Membrane, Mapei Mapelastic) applied to all surfaces within the shower enclosure, extended a minimum of 200mm beyond the shower perimeter, is the correct specification.

Wet room floors: full tanking of the floor and at least 150mm up all walls, continuous with the drain flange. See the wet room specification guide for details.

Shower trays: proprietary acrylic or stone resin trays are a self-contained waterproof surface. They require a silicone joint (not grout) at the wall junction — grout at this junction will crack with movement and allow water ingress.

Silicone joints

Movement joints (silicone rather than grout) are required: - At all internal corners (floor/wall junction, wall/wall junction) - At the junction between tiles and any other material (bath edge, shower tray, door frame) - At expansion joints in large floor areas (every 4–5m in a large open floor, or at any structural joint in the building)

Grouting these positions instead of siliconing them is the single most common tiling specification error. The movement at internal corners and junctions is sufficient to crack cementitious grout within 1–2 years. The silicone joint must match the grout colour — all major grout manufacturers produce matching silicone sealant ranges.

Quality indicators

When inspecting completed tiling:

• —Tap test: tap each tile with a coin or knuckle. A hollow sound indicates inadequate adhesive coverage. In a quality installation, all tiles should sound solid.
• —Lippage: adjacent tiles should be flush at joints. Run a finger across the joint — any step greater than 1mm is visible and collects dirt. Causes include uneven substrate, poorly sorted tiles (thickness variation within a batch), or inadequate bed adjustment by the tiler.
• —Joint consistency: grout joint width should be consistent throughout. Variable joints indicate non-rectified tiles or poor setting-out.
• —Silicone joints: check all corners and junctions for silicone (not grout). Grout at internal corners is a defect.
• —Grout consistency: grout should be fully filled to the depth of the joint, with no voids or crumbling. Inadequately filled joints allow water penetration and collect dirt.