What Type of Grout Should You Use? Cement, Epoxy, Furan, and Polyurethane Grout Compared

Summary

Grout selection is one of the most frequently underspecified decisions in tiling work. Many tilers specify by colour or brand familiarity without considering the chemical and mechanical requirements of the application. Selecting the wrong grout type for the conditions results in staining, cracking, biological growth in the joints, and in severe cases, chemical attack that causes the grout to disintegrate within months of application.

The European harmonised standard BS EN 13888 classifies grouts into three fundamental types: CG (cementitious grout), RG (reaction resin grout — primarily epoxy), and FU (furan grout, now relatively uncommon). Within CG, a series of suffix letters indicates enhanced performance characteristics. Understanding this classification system is the most efficient way to specify correctly.

Movement joints are a separate but related issue: no tile installation is complete without correctly placed movement joints at perimeter, changes of plane, and in large field areas. Using grout in movement joint positions is a common error that causes the surrounding tiles to crack as the substrate moves. Movement joints must be filled with a flexible sealant (silicone or polyurethane), never grout.

Key Facts

• CG1 — Standard cementitious grout; suitable for walls and floors with normal requirements; minimum joint width 2mm
• CG2 — Improved cementitious grout; higher flexural strength and lower water absorption than CG1
• CG1A / CG2A — Suffix 'A' = abrasion-resistant; relevant for floors subject to heavy foot traffic or wheeled traffic
• CG2W — Suffix 'W' = enhanced water resistance; for swimming pools, wet rooms, showers
• CG2F — Suffix 'F' = fast-setting; achieves service strength faster; useful for commercial projects or where early trafficking is needed
• CG2WA — The most common general-purpose specification: improved strength, water resistance, and abrasion resistance; recommended as the minimum for most domestic floor tiling and all shower/wet room applications
• RG (Reaction Resin Grout) — Two or three-component epoxy system; chemically resistant; no water absorption; suitable for food processing, laboratory, industrial, and pool applications; much more difficult to apply and clean than cementitious
• FU (Furan grout) — Resin-based; highly resistant to aggressive acids and alkalis; used in highly specialised industrial settings; rarely specified in general construction
• Epoxy grout advantages — Virtually non-porous; no colour fading; resists staining from wine, oil, coffee, acids; does not support biological growth
• Epoxy grout disadvantages — Short working time (pot life 20–30 minutes); must be applied quickly and cleaned off all tile faces before it cures; smeared epoxy that has cured is extremely difficult to remove; significantly higher cost
• Joint width minimum — 2mm minimum for all grout types; tiles laid butt-jointed (zero gap) cannot be grouted and will crack as the substrate moves
• Movement joints — Required at: perimeter (wall/floor junction), changes of plane, structural joints in the substrate, around penetrations (pipes, columns), and in large uninterrupted field areas (every 3–4.5m in floors [verify with BS 5385])
• Movement joint sealant classification — ISO 13007-4 defines S1 (standard) and S2 (improved deformability) movement joint sealants; S2 sealants should be used in areas of significant movement
• Grout/sealant interface — Where grout meets a movement joint, the sealant must be applied over backing rod, not over hardened grout — sealant bonded three-way (to both tiles and the grout) will fail; always allow a clean break

Quick Reference Table

Detailed Guidance

Cementitious Grout — Selection and Application

Cementitious grout is a mixture of Portland cement, fine aggregate (typically graded quartz sand), and admixtures that improve workability, water resistance, and flexibility. It is the standard choice for the vast majority of ceramic, porcelain, and natural stone tiling applications in UK construction.

Unsanded vs sanded grout: In the UK, cementitious grout products are generally formulated with fine aggregate for joints above 3mm and with very fine aggregate (or no aggregate) for narrow joints (2–3mm). Some manufacturers specify different products for narrow vs wide joints. Wide joints (above 12mm) require coarser aggregate to resist shrinkage cracking. Always check the manufacturer's joint width guidance for the specific product.

Colour additives: Cementitious grouts use pigment to achieve colour. On porous natural stone (limestone, marble), the pigment can leach into the tile edge and the tile field, causing permanent staining. Use a grout release (film-forming sealant applied to the tile surface before grouting) on porous stone, and test on a spare tile. White or light grey grout is less likely to stain porous stone than dark pigmented grout.

Application technique: Mix to the manufacturer's consistency (typically to a smooth paste that holds its shape). Apply with a rubber grout float, working at 45° to the joint direction to prevent dragging grout out of joints. Remove excess from the tile face in stages — first with the float, then with a lightly damp sponge after initial set, and finally with a clean dry cloth. Do not apply excess water during cleaning — this washes cement out of the joints and weakens them.

Epoxy Grout — Application and Limitations

Epoxy grout consists of an epoxy resin base (Part A) and a hardener (Part B), sometimes with a filler component (Part C). The components are mixed immediately before use and must be applied and cleaned within the pot life — typically 20–30 minutes at 20°C. Higher temperatures reduce pot life significantly.

Application considerations:

• Work in small areas at a time, consistent with the pot life
• Remove all grout from the tile faces before it cures — any residue left overnight will bond permanently; epoxy remover (chemical, abrasive) may be needed and risks damaging tiles
• Use a foam float for application and a slightly damp sponge with firm pressure for cleaning; do not use excessive water
• Smooth the joints with a shaped tool or finger while the epoxy is still workable

Where epoxy is the correct choice:

• Commercial kitchens, food preparation areas (required to prevent bacterial growth in porous cementitious joints)
• Swimming pools and spa areas (chemical resistance to pool treatment chemicals)
• Laboratory floors and bench tops
• Areas subject to oils, fats, acids, or strong cleaning chemicals
• Areas where persistent staining of cementitious grout is a recognised problem

Polyurethane grout (flexible grout sealant): Products such as Mapesil AC, Bostik 600, and similar are single-component flexible joints described as "polyurethane grouting" or "flexible grout sealant". These are essentially sealant products with a fine grain texture to simulate the appearance of cementitious grout but with the flexibility of a sealant. They are useful for thin joints, movement-joint areas, and around sanitary ware where conventional grout would crack. They are not classified under BS EN 13888 and are proprietary products — check manufacturer data for performance characteristics.

Joint Width and Movement Joints

Minimum joint width: A minimum 2mm joint is required for all grout types. Tiles laid with zero joint (tight butt-jointed) will crack along the joint lines as the substrate moves, because there is no space for movement accommodation and no grout bond line to transfer stress. Modern rectified (precision-cut) porcelain tiles can be laid with very small joints (2–3mm) but not truly zero joint.

Why movement joints fail when filled with grout: Movement joints exist to accommodate thermal, moisture, and structural movement in the substrate and the tile installation. If these joints are filled with grout (a rigid material), the movement stress concentrates at the grout line and causes cracking in the tiles, grout, or substrate. Always fill movement joint positions with the correct sealant product.

Movement joint positioning: The UK industry standard for positioning is derived from BS 5385 (wall and floor tiling — code of practice) [verify current parts of BS 5385 relevant to your application]. Key positions:

• Perimeter joints (floor-to-wall, and at changes of level) — must be continuous
• At changes of plane (internal/external corners)
• Over structural joints or movement joints in the backing structure
• Field joints in large floor areas — typically every 3–4.5m in floors (check current BS 5385 guidance)
• Around penetrations (pipes, columns, door thresholds)

Sealant specification: Movement joints in wet areas (showers, pool surrounds) require a sanitary-grade silicone with fungicidal additive (to prevent black mould). For external use, use a movement joint sealant rated for UV exposure. ISO 13007-4 classifies sealants into S1 (standard deformability) and S2 (higher deformability) — use S2 for joints subject to significant movement.

Frequently Asked Questions

Can I use silicone instead of grout in a corner?

Yes, and in most cases this is the correct approach. Changes of plane (floor-to-wall, internal corners) should use a flexible sealant (silicone or polyurethane), not grout. Grout in corners will crack within months as the substrate moves. Use a colour-matched silicone for a neat appearance.

Why does my grout go black in the shower?

Cementitious grout in showers provides a habitat for mould (Aspergillus, Cladosporium, and other fungi) in the porous structure of the grout joint. Prevention: use a CG2W or CG2WA grout with a mildewcide additive, ensure adequate ventilation in the shower enclosure to reduce dwell time of moisture, seal the grout after installation with a penetrating grout sealer, and clean regularly. If mould is persistent, consider using an epoxy grout for new installations.

Is epoxy grout suitable for domestic showers?

Yes, and it is increasingly specified for domestic showers where staining and mould resistance are priorities. The application difficulty is the main barrier — for a large shower enclosure, the tight working time of epoxy requires either experience or additional labour. Mapei, Sika, and Laticrete all produce domestic-grade epoxy grouts that are somewhat more forgiving than industrial versions.

What is the difference between grout and tile adhesive? Can I use one for the other?

No. Grout and tile adhesive are chemically different products designed for different purposes. Adhesive is used to bond tiles to the background and must accommodate peel and shear stresses. Grout is used to fill the joints between tiles and must accommodate surface moisture, cleaning chemicals, and minor differential movement. Never use adhesive as grout or vice versa.

Regulations & Standards

• BS EN 13888 — Grout for tiles: definitions, requirements, evaluation of conformity, marking, and labelling; the primary classification standard (CG1, CG2, RG categories)

• BS 5385 (various parts) — Wall and floor tiling: code of practice; movement joint specification, adhesive and grout selection [verify current parts applicable to floor tiling, wall tiling, and external]

• ISO 13007-4 — Ceramic tiles: grouts and adhesives — requirements and test methods for grouts; defines S1/S2 movement joint sealant classification

• British Standards Institution — BS EN 13888 — Classification standard for grouts

• Mapei UK — Grout selection guide — Product selection guidance with BS EN 13888 classification

• Tile Association (TTA) — Grouting guidance — Industry guidance on grout and movement joint specification

• Sika UK — Tile grout technical data — Epoxy and cementitious grout systems

• Schluter Systems — Movement joint guidance — Movement joint profiles and positioning

• wall tie types — Another cross-reference article on component selection

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