Grout Selection Guide: Joint Width, Flexible vs Standard and Anti-Mould Options

Quick Answer: Cement-based grout is classified by BS EN 13888 as CG1 (basic) or CG2 (improved); reaction-resin grouts (epoxy, polyurethane) are class RG. Joint width determines particle size — narrow-joint (<3mm) for non-rectified tiles 1–3mm joint, wide-joint (3–15mm) for floor tiles, and >15mm needs specialist large-bed grout. Use flexible CG2 with anti-mould additives in wet areas; epoxy RG for chemical exposure, food preparation surfaces, or where stain resistance is paramount.

Summary

Grout is the unsung material in tile installation — it occupies 5–10% of the visible surface area but accounts for 80% of tile-installation complaints. The two failure modes are mould (cement grout in wet areas) and cracking (rigid grout on flexing substrates). Both are eliminated by correct grout class selection at the start, costing £5–£15 more per typical bathroom over the cheapest option but eliminating the £400–£800 re-grout call-back two years later.

This guide covers the BS EN 13888 grout classification system, joint width selection, the cement-based versus reaction-resin choice, anti-mould additives, colour matching and pigment stability, and the practical application sequence including time gaps from tile install to grout to silicone. It includes a comparison table for the four common scenarios (standard wall, floor, wet area, kitchen splashback) with the right grout for each.

The most common selection mistake: using a standard CG1 cement grout in a shower or wet area. Cement grout is porous and absorbs water and soap residue, then becomes a substrate for mould growth. Within 18 months the joints look black and the customer thinks the tiler did a poor job. The fix is anti-mould additive (BAL Microflex, Mapei Mapegrout, etc.) in mix or, better, switching to epoxy resin grout (RG) in showers — costing £25–£40 more on a typical bathroom but eliminating mould complaints permanently.

Key Facts

• BS EN 13888:2009 — grout classification standard
• CG1 — basic cementitious grout; meets minimum compressive strength
• CG2 — improved cementitious grout; high abrasion resistance, low water absorption
• W suffix (CG2W) — water absorption ≤5g after 30 minutes
• A suffix (CG2A) — high abrasion resistance for floors
• RG — reaction resin grout (epoxy, polyurethane)
• Polymer-modified — most modern grouts include polymer for improved flexibility and water resistance
• Joint width — narrow (1–3mm) — for rectified porcelain, fine-cut natural stone
• Joint width — standard (3–6mm) — for most floor and wall tiles
• Joint width — wide (6–15mm) — for handmade tiles, brick-effect, irregular tiles
• Joint width — extra wide (>15mm) — for large-format external paving, requires specialist large-bed grout
• Cement grout cure time — 24 hours for foot traffic, 7 days for full chemical resistance
• Epoxy grout cure — 24 hours for foot traffic, 24 hours for chemical resistance, but harder to clean off tile face
• Pot life — cement grout 60–90 min; epoxy grout 30–45 min typical
• Silicone joints — replace grout at all internal corners, perimeters, and around penetrations (BS 5385); never grout these
• Movement joint silicone — colour-matched to grout; non-anti-mould (silicone with anti-mould additive can stain)
• Grout coverage — varies by tile size, joint width, depth; ~0.5kg/m² for 100×100mm × 4mm joint × 6mm depth
• Anti-mould additive (in cement grout) — fungicide added at mix; effective 2–5 years; needs reapplication or premium epoxy
• Pigment stability — dark pigments (charcoal, black, brown) more prone to fading and "haze" than mid-tones; whites and creams most stable

Quick Reference Table — Grout Selection

Detailed Guidance

Joint width selection

The joint between tiles serves two functions: cosmetic (tolerance for tile dimension variation) and engineering (movement accommodation). Wider joints handle more movement and tile-size variation but show more grout.

The bigger error is going too narrow rather than too wide. Tiles batched within "1mm tolerance" can vary in dimension by 1.5–2mm in practice. A 1mm joint cannot absorb this variation and the grout lines will look wavy.

Cement grout (CG1, CG2) — the workhorse

Cement-based grouts are mineral powders (Portland cement + sand + polymer + pigment) mixed with water. CG1 is the basic class; CG2 is improved with higher strength, lower water absorption, and better resistance to chemical attack.

The W and A suffixes:

• W (water absorption ≤5g after 30 min) — essential for wet areas
• A (high abrasion resistance) — essential for trafficked floors

Most modern bagged grouts on the UK market are CG2W or CG2WA. Pure CG1 is becoming rare; budget bargain grouts at £4–£6/bag are usually CG1. Premium polymer-modified CG2WA grouts at £12–£25/bag.

For wet areas, the issue with cement grout is porosity. Even CG2W cement grout absorbs water and soap scum gradually, providing a substrate for mould (Aspergillus, Cladosporium, Stachybotrys) to colonise. The visible result is the black-line bathroom shower joints familiar from any 5+ year old bathroom.

Anti-mould additives (BAL Microflex, Mapei Microadditivo, Tile Doctor MoldGuard) added at mix delay this for 2–5 years. They work, but they wear off — the fungicide is consumed over time and re-treatment is needed.

Reaction-resin (RG) grouts — epoxy and polyurethane

Reaction-resin grouts (RG class per BS EN 13888) are two-part chemical systems: a resin and a hardener that cure by chemical reaction rather than by water evaporation.

Epoxy grout (most common RG):

• Non-porous, mould-resistant for life
• Chemically inert — coffee, wine, tomato sauce, bleach all wipe off
• Premium cost: £35–£75 per 5kg pack vs £6–£15 for cement
• Harder to apply — sticky, short pot life (30–45 min), residue must be removed within 24 hours or it bonds to tile face permanently
• Best for: shower floors, kitchen worktops, food preparation areas, swimming pool surrounds, hospitals/labs

Polyurethane grout (less common RG):

• Highly flexible — accommodates significant movement
• Resistant to UV degradation (epoxy can yellow in sunlight)
• Cost similar to epoxy
• Best for: external paving, expansion-prone substrates

For a typical shower (2.4m² wall + 1.2m² floor = 3.6m² tiled area, ~3kg of grout), epoxy adds £20–£35 to material cost vs cement grout. The customer call-back rate at year 2–3 with cement grout in showers is high enough that premium installers default to epoxy in all wet areas.

Application sequence and timing

Standard sequence:

1. Tile install — bed in adhesive, leave 24 hours minimum (4 hours rapid-set)
2. Joint check — clean any adhesive from grout joints
3. Grout application — mix small batches; spread with rubber float at 45° angle to joints
4. Initial wipe — within 15–30 min of placement, wipe with damp sponge in circular motion to flush joints level and remove surface grout
5. Haze removal — 1–4 hours later, polish with dry microfibre or soft cloth to remove cement haze
6. Cure — 24 hours before light foot traffic; 7 days before heavy use or wet exposure
7. Silicone perimeters — apply at all internal corners, perimeters, around penetrations (24 hours after grout cure)

Rushing the haze removal stage is the most common fault — leaves white "blooming" on tile face that is hard to remove later. Polishing too early can pull grout from joints; too late and the haze sets permanently. The 1–4 hour window is the sweet spot.

Anti-mould additives — what works

Silver-additive grouts are the newest premium category, marketed for hospitals and food prep. Independent verification of long-term performance is limited; claimed life expectancy is not yet supported by 10+ year field data.

The most reliable solution remains: epoxy grout in wet areas, anti-mould cement grout elsewhere with a refresh sealer at year 2–3 if needed.

Silicone — where grout doesn't go

BS 5385-1 and -3 require flexible silicone (not grout) at:

• All internal corners (wall-to-wall, wall-to-floor)
• Perimeter of tiled floors
• Around penetrations (waste pipes, soil pipes)
• Where two substrates meet (tile-to-bath, tile-to-shower tray, tile-to-worktop)
• At expansion joints (every 4–5m on floors, 4m on walls)

Use neutral-cure silicone matched to grout colour. Anti-mould silicones contain fungicides similar to anti-mould grout and provide 2–5 years protection. Bath and shower silicones are colour-stable; kitchen silicones may be food-grade for worktop applications.

Colour selection — the cosmetic decision

Standard grout colours: white, ivory, beige, light grey, mid grey, dark grey, charcoal, black, brown.

Practical considerations:

• White grout stains visibly and shows mould early — high maintenance but timeless aesthetic
• Mid-grey is the universal default — hides minor staining and tile dust, works with most tile colours
• Charcoal/black photographs beautifully on Instagram but shows lime scale (white deposits) starkly — high maintenance in hard-water areas
• Coloured grout (red, blue, green) — pigment fades in direct sun (UV degradation); use only indoors or with UV-stable epoxy

For floor tiles, mid-grey or beige hides foot traffic dirt better than light or dark extremes. For wall tiles, match grout to the lightest tone in the tile pattern for a unified appearance, or contrast deliberately for grid emphasis.

For homeowners — what should I expect

A grout specification on a typical bathroom quote should call out:

• Class (CG2W minimum for walls, CG2WA for floor)
• Anti-mould additive or epoxy grout in shower zone
• Silicone at all corners and perimeters (not grout)
• Colour selected with samples

If the quote just says "grout included", ask which class and whether the wet area gets epoxy or anti-mould additive. The £20–£35 differential is trivial; the long-term complaint avoidance is significant.

Realistic grout cost for a typical 8m² bathroom:

• Cement CG2W throughout: £15–£40 in materials
• Cement CG2WA + epoxy in shower: £35–£75
• Full epoxy throughout: £80–£140

Labour cost is broadly the same regardless of grout class — the difference is application technique and cleaning effort, not time.

Frequently Asked Questions

Why does my bathroom grout go black so quickly?

Almost always mould growth on porous cement grout. Soap residue, body oils, and shampoo all deposit on the grout, creating a nutrient layer for fungi. With shower humidity, the conditions are ideal. The fix at re-grout is to switch to epoxy grout in the shower area, or use a silicone-based grout sealer applied annually. Bleach kills the surface mould temporarily but doesn't address the porosity issue.

Can I re-grout over existing grout?

Only if the existing grout is sound. A "skim re-grout" — raking out the top 2–3mm and applying new grout over the rest — is acceptable cosmetic refresh provided no underlying tile failure. If grout is cracking, debonding, or showing through to the adhesive bed, full grout removal and re-grout is needed. A grout removal blade for an oscillating tool is the standard kit (£8–£15).

What's the difference between sanded and unsanded grout?

Sanded grout includes silica sand for joints 3mm and wider; unsanded grout has finer particles for narrow joints (<3mm). Use sanded for floors, unsanded for narrow-joint wall tiling. Modern UK manufacturers usually classify by "narrow joint" and "wide joint" rather than sanded/unsanded, but the principle is the same.

Do I need primer before applying grout?

For cement grout, no — the grout slurry penetrates the joint walls naturally. For epoxy grout on porous substrates (some natural stones), a sealer applied to tile faces before grouting prevents grout adhesion to the tile face and makes cleanup easier. Test on an offcut first if unsure about staining risk.

Can I use bathroom silicone in the kitchen?

Generally yes, but check the food contact rating if the silicone is near food preparation surfaces. Standard kitchen and bathroom silicones are similar formulations; specialist food-grade silicones are available for direct food contact. The mould-resistance characteristics are the same.

Regulations & Standards

• BS EN 13888:2009 — grouts for tiles: classification and tests

• BS 5385-1:2018 — internal wall tiling design and installation

• BS 5385-3:2014 — internal floor tiling design and installation

• BS 5385-4:2015 — special conditions: heated floors, swimming pools

• BS EN 14411 — ceramic tile classification

• BS 8000-11 — workmanship on building sites: tiling

• TTA Specifying Tile Installations — Tile Association best-practice guidance

• EN 13501-1 — fire performance for grouts in fire-rated installations

• The Tile Association — UK tiling industry body

• BS EN 13888 BSI Standards — grout classification

• Mapei UK Technical Bulletins — grout selection guides

• BAL UK Tile Adhesives & Grouts — UK technical resource

• Schluter Profiles & Sealants — silicone and movement joint guidance

• matching adhesive selection for substrate

• grout cracking diagnostics including movement and substrate failures

• wet room construction including epoxy grout specification

• condensation and mould issues that can mimic grout failures