Grout Types and Selection: Cementitious, Epoxy, Joint Widths and Wet Areas

Quick Answer: Grouts are classified under BS EN 13888 as CG (cementitious) or RG (reaction resin / epoxy). Use CG2 polymer-modified grout for most wall and floor work; epoxy RG for swimming pools, commercial kitchens, and worktops. Joint width determines sanded vs unsanded grout — unsanded for joints ≤3mm, sanded for 3–10mm. Wet area perimeter joints use sanitary silicone, never grout. Apply with rubber float, work diagonally across joints, clean off with damp sponge before grout hardens.

Summary

Grout selection is shorter and simpler than adhesive selection, but the choices still matter. The wrong grout type — sanded grout in a 2mm joint, cementitious grout in a swimming pool, no-anti-mould grout in a steamy bathroom — visibly fails within months. Most failures are not the grout itself but the application: too thick, too thin, contaminated, or not cured before traffic.

The BS EN 13888 classification mirrors the adhesive standard. CG (cementitious grout) with classes 1 and 2 for standard and improved; W for reduced water absorption; A for high abrasion resistance. RG (reaction resin grout) is two-pack epoxy or polyurethane, chemically resistant and waterproof in itself. For most domestic work, CG2 W (improved cementitious with reduced water absorption) is the default. For commercial wet, chemical or food-prep environments, RG epoxy is the right choice despite the higher cost and harder application.

Joint width drives the choice between sanded and unsanded. Unsanded grout (no aggregate) is used for narrow joints (≤3mm) including most wall tile and natural stone where sand could scratch the surface. Sanded grout has fine aggregate to bulk it out and reduce shrinkage in wider joints (3–10mm) — typical floor tile spacings. Beyond 10mm, dedicated wide-joint grouts or proprietary fillers are needed. See tile adhesive selection for adhesive selection and tile expansion joints for movement joints (which are NOT grouted).

Key Facts

• BS EN 13888 — Grouts for tiles; definitions and classification
• CG — Cementitious grout — Portland cement + polymers; class 1 standard, class 2 improved
• RG — Reaction resin grout — epoxy or polyurethane two-pack; chemical resistance, waterproof
• W — reduced water absorption — CG2 W class; for damp or wet areas
• A — high abrasion resistance — CG2 A class; for commercial floors
• Unsanded grout — for joints ≤3mm; smooth finish; for wall tile, natural stone, polished surfaces
• Sanded grout — for joints 3–10mm; aggregate reduces shrinkage; typical for floor tile
• Wide joint grout — proprietary products for joints >10mm; e.g. flagstone, rustic floor tile
• Epoxy grout — RG class; 100% solids; impervious; stain resistant; chemical resistant; harder application
• Joint depth — typical 3–6mm depth; grout must penetrate full depth for strength
• Mixing — cementitious: water per manufacturer ratio, paddle mix, slake 5 min, remix; epoxy: A + B + filler, mix immediately, short pot life
• Application tool — rubber-faced float; diagonal action across joints
• Cleaning — damp sponge in clean water; clean immediately before grout hardens
• Cure time — cementitious typically 24 hours light traffic, 7 days full; epoxy 24 hours
• Polymer modification — most modern grouts include latex/polymer additives; reduces water absorption and improves flexibility
• Anti-mould additive — for wet area cementitious grouts; many include fungistat
• Sanitary silicone — for perimeter and corner joints in wet areas; never grout
• Colour selection — neutral grey shows least dirt; matching tile colour hides grout lines; contrast emphasises pattern

Quick Reference Table

Detailed Guidance

Cementitious grout — the workhorse

Cementitious grout is Portland cement, fine sand (in sanded grouts), pigment, and polymer additives. Mixed with clean water, applied with a rubber float, cleaned off with a damp sponge.

Process:

1. Check tile adhesive is cured — at least 24 hours from tiling; longer in cold conditions
2. Vacuum the joints clear — remove any adhesive that squeezed up; lift any sticky residue
3. Mix grout — water per manufacturer's ratio (typically 1:3 by weight, water to powder); paddle mix at low speed (300 rpm); avoid whipping in air; slake for 5 minutes; remix briefly; check consistency (stiff peak that doesn't slump)
4. Apply with rubber float — work diagonally across the joints (45° to joint direction); force grout into joints; remove excess in same diagonal stroke
5. Initial set — typically 20–30 minutes; surface looks dull and firm; test with a fingernail (should leave a slight imprint but not be soft)
6. First clean — slightly damp sponge (wrung out hard); circular motion; rinse sponge frequently in clean water; aim to leave 1–2mm of grout below the tile face
7. Second clean — when grout is firmer (typically 1–2 hours later); cleaner water; remove haze; check joint profile
8. Polish off — when grout is fully set (typically 24 hours); soft cloth removes final haze
9. Cure — 24 hours before light traffic, 7 days before heavy use or sealing

Epoxy grout — premium chemistry

Epoxy grout (RG class) is a two-pack resin system: a base resin (A), a hardener (B), and a coloured filler. Mixed in proportion specified by manufacturer, used within 30–60 minutes pot life.

Properties:

• Completely waterproof (does not absorb water)
• Stain resistant
• Resistant to acids, fats, chemicals
• Permanently coloured (no fade)
• Hard, dimensionally stable

Trade-offs:

• 3–5× the cost of cementitious
• Short pot life
• Harder to clean off (white spirit or proprietary cleaner; cannot use water alone)
• Skin/eye irritation — full PPE
• Requires precise mixing
• Less forgiving in joint widths and profile

Used where the cost is justified: swimming pools, commercial kitchens, food-prep counters, hospital wet rooms.

Joint widths — sanded vs unsanded

The joint width drives the choice between sanded and unsanded grout. The rules:

• ≤3mm — unsanded (smooth) grout; no aggregate could scratch; less rigid but joints are too narrow for shrinkage problems
• 3–10mm — sanded grout; fine aggregate bulks out the joint and reduces shrinkage cracking
• >10mm — proprietary wide-joint grout; usually pourable; for flagstones and reclaimed tile
• Movement / perimeter joints — NEVER grouted; sanitary silicone or proprietary movement profile

Using sanded grout in a 2mm joint: cannot bed properly, grout pulls out, mineral grit shows. Using unsanded grout in a 10mm joint: shrinks excessively, cracks, separates from tile edges.

Wet area considerations

Bathroom and wet room grouts need:

• Anti-mould additive — most modern wet-area grouts include a fungistat that resists Aspergillus and similar mould; check the bag
• Reduced water absorption — CG2 W class; absorbs <3% water per BS EN 12808
• Compatible silicone — perimeter joints and corner joints in showers use sanitary silicone (mildew-resistant), colour-matched to grout — never grout

Steam rooms, saunas and intensively used wet areas may need epoxy grout for full water resistance and easier cleaning.

Underfloor heating

Heated floors stress grouts. Use:

• CG2 W with polymer modification (flexible)
• Anti-fracture matting (Schluter Ditra, Dural Durabase) below tile if substrate is questionable
• Movement joints per BS 5385 (every 5m for heated floors)
• Cure adhesive and grout completely (7 days) before turning UFH back on
• First UFH run after grouting: increase temperature 5°C per day to avoid thermal shock

Natural stone

Natural stone — marble, travertine, limestone, slate — is more sensitive to grout selection:

• White or light stone — use white grout to avoid pigment showing through translucent stone
• Porous stone (limestone, travertine) — seal the stone BEFORE grouting (a single coat of impregnating sealer); otherwise grout slurry stains the stone surface
• Reactive stone (green marble, metallic-veined slate) — check with supplier; some stones react with cement and need epoxy grout
• Polished stone — avoid sanded grout (scratches); use unsanded only

Cleaning during application

Clean grout off the tile face before it hardens — once cured, cementitious grout is much harder to remove (proprietary cement haze remover or careful scraping). Epoxy grout is essentially impossible to remove from the tile face once cured.

The cleaning sequence:

1. First pass — sponge slurry — heavy water, removes most of the bulk grout; firm pressure leaves the joint slightly hollow
2. Second pass — surface clean — lighter water, smooths the joint profile, removes most of the haze
3. Third pass — haze removal — almost dry sponge or cloth; removes the residual film
4. Final polish — dry soft cloth when fully cured

Each pass: clean sponge and clean water. A dirty sponge re-deposits grout slurry — slowing cleanup and reducing finished quality.

Sealing grout

Cementitious grout absorbs water and dirt over time. Sealing extends life and ease of cleaning:

• Impregnating sealer — penetrates the grout, leaves no film; restored periodically (annually for high-use)
• Surface sealer — coats the grout surface; less common; can become slippery on floors

Apply sealer at least 7 days after grouting (full cure). Mask the tile to avoid sealer on the face. Allow 24 hours before traffic.

Epoxy grout does not need sealing — it's already impervious.

Frequently Asked Questions

Can I use grout to seal the gap between tiles and the bath?

No — this joint is a perimeter movement joint and MUST be sealed with flexible sanitary silicone, not grout. Grout in this position cracks within months as the bath flexes, and water enters behind the bath causing rot and mould. The correct detail: 3–5mm gap left during tiling, filled with sanitary silicone after grouting.

What grout joint width should I use?

The tile manufacturer usually specifies a minimum. Typical values:

• Mosaic — 2mm
• Standard wall tile (200–300mm) — 2–3mm
• Standard floor tile (300mm) — 3mm
• Larger floor tile (600mm+) — 3–5mm
• Rustic / handmade tile — 5–8mm (irregular tile needs wider joints)
• Reclaimed / Victorian — 5–10mm

Narrower than the manufacturer's minimum and tiles don't have room to expand or settle into the bed; wider than typical for the tile shows as gappy and visually unbalanced.

How long after grouting can I use the shower?

24 hours minimum for the grout to set hard, 72 hours preferable. Cementitious grout continues to cure for 7 days — early exposure to repeated hot water can weaken the bond. Modern improved-class grouts (CG2 W) tolerate earlier use better than basic CG1, but 72 hours is a safe default.

Why does grout discolour over time?

Several causes:

• Soap scum and limescale in bathrooms — clean with mild acidic cleaner (cream cleaner, white vinegar)
• Mould growth — bleach solution kills mould; if recurring, ventilation needs improvement
• Dirt absorption — sealing helps; light grey grout shows dirt less than white
• Pigment fade in coloured grout exposed to UV (external tiling) — choose UV-stable products

Once mould has penetrated the grout, the only permanent fix is to rake out the joint and re-grout.

Can I change grout colour without re-tiling?

Yes — grout colourants paint over existing grout. Reasonable results for changing white/grey grout to a darker tone. Less reliable going from dark to light. The colourant film wears off over 2–5 years and needs reapplication. For permanent colour change, raking out and re-grouting is the durable solution.

Is epoxy grout worth the cost for a domestic bathroom?

Usually no. Standard CG2 W with anti-mould additive performs well for 5–10 years in domestic bathrooms with adequate ventilation. Epoxy is justified for steam showers, swimming pools, hot tubs, and commercial environments — domestically, the difference doesn't usually justify the 3–5× cost.

What about coloured grout — black, brown, white?

Coloured grouts use mineral pigments and perform identically to grey or white. Considerations:

• White grout — shows every speck of dirt; only on light tile with good cleaning regime
• Black/dark grout — dramatic appearance with light tile; hides dirt; check the dye for any colour bleed onto porous tile (test on offcut)
• Coloured grout on natural stone — pigment may bleed into porous stone; seal the stone before grouting

Regulations & Standards

• BS EN 13888:2009 — Grouts for tiles; definitions and specifications

• BS EN 12808-1 to -5 — Test methods for grouts (abrasion, flexural and compressive strength, water absorption, shrinkage)

• BS 5385-1 to -5 — Wall and floor tiling installation

• BS EN 12004 — Adhesives for tiles

• BS 6213 — Selection of construction sealants

• BS 8000-11 — Workmanship on building sites — tiling

• CoSHH Regulations 2002 — control of substances hazardous to health (epoxy resins)

• BS 7976 — Pendulum testers (slip resistance — relevant for grout choice on floors)

• BBA / Kiwa certification — third-party product certification

• BS EN 13888 (BSI Knowledge)

• The Tile Association — grout guidance

• BAL grout technical data

• Mapei grout selection guide

• Schluter Systems Kerdi waterproofing

• tile adhesive selection — adhesive selection

• tile cutting — tile cutting techniques

• tile expansion joints — movement joint design (NOT grouted)

• grout quantities — grout quantity calculator

• natural stone — natural stone tiling

• wet room construction — wet room construction