Match the Grout to the Tile and the Room

Grout is the unsung hero of any tiling job. It holds the edges of each tile in place, spreads the load of foot traffic and controls where water travels — or at least, it should. Choose the wrong type and you will be re-grouting a shower within a couple of years.

For most ceramic and porcelain walls and floors, a cementitious grout is the sensible default. It is inexpensive, widely available and forgiving to work with. Within that family, though, there are two broad grades. Fine, unsanded grout is designed for joints under 3mm and gives a smooth, almost seamless finish on polished porcelain or glazed wall tiles. Sanded, wide-joint grout contains a fine aggregate that stops the mix shrinking and cracking in joints of 3mm and above, which covers most floor tiling.

Where the choice really matters is in wet areas and with natural stone:

• Showers, wet rooms and bathroom floors: use an epoxy grout or a high-performance cementitious grout with a built-in water repellent. Epoxy is harder to apply but resists mould, staining and water almost completely.
• Natural stone — limestone, travertine, slate, marble: avoid sanded grout, which can scratch softer surfaces. Use a fine grout or an epoxy formulated for stone.
• Underfloor heating or timber substrates: use a flexible grout, either a pre-mixed flexible product or a cementitious grout with a polymer additive. Thermal movement will crack a rigid joint.
• Kitchen splashbacks: ordinary cementitious grout is fine here, and a stain-resistant additive is a worthwhile upgrade behind a hob.

Get the Joint Width and Depth Right

Joint width is a design decision as much as a structural one. As a rough guide, wall tiles look best at 2–3mm, floor tiles at 3–5mm, and stone or heavily textured tiles often suit 5mm or more. Rectified porcelain, which is cut to a precise size, can take a 2mm joint and still look crisp.

Depth matters just as much as width. Grout needs to fill the joint properly, but it should sit about 2–3mm below the tile face — deep enough to grip, shallow enough to clean. Pushing grout in too deep leaves weak, hollow joints. Overfilling leaves a ridge that catches dirt forever.

One thing many DIY tilers miss: grout is not for movement. Where tiles meet a wall, a bath, a shower tray or a different floor surface, use a colour-matched silicone sealant instead. Grout in those junctions will crack, and once it cracks water gets behind the tiles.

Mixing and Applying for Joints That Last

Mix grout by weight where the instructions give a ratio, not by eye. Use a clean bucket, cold water and a drill paddle on a low speed. Let the mix stand for five to ten minutes, then stir again — this "slake" step lets the polymers hydrate fully. Never add extra water to a batch that is starting to stiffen; mix a new one instead. Work in batches of 2–3kg so nothing sits around long enough to set.

Application is straightforward but rewards patience:

• Work in small areas, roughly one to two square metres at a time.
• Push grout across the joints with a rubber float held at about 45 degrees, moving diagonally to the joint lines rather than along them.
• Make a second pass at a steeper angle to top up any joints that have sunk.
• Leave it to firm up — usually 15–30 minutes — then wipe with a damp sponge, rinsing often. A wet sponge pulls the grout out; a dirty one smears it everywhere.
• Once the surface is dry, buff off any haze with a clean, dry cloth.

Seal Porous Grout — but Only at the Right Time

Cementitious grout is porous by nature. That is fine on a hallway floor, less so in a shower or behind a kitchen hob. A penetrating sealer soaks into the grout and lines the pores, so spills sit on the surface long enough to wipe away.

Timing is everything. Fresh grout must cure before sealing — typically five to seven days, though some products allow it sooner. Test by dripping water onto a joint: if it darkens and soaks in, the grout is ready to take a sealer; if the water beads, it is not fully cured. Sealing too early traps moisture inside, which causes cloudy patches, efflorescence and, in the worst cases, loose tiles.

Apply sealer with a small foam brush or roller, working along the joints. Give it ten to fifteen minutes to absorb, wipe off any excess on the tile face, and apply a second coat once the first is touch-dry. In bathrooms and kitchens, reapply annually — a quick water-drip test will tell you when it is due. Epoxy grout does not need sealing at all, and sealing it is a waste of time and money.

Cleaning and Long-Term Care

Most grout problems come down to cleaning products rather than the grout itself. Acidic cleaners — the kind sold for limescale — will eat into cementitious grout and dull natural stone. Stick to a pH-neutral cleaner and a soft brush for routine work.

For stained joints, a paste of bicarbonate of soda and water, left for ten minutes before scrubbing, lifts most marks without damaging the surface. Steam cleaners work well on floors, though keep the nozzle moving and avoid blasting a single joint for too long.

If black mould keeps returning to a shower, the cause is usually ventilation or a failed silicone joint rather than the grout. Improving extraction and replacing worn sealant will do more than any amount of scrubbing. And if a small area of grout has crumbled, rake it out to a depth of about 3mm, dust it off and re-grout that section — a neat patch is invisible when the colour and texture match.