How to Regrout a Bathroom: Fix Crumbling Grout in a Single Weekend

That Black Line Between Your Tiles Is Not Just Ugly

Pull back the shower curtain in most British bathrooms built before 2010 and you will spot it: grout that has gone from white to grey to something resembling a Petri dish. The discolouration is not cosmetic. Cracked, crumbling grout lets moisture seep behind tiles, feeding mould colonies in the adhesive layer and, eventually, rotting the plasterboard underneath. A full bathroom re-tile costs £1,200–£2,500 with a tradesperson. Regrouting the same bathroom yourself? About £40 in materials and a weekend of effort.

What You Actually Need (and What You Don't)

Hardware shops love upselling grout-related gadgets. Ignore most of them. The essential kit is short:

• Grout rake or oscillating multi-tool with a grout blade — a manual rake (£4–£8 at Screwfix) works for small areas. For a full bathroom, a multi-tool saves hours. The Bosch GOP 30-28 (around £90) handles it cleanly, but any oscillating tool with a thin carbide blade will do.
• Grout float — rubber-faced, about £6. Do not substitute a sponge or your finger.
• Unsanded grout for wall tiles, sanded grout for floor tiles — BAL Micromax2 (roughly £12 for 2.5 kg) is a reliable mid-range option. Mapei Keracolor is another solid pick. Avoid the cheapest own-brand bags; they cure unevenly and stain within months.
• Grout sealant — UniBond or Everbuild after curing. This step is non-negotiable if you want the grout to last more than two years.
• Spray bottle, sponge, bucket, old towels — you probably own these already.

Skip the steam cleaners and "grout pens." Pens are a temporary cosmetic fix that peels within weeks in a wet environment. Steaming softens adhesive behind the tiles — the opposite of what you want.

Removing the Old Grout Without Cracking Tiles

This is the tedious part, and there is no shortcut. Hold the grout rake at roughly 45 degrees and drag along the joint, applying firm but controlled pressure. You need to remove at least two-thirds of the grout depth — shallow scratching will not give the new grout enough purchase to bond.

With an oscillating tool, let the blade do the work. Pressing hard does not speed things up; it just increases the risk of chipping a tile edge. Keep the blade centred in the joint. If a tile is already loose or hollow-sounding when you tap it, remove and re-fix it before regrouting — fresh grout will not stabilise a debonded tile.

Vacuum the joints thoroughly once you have raked them. Dust left in the channel prevents adhesion. A cordless vacuum with a crevice nozzle is ideal. Wipe down with a damp sponge afterwards and let the joints dry for at least an hour.

Mixing and Applying New Grout

Follow the packet ratios exactly. Grout that is too wet slumps out of vertical joints; too dry and it crumbles during application. You want a thick, peanut-butter-like consistency. Mix only as much as you can apply in 20–30 minutes — grout begins to set quickly, and remixing with water weakens the cure.

Load the grout float and sweep it diagonally across the tile surface, pressing grout firmly into the joints. Diagonal strokes prevent the float from digging grout back out of the lines. Work in sections of about one square metre at a time.

After filling a section, wait 10–15 minutes (the grout will start to firm up and lose its sheen), then wipe the tile faces with a barely damp sponge in circular motions. Rinse the sponge frequently. Two passes are usually enough — over-wiping pulls grout from the joints.

The 72-Hour Cure Window

Fresh grout needs at least 24 hours before any water exposure, and 72 hours before you seal it. Yes, that means no showers in that bathroom for a day. Plan accordingly — do the work on a Saturday morning and the bathroom is back in service by Sunday evening for everything except sealing.

Apply sealant with a small brush or the applicator bottle, covering every grout line. One coat is minimum; two coats are better for shower enclosures. Everbuild Forever White (about £8) or UniBond Anti-Mould (roughly £7) are both decent options available at B&Q and Wickes.

Corners and Tile-to-Bath Joints: Use Silicone, Not Grout

This catches people out every time. The joint where tiles meet the bathtub, shower tray, or worktop should never be grouted — it needs flexible silicone sealant. These junctions move slightly with temperature changes and weight. Rigid grout cracks there within months, reopening the gap for water ingress.

Cut the silicone nozzle at a 45-degree angle, apply a steady bead, and smooth it with a wet finger or a sealant tool. Mask either side with painter's tape first for a clean line. Remove the tape before the silicone skins over.

Common Mistakes That Undo Your Work

The most frequent failure is skipping the sealant. Unsealed grout absorbs moisture, stains, and eventually cracks — exactly the problem you just spent a weekend fixing. The second most common mistake is regrouting over old grout without raking it out properly. New grout bonded to a thin layer of old, contaminated grout will not hold.

Some people also mix different grout brands or types in the same bathroom, expecting them to match. They will not. Colour varies between manufacturers even when the label says "white." Buy enough of one product to finish the entire room.

There is an argument for hiring a professional for very large bathrooms or complex mosaic tile patterns, where the margin for error is smaller. But for a standard tiled bathroom with rectangular tiles, regrouting is firmly in DIY territory. The materials cost under £50, the tools are reusable, and the result — clean, sealed grout lines — adds visible value to the room without touching a single tile.