How to clean grout between tiles that has gone grey

30 to 60 minutes. Costs a few cents to a few dollars.

Try this first (costs nothing): scrub a small patch with a stiff brush, warm water and washing-up liquid. Rinse and dry. Most grout looks worse than it is. Dirt, soap scum and body oils sit in the pores.

This guide covers how to clean grout in steps, from free to replace.

Step 1: Soap and a stiff brush

1. Wet the area and spread soapy water over the lines.
2. Scrub with a stiff nylon brush along each line.
3. Rinse with clean water and wipe dry.

A toothbrush works for small areas. Do not use a wire brush. It scratches tiles and wears away grout.

Ruling: soap and a brush clears light dirt on most walls and floors.

Step 2: Baking soda paste

Mix baking soda with a little water to a thick paste. Spread it on stained grout, leave 10 minutes, scrub, rinse and dry.

Use one product at a time. Do not follow it with vinegar or bleach in the same session. Mixed cleaners can give off harmful gases. Check the label on any other cleaner and use it separately, rinsing between.

Ruling: paste is a gentle second step for stains.

Step 3: Mould in the grout

Black or green spots in a damp bathroom are mould. Wash with soapy water, rinse and dry, and keep the room ventilated. If you use a bathroom mould cleaner, follow the label, wear gloves and open a window. For ongoing mould or health worries, ask a landlord, a surveyor or a doctor.

Ruling: dry is the main cure. Run the fan after showers.

Step 4: Reseal, touch up or regrout

• Grout pen or paint: makes old grout look fresh. Good for colour loss.
• Sealer: reduces how fast new dirt sinks in. Apply to clean, dry grout.
• Regrout: needed when grout is cracked, crumbling, or sunken.

Ruling: if it crumbles when you scrape with a fingernail, regrout.

If that does not work

• Stains that stay: they are deep. A grout pen is a fair cover.
• Loose tiles or hollow sounds: water may be behind them. Ask a tiler.
• Mould that returns within days: there is a damp source.

For another scale-based cleaning job, see how to remove limescale from a shower head.

Worked example

Say a shower wall has grey lines across 10 square metres of tile. Round numbers. A soapy scrub takes about an hour with a stiff brush. The lines are lighter but not white. A baking soda paste on the dirtiest 2 square metres takes another 20 minutes. The rest looks fine at arm's length. The bottom row still looks brown, so you use a grout pen, which takes 30 minutes and evens the look.

Common mistakes

• Using bleach and vinegar together or one after another.
• Wire brushes.
• Cleaning wet grout and sealing it. Seal only when dry.
• Ignoring damp.

Fix it, replace it or live with it

Fix it with soap, paste and a pen. Replace it by regrouting when it crumbles. Live with it when the stain is only on a floor in a busy hall.

What it costs

A stiff brush, a box of baking soda and a grout pen come to a few dollars. Regrouting by yourself costs a tub of grout and a rake tool. Hiring a tiler costs more and is worth it for a whole bathroom. Time is the main cost, so work in small sections.

Floor grout and wall grout

Floor grout takes more dirt, such as mud and grease, and may need a second go. Wall grout in a shower takes soap scum and damp. Treat the floor with a brush and soapy water, and let it dry. For walls, wipe dry after each shower with a squeegee. Light grout stains easily. Dark grout hides dirt but can fade.

Where the answer flips

If the grout is pink or orange, that is a bacteria film, not dirt. Scrubbing with soapy water and rinsing is still the safe step, then ask a professional if it returns. If grout is cracked in a shower, water can get behind tiles, so repair soon.

Quick routine for a shower wall

After each shower, run a squeegee down the tiles. Open the window or run the fan for 15 minutes. Once a week, spray a mild soapy mix on the grout and brush it. Rinse and dry. Once a month, check the sealant along the bath for black spots. That takes five minutes a week and keeps grout pale for much longer than a big scrub every year. For the sealant side, see how to remove mould from bathroom sealant.

Sealing, step by step

1. Clean the grout and let it dry for at least a day.
2. Apply sealer with the small brush or applicator, along the lines only.
3. Wipe any off the tile before it dries.
4. Wait the time on the label before wetting it.
5. Test later with a drop of water. If it beads, it works. If it sinks in, seal again.

Sealer is not needed everywhere. It helps most in wet rooms and on kitchen floors. Always follow the product label.