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Grout Cracking Or Going Black
Joints are crumbling, cracking out in lines, or staining black however hard you scrub them.
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What's happening?
Grout is a filler, not a waterproof layer, and it fails in two distinct ways that need different answers. Crumbling or powdery grout is usually a mixing or curing problem — too much water, cleaned off too soon, or the wrong grout for a wet area. Cracking in a straight line at a corner, at the top of a bath or where a wall meets a floor is not a grout defect at all: those junctions move, and anything rigid in them will always crack. They should be flexible sealant, not grout. Black staining is a third thing again: it is mould growing in the grout's pores, fed by moisture that never dries, and it comes back after cleaning because the conditions that grew it have not changed.
What usually causes it?
- Grout used where a flexible movement joint belongs — internal corners, tile-to-bath, tile-to-tray, wall-to-floor
- Grout mixed too wet, or washed off before it had cured, leaving a weak and porous joint
- Movement in the background — a springy floor, a stud wall or underfloor heating cycling hot and cold
- Poor ventilation, so surfaces stay damp for hours after every shower and mould has somewhere to live
- Water tracking behind the tiles from a failed seal, keeping the joints permanently wet from behind
- Standard grout used in a shower where a water-resistant or epoxy grout was needed
- Silicone or grout applied over an existing mouldy joint instead of the old one being cut out
Is it dangerous?
Not in itself. Cracked grout is a route for water, not a hazard, and the mould in a bathroom joint is a surface growth rather than a structural problem. Persistent damp and mould in a home is a recognised health risk, especially for children, older people and anyone with asthma or a weakened immune system, so it is worth fixing the cause rather than repainting over it. The practical risk is what open joints let through: a shower wall with cracked joints will wet the board behind it.
Can I fix it myself?
Yes, for most of it. Rake the failed grout out to full depth with a grout rake or an oscillating tool — skimming fresh grout over old is the single commonest reason repairs fail — then re-grout with a product rated for wet areas. Where the crack is at a corner or a tile-to-bath junction, do not re-grout at all: cut it out and replace it with sanitary silicone, which is designed to move. For black staining, clean with a proprietary mould remover, dry the room properly and then deal with the ventilation, or it will return within weeks.
When should you call a bathroom fitter?
Call a bathroom fitter if the grout has already been redone and cracked again, if joints are cracking across a whole floor or wall rather than at one junction, if tiles sound hollow around the failed joints, or if the mould returns straight after cleaning. Repeat failure means movement or trapped water, and neither is a grouting job — it is a substrate, joint-design or ventilation job with grouting at the end of it.
What will the bathroom fitter actually do?
- Identify which joints are grout failures and which are movement joints wrongly filled with grout
- Rake out failed joints to full depth and check the substrate and any waterproofing behind them
- Correct the background where boards are soft, wet or moving before anything is refilled
- Re-grout with the right grade of grout for the area and the joint width
- Replace all changes of plane with sanitary-grade flexible sealant, colour matched to the grout
- Check the room's ventilation and report where the extract rate or the run-on is the reason mould keeps returning
Which joints should never be grout
- Internal corners where two tiled walls meet.
- Where tiling meets the rim of a bath, a shower tray or a basin.
- Where a tiled wall meets a tiled floor.
- Perimeters of a tiled floor, against skirting or a threshold.
- Any junction between two different materials or two different structures.
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