- 27 Jun 2026
- 4 min read
Black staining gets scrubbed, treated with bleach, and painted over. The stain comes back. That is because the black is almost never the problem — it is the symptom. Whether it appears on external stonework or on the grout and silicone in your bathroom, the cause is the same: persistent moisture with nowhere to go.
Black Staining on External Stone Walls
On Yorkshire gritstone, sandstone, and brick, black biological growth — algae, lichen, and certain mould species — colonises surfaces that stay damp. The usual causes are:
- Failed pointing allowing water to track into the wall and slow the drying cycle
- Blocked or overflowing gutters depositing water repeatedly against the same section of wall
- Failed water repellent treatment — once expired, stone absorbs and retains water
- Inadequate DPC or rising damp from ground level
Cleaning the surface removes the stain temporarily. The growth returns within one or two seasons because the underlying water path has not been addressed. The correct approach is to identify the water source first — through a building leak and damp inspection — and fix the cause before any surface treatment.
Why Sealant Over a Wet Surface Makes It Worse
Some property owners apply a sealant coating to stained stonework to stop water getting in. If the wall is already holding moisture, trapping it inside accelerates biological growth, causes efflorescence, and in winter creates freeze-thaw spalling behind the surface. The wall must be dry before any treatment is applied. On a saturated or water-damaged wall, that may require weeks of drying time after the defect is repaired.
Black Mould and Staining in Bathrooms and Kitchens
In bathrooms and kitchens, black mould on grout lines, silicone beads, and wall tiles is a water problem — specifically, water getting behind the surface layer. The most common entry points are:
- Failed bath or shower silicone — cracked, detached, or degraded beads allow water behind the bath panel or into the wall cavity
- Failed basin silicone — water tracks behind the pedestal or into the unit below, creating hidden damp
- Kitchen splashback silicone — when the bead cracks or lifts at the wall junction, water ingress behind tiles causes adhesion failure and mould growth on the cavity surface
Bleach treatments kill the surface mould temporarily. Within weeks it returns, because the water is still getting in. This is one of the most misdiagnosed problems in domestic property — and one of the most straightforward to fix permanently.
What a Proper Fix Looks Like
The old sealant must be completely removed — not just cut back or painted over. Full strip-out of all old bead material, any contaminated substrate cleaned and dried, then a fresh application of high-quality, mould-resistant silicone. NSJ's internal sealant replacement service covers baths, showers, basins and kitchen splashbacks. We use professional-grade products, not retail silicone, and remove everything before resealing.
If your bathroom or kitchen has persistent black mould around the sealant lines, it is a water problem — not a cleaning problem. Explore the full range of bathroom and kitchen leak and mould issues we fix, or contact us to book an assessment. We cover Leeds, Bradford, Harrogate and across West and North Yorkshire.