Vertical black lines almost always mark where water repeatedly runs down the same path. Read the top of the streak and you will normally find the cause: a leaking gutter joint, a blocked outlet, a missing drip on a sill, or a coping without a throat.
Read the streak, not the stain
A streak that starts at a gutter joint is a plumbing fault. One that starts under every sill on the same elevation is a detailing issue. Cleaning either without fixing the source buys you a season at best.
This guide sits under our Wall Cleaning & Softwash work, covering 20-mile radius of Plymouth.
Why the stain is dark
Constant wetting supports a mixed growth of algae and darker fungal colonies, then traffic pollution and roof grit bind into it. The result reads as soot but wipes off as biological growth.
The order of work
We clear and check gutters first, then softwash the elevation, so the wall is not re-stained a fortnight later. Where the cause is detailing we tell you what needs a builder rather than a cleaner.
Questions we get on this
- Do you fix the gutter as well?
- We clear, flush and re-seat gutters and can report on a failed joint. Full replacement or fascia work is a builder's job and we will say so.
- Will the wall come back to matching colour?
- Usually yes on render and paint. Very old, heavily weathered stone can hold a shadow, and we will tell you before we start.
- How long does an elevation take?
- Most semi-detached elevations are half a day including protection and rinse down.
Related reading
- Salt and sea air: what it does to coastal exteriors
- Why only one side of the house goes green
- White powder on brickwork: efflorescence explained
- Cracked render: what to fix before cleaning