Black streaks on render running down from the roofline or pooling in patches lower down the wall usually have one of two causes, and telling them apart matters because the fix is different for each. Vertical streaks tracking down from above are almost always gutter overspill, while broader black or dark grey staining spread across a wall is more often pollution deposits or a darker strain of algae.
This guide explains how to tell the difference, and how each type is safely removed without damaging the render's surface finish.
Streaks from gutter overspill
If black or dark brown streaks run vertically from just below the gutter line downward, tapering as they go, that's a strong sign the gutter above is overflowing or has a leaking joint, and dirty water carrying moss, silt and roof grime is running straight down the render face. This is one of the most common causes of render staining we see, and it's a symptom worth investigating at the source, not just cleaning off the wall.
Clearing and checking the gutter that's causing the overspill (see our guide on blocked gutter signs) usually stops new streaks forming, while the existing staining is removed with a soft wash treatment on the render itself. Clean the wall without fixing the gutter and the streaks will simply return within a season or two.
Broader black staining: pollution and dark algae
General black or grey-black discolouration spread more evenly across a wall, rather than in distinct vertical streaks, is more often a combination of airborne pollution deposits and darker algae or lichen species establishing on the render surface. This tends to build up gradually over several years rather than appearing suddenly, and is often worst on walls facing busier roads or in more sheltered, less rain-washed spots.
It responds well to the same soft wash biocide treatment used for green algae, though heavier, longer-established black staining sometimes needs a slightly stronger dwell time or a repeat treatment to fully lift.
Why pressure washing render is the wrong approach
Render, particularly modern thin-coat systems like K-Rend, silicone and monocouche renders, has a relatively thin decorative top coat that pressure washing can etch, pit or strip away entirely, leaving a patchy, uneven finish that's far more obvious and expensive to fix than the original staining ever was. This is one of the most common DIY and cowboy-contractor mistakes we see reversed on call-outs.
A soft wash, low pressure, biocide-led, with the chemical doing the work rather than the water pressure, is the method that removes staining without touching the render's surface integrity.
What it costs to have treated
Render soft washing typically runs from around £300-£700+ for a semi-detached property depending on size, access and how heavy the staining is, with larger detached homes or full-house treatments costing more. Where gutter repair work is needed alongside the clean to stop overspill recurring, that's usually priced separately.
If a streak keeps returning within months of a clean, it's worth checking the gutter above before paying for a repeat wash, since treating the symptom repeatedly without the cause costs more over time than fixing the gutter once.
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